Implement [<ArgumentFlag>] for two-case DUs (#242)

.gitignore

@@ -11,3 +11,4 @@ result
 analysis.sarif
 .direnv/
 .venv/
+.vs/

ConsumePlugin/Args.fs

@@ -111,7 +111,7 @@ type ChildRecordWithPositional =
     {
         Thing1 : int
         [<PositionalArgs>]
-        Thing2 : string list
+        Thing2 : Uri list
     }
 
 [<ArgParser true>]
@@ -128,3 +128,53 @@ type ParentRecordSelfPos =
         [<PositionalArgs>]
         AndAnother : bool list
     }
+
+[<ArgParser true>]
+type ChoicePositionals =
+    {
+        [<PositionalArgs>]
+        Args : Choice<string, string> list
+    }
+
+[<ArgParser true>]
+type ContainsBoolEnvVar =
+    {
+        [<ArgumentDefaultEnvironmentVariable "CONSUMEPLUGIN_THINGS">]
+        BoolVar : Choice<bool, bool>
+    }
+
+[<RequireQualifiedAccess>]
+module Consts =
+    [<Literal>]
+    let FALSE = false
+
+    [<Literal>]
+    let TRUE = true
+
+type DryRunMode =
+    | [<ArgumentFlag(Consts.FALSE)>] Wet
+    | [<ArgumentFlag true>] Dry
+
+[<ArgParser true>]
+type WithFlagDu =
+    {
+        DryRun : DryRunMode
+    }
+
+[<ArgParser true>]
+type ContainsFlagEnvVar =
+    {
+        // This phrasing is odd, but it's for a test. Nobody's really going to have `--dry-run`
+        // controlled by an env var!
+        [<ArgumentDefaultEnvironmentVariable "CONSUMEPLUGIN_THINGS">]
+        DryRun : Choice<DryRunMode, DryRunMode>
+    }
+
+[<ArgParser true>]
+type ContainsFlagDefaultValue =
+    {
+        [<ArgumentDefaultFunction>]
+        DryRun : Choice<DryRunMode, DryRunMode>
+    }
+
+    static member DefaultDryRun () = DryRunMode.Wet

WoofWare.Myriad.Plugins.Attributes/ArgParserAttributes.fs

@@ -61,3 +61,12 @@ type ParseExactAttribute (format : string) =
 /// `TimeSpan.ParseExact (s, @"hh\:mm\:ss", CultureInfo.InvariantCulture).
 type InvariantCultureAttribute () =
     inherit Attribute ()
+
+/// Attribute placed on a field of a two-case no-data discriminated union, indicating that this is "basically a bool".
+/// For example: `type DryRun = | [<ArgumentFlag true>] Dry | [<ArgumentFlag false>] Wet`
+/// A record with `{ DryRun : DryRun }` will then be parsed like `{ DryRun : bool }` (so the user supplies `--dry-run`),
+/// but that you get this strongly-typed value directly in the code (so you `match args.DryRun with | DryRun.Dry ...`).
+///
+/// You must put this attribute on both cases of the discriminated union, with opposite values in each case.
+type ArgumentFlagAttribute (flagValue : bool) =
+    inherit Attribute ()

WoofWare.Myriad.Plugins.Attributes/SurfaceBaseline.txt

@@ -7,6 +7,8 @@ WoofWare.Myriad.Plugins.ArgumentDefaultEnvironmentVariableAttribute inherit Syst
 WoofWare.Myriad.Plugins.ArgumentDefaultEnvironmentVariableAttribute..ctor [constructor]: string
 WoofWare.Myriad.Plugins.ArgumentDefaultFunctionAttribute inherit System.Attribute
 WoofWare.Myriad.Plugins.ArgumentDefaultFunctionAttribute..ctor [constructor]: unit
+WoofWare.Myriad.Plugins.ArgumentFlagAttribute inherit System.Attribute
+WoofWare.Myriad.Plugins.ArgumentFlagAttribute..ctor [constructor]: bool
 WoofWare.Myriad.Plugins.ArgumentHelpTextAttribute inherit System.Attribute
 WoofWare.Myriad.Plugins.ArgumentHelpTextAttribute..ctor [constructor]: string
 WoofWare.Myriad.Plugins.CreateCatamorphismAttribute inherit System.Attribute

WoofWare.Myriad.Plugins.Attributes/Test/WoofWare.Myriad.Plugins.Attributes.Test.fsproj

@@ -1,10 +1,15 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
     <PropertyGroup>
-        <TargetFramework>net8.0</TargetFramework>
+      <TargetFramework>net8.0</TargetFramework>
 
-        <IsPackable>false</IsPackable>
-        <IsTestProject>true</IsTestProject>
+      <IsPackable>false</IsPackable>
+      <IsTestProject>true</IsTestProject>
+      <!--
+        Known high severity vulnerability
+        I have not yet seen a single instance where I care about this warning
+      -->
+      <NoWarn>$(NoWarn),NU1903</NoWarn>
     </PropertyGroup>
 
     <ItemGroup>

WoofWare.Myriad.Plugins.Attributes/version.json

@@ -1,5 +1,5 @@
 {
-  "version": "3.2",
+  "version": "3.3",
   "publicReleaseRefSpec": [
     "^refs/heads/main$"
   ],
@@ -12,4 +12,4 @@
     "./",
     ":^Test"
   ]
-}
+}

WoofWare.Myriad.Plugins.Test/TestArgParser/TestArgParser.fs

@@ -367,18 +367,19 @@ Required argument '--exact' received no value"""
         let parsed =
             ParentRecordChildPos.parse'
                 getEnvVar
-                [ "--and-another=true" ; "--thing1=9" ; "--thing2=some" ; "--thing2=thing" ]
+                [
+                    "--and-another=true"
+                    "--thing1=9"
+                    "--thing2=https://example.com"
+                    "--thing2=http://example.com"
+                ]
 
-        parsed
-        |> shouldEqual
-            {
-                Child =
-                    {
-                        Thing1 = 9
-                        Thing2 = [ "some" ; "thing" ]
-                    }
-                AndAnother = true
-            }
+        parsed.AndAnother |> shouldEqual true
+        parsed.Child.Thing1 |> shouldEqual 9
+
+        parsed.Child.Thing2
+        |> List.map (fun (x : Uri) -> x.ToString ())
+        |> shouldEqual [ "https://example.com/" ; "http://example.com/" ]
 
     [<Test>]
     let ``Can consume stacked record, child has no positionals, parent has positionals`` () =
@@ -421,3 +422,131 @@ Required argument '--exact' received no value"""
 --thing1  int32
 --thing2  string
 --and-another  bool (positional args) (can be repeated)"""
+
+    [<Test>]
+    let ``Positionals are tagged with Choice`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        ChoicePositionals.parse' getEnvVar [ "a" ; "b" ; "--" ; "--c" ; "--help" ]
+        |> shouldEqual
+            {
+                Args = [ Choice1Of2 "a" ; Choice1Of2 "b" ; Choice2Of2 "--c" ; Choice2Of2 "--help" ]
+            }
+
+    let boolCases =
+        [
+            "1", true
+            "0", false
+            "true", true
+            "false", false
+            "TRUE", true
+            "FALSE", false
+        ]
+        |> List.map TestCaseData
+
+    [<TestCaseSource(nameof (boolCases))>]
+    let ``Bool env vars can be populated`` (envValue : string, boolValue : bool) =
+        let getEnvVar (s : string) =
+            s |> shouldEqual "CONSUMEPLUGIN_THINGS"
+            envValue
+
+        ContainsBoolEnvVar.parse' getEnvVar []
+        |> shouldEqual
+            {
+                BoolVar = Choice2Of2 boolValue
+            }
+
+    [<Test>]
+    let ``Bools can be treated with arity 0`` () =
+        let getEnvVar (_ : string) = failwith "do not call"
+
+        ContainsBoolEnvVar.parse' getEnvVar [ "--bool-var" ]
+        |> shouldEqual
+            {
+                BoolVar = Choice1Of2 true
+            }
+
+    [<TestCaseSource(nameof boolCases)>]
+    let ``Flag DUs can be parsed from env var`` (envValue : string, boolValue : bool) =
+        let getEnvVar (s : string) =
+            s |> shouldEqual "CONSUMEPLUGIN_THINGS"
+            envValue
+
+        let boolValue = if boolValue then DryRunMode.Dry else DryRunMode.Wet
+
+        ContainsFlagEnvVar.parse' getEnvVar []
+        |> shouldEqual
+            {
+                DryRun = Choice2Of2 boolValue
+            }
+
+    let dryRunData =
+        [
+            [ "--dry-run" ], DryRunMode.Dry
+            [ "--dry-run" ; "true" ], DryRunMode.Dry
+            [ "--dry-run=true" ], DryRunMode.Dry
+            [ "--dry-run" ; "True" ], DryRunMode.Dry
+            [ "--dry-run=True" ], DryRunMode.Dry
+            [ "--dry-run" ; "false" ], DryRunMode.Wet
+            [ "--dry-run=false" ], DryRunMode.Wet
+            [ "--dry-run" ; "False" ], DryRunMode.Wet
+            [ "--dry-run=False" ], DryRunMode.Wet
+        ]
+        |> List.map TestCaseData
+
+    [<TestCaseSource(nameof dryRunData)>]
+    let ``Flag DUs can be parsed`` (args : string list, expected : DryRunMode) =
+        let getEnvVar (_ : string) = failwith "do not call"
+
+        ContainsFlagEnvVar.parse' getEnvVar args
+        |> shouldEqual
+            {
+                DryRun = Choice1Of2 expected
+            }
+
+    [<TestCaseSource(nameof dryRunData)>]
+    let ``Flag DUs can be parsed, ArgumentDefaultFunction`` (args : string list, expected : DryRunMode) =
+        let getEnvVar (_ : string) = failwith "do not call"
+
+        ContainsFlagDefaultValue.parse' getEnvVar args
+        |> shouldEqual
+            {
+                DryRun = Choice1Of2 expected
+            }
+
+    [<Test>]
+    let ``Flag DUs can be given a default value`` () =
+        let getEnvVar (_ : string) = failwith "do not call"
+
+        ContainsFlagDefaultValue.parse' getEnvVar []
+        |> shouldEqual
+            {
+                DryRun = Choice2Of2 DryRunMode.Wet
+            }
+
+    [<Test>]
+    let ``Help text for flag DU`` () =
+        let getEnvVar (_ : string) = failwith "do not call"
+
+        let exc =
+            Assert.Throws<exn> (fun () ->
+                ContainsFlagDefaultValue.parse' getEnvVar [ "--help" ]
+                |> ignore<ContainsFlagDefaultValue>
+            )
+
+        exc.Message
+        |> shouldEqual
+            """Help text requested.
+--dry-run  bool (default value: false)"""
+
+    [<Test>]
+    let ``Help text for flag DU, non default`` () =
+        let getEnvVar (_ : string) = failwith "do not call"
+
+        let exc =
+            Assert.Throws<exn> (fun () -> WithFlagDu.parse' getEnvVar [ "--help" ] |> ignore<WithFlagDu>)
+
+        exc.Message
+        |> shouldEqual
+            """Help text requested.
+--dry-run  bool"""

WoofWare.Myriad.Plugins.Test/WoofWare.Myriad.Plugins.Test.fsproj

@@ -4,6 +4,11 @@
     <TargetFramework>net8.0</TargetFramework>
     <IsPackable>false</IsPackable>
     <IsTestProject>true</IsTestProject>
+    <!--
+      Known high severity vulnerability
+      I have not yet seen a single instance where I care about this warning
+    -->
+    <NoWarn>$(NoWarn),NU1903</NoWarn>
   </PropertyGroup>
 
   <ItemGroup>

WoofWare.Myriad.Plugins/ArgParserGenerator.fs

@@ -12,6 +12,23 @@ type internal ArgParserOutputSpec =
         ExtensionMethods : bool
     }
 
+type internal FlagDu =
+    {
+        Name : Ident
+        Case1Name : Ident
+        Case2Name : Ident
+        /// Hopefully this is simply the const bool True or False, but it might e.g. be a literal
+        Case1Arg : SynExpr
+        /// Hopefully this is simply the const bool True or False, but it might e.g. be a literal
+        Case2Arg : SynExpr
+    }
+
+    static member FromBoolean (flagDu : FlagDu) (value : SynExpr) =
+        SynExpr.ifThenElse
+            (SynExpr.equals value flagDu.Case1Arg)
+            (SynExpr.createLongIdent' [ flagDu.Name ; flagDu.Case2Name ])
+            (SynExpr.createLongIdent' [ flagDu.Name ; flagDu.Case1Name ])
+
 /// The default value of an argument which admits default values can be pulled from different sources.
 /// This defines which source a particular default value comes from.
 type private ArgumentDefaultSpec =
@@ -20,19 +37,27 @@ type private ArgumentDefaultSpec =
     /// From calling the static member `{typeWeParseInto}.Default{name}()`
     /// For example, if `type MyArgs = { Thing : Choice<int, int> }`, then
     /// we would use `MyArgs.DefaultThing () : int`.
+    ///
     | FunctionCall of name : Ident
 
-type private Accumulation =
+type private Accumulation<'choice> =
     | Required
     | Optional
-    | Choice of ArgumentDefaultSpec
-    | List
+    | Choice of 'choice
+    | List of Accumulation<'choice>
 
-type private ParseFunction =
+type private ParseFunction<'acc> =
     {
         FieldName : Ident
         TargetVariable : Ident
         ArgForm : string
+        /// If this is a boolean-like field (e.g. a bool or a flag DU), the help text should look a bit different:
+        /// we should lie to the user about the value of the cases there.
+        /// Similarly, if we're reading from an environment variable with the laxer parsing rules of accepting e.g.
+        /// "0" instead of "false", we need to know if we're reading a bool.
+        /// In that case, `boolCases` is Some, and contains the construction of the flag (or boolean, in which case
+        /// you get no data).
+        BoolCases : Choice<FlagDu, unit> option
         Help : SynExpr option
         /// A function string -> %TargetType%, where TargetVariable is probably a `%TargetType% option`.
         /// (Depending on `Accumulation`, we'll remove the `option` at the end of the parse, asserting that the
@@ -42,21 +67,25 @@ type private ParseFunction =
         /// If `Accumulation` is `List`, then this is the type of the list *element*; analogously for optionals
         /// and choices and so on.
         TargetType : SynType
-        Accumulation : Accumulation
+        Accumulation : 'acc
     }
 
+[<RequireQualifiedAccess>]
+type private ChoicePositional =
+    | Normal
+    | Choice
+
+type private ParseFunctionPositional = ParseFunction<ChoicePositional>
+type private ParseFunctionNonPositional = ParseFunction<Accumulation<ArgumentDefaultSpec>>
+
 type private ParserSpec =
     {
-        NonPositionals : ParseFunction list
+        NonPositionals : ParseFunctionNonPositional list
         /// The variable into which positional arguments will be accumulated.
         /// In this case, the TargetVariable is a `ResizeArray` rather than the usual `option`.
-        Positionals : ParseFunction option
+        Positionals : ParseFunctionPositional option
     }
 
-type private ArgToParse =
-    | Positional of ParseFunction
-    | NonPositional of ParseFunction
-
 type private HasPositional = HasPositional
 type private HasNoPositional = HasNoPositional
 
@@ -67,8 +96,8 @@ module private TeqUtils =
 
 [<RequireQualifiedAccess>]
 type private ParseTree<'hasPositional> =
-    | NonPositionalLeaf of ParseFunction * Teq<'hasPositional, HasNoPositional>
-    | PositionalLeaf of ParseFunction * Teq<'hasPositional, HasPositional>
+    | NonPositionalLeaf of ParseFunctionNonPositional * Teq<'hasPositional, HasNoPositional>
+    | PositionalLeaf of ParseFunctionPositional * Teq<'hasPositional, HasPositional>
     /// `assemble` takes the SynExpr's (e.g. each record field contents) corresponding to each `Ident` in
     /// the branch (e.g. each record field name),
     /// and composes them into a `SynExpr` (e.g. the record-typed object).
@@ -142,7 +171,7 @@ module private ParseTree =
 
         go None ([], None) subs
 
-    let rec accumulatorsNonPos (tree : ParseTree<HasNoPositional>) : ParseFunction list =
+    let rec accumulatorsNonPos (tree : ParseTree<HasNoPositional>) : ParseFunctionNonPositional list =
         match tree with
         | ParseTree.PositionalLeaf (_, teq) -> exFalso teq
         | ParseTree.BranchPos (_, _, _, _, teq) -> exFalso teq
@@ -150,7 +179,10 @@ module private ParseTree =
         | ParseTree.Branch (trees, _, _) -> trees |> List.collect (snd >> accumulatorsNonPos)
 
     /// Returns the positional arg separately.
-    let rec accumulatorsPos (tree : ParseTree<HasPositional>) : ParseFunction list * ParseFunction =
+    let rec accumulatorsPos
+        (tree : ParseTree<HasPositional>)
+        : ParseFunctionNonPositional list * ParseFunctionPositional
+        =
         match tree with
         | ParseTree.PositionalLeaf (pf, _) -> [], pf
         | ParseTree.NonPositionalLeaf (_, teq) -> exFalso' teq
@@ -164,7 +196,7 @@ module private ParseTree =
     /// Collect all the ParseFunctions which are necessary to define variables, throwing away
     /// all information relevant to composing the resulting variables into records.
     /// Returns the list of non-positional parsers, and any positional parser that exists.
-    let accumulators<'a> (tree : ParseTree<'a>) : ParseFunction list * ParseFunction option =
+    let accumulators<'a> (tree : ParseTree<'a>) : ParseFunctionNonPositional list * ParseFunctionPositional option =
         // Sad duplication of some code here, but it was the easiest way to make it type-safe :(
         match tree with
         | ParseTree.PositionalLeaf (pf, _) -> [], Some pf
@@ -176,6 +208,18 @@ module private ParseTree =
             let nonPos2, pos = accumulatorsPos tree
             nonPos @ nonPos2, Some pos
 
+        |> fun (nonPos, pos) ->
+            let duplicateArgs =
+                Option.toList (pos |> Option.map _.ArgForm) @ (nonPos |> List.map _.ArgForm)
+                |> List.groupBy id
+                |> List.choose (fun (key, v) -> if v.Length > 1 then Some key else None)
+
+            match duplicateArgs with
+            | [] -> nonPos, pos
+            | dups ->
+                let dups = dups |> String.concat " "
+                failwith $"Duplicate args detected! %s{dups}"
+
     /// Build the return value.
     let rec instantiate<'a> (tree : ParseTree<'a>) : SynExpr =
         match tree with
@@ -217,15 +261,28 @@ module internal ArgParserGenerator =
 
         result.ToString ()
 
+    let private identifyAsFlag (flagDus : FlagDu list) (ty : SynType) : FlagDu option =
+        match ty with
+        | SynType.LongIdent (SynLongIdent.SynLongIdent (ident, _, _)) ->
+            flagDus
+            |> List.tryPick (fun du ->
+                let duName = du.Name.idText
+                let ident = List.last(ident).idText
+                if duName = ident then Some du else None
+            )
+        | _ -> None
+
     /// Builds a function or lambda of one string argument, which returns a `ty` (as modified by the `Accumulation`;
     /// for example, maybe it returns a `ty option` or a `ty list`).
     /// The resulting SynType is the type of the *element* being parsed; so if the Accumulation is List, the SynType
     /// is the list element.
-    let rec private createParseFunction
+    let rec private createParseFunction<'choice>
+        (choice : ArgumentDefaultSpec option -> 'choice)
+        (flagDus : FlagDu list)
         (fieldName : Ident)
         (attrs : SynAttribute list)
         (ty : SynType)
-        : SynExpr * Accumulation * SynType
+        : SynExpr * Accumulation<'choice> * SynType
         =
         match ty with
         | String -> SynExpr.createLambda "x" (SynExpr.createIdent "x"), Accumulation.Required, SynType.string
@@ -237,6 +294,12 @@ module internal ArgParserGenerator =
                     (SynExpr.createIdent "x")),
             Accumulation.Required,
             ty
+        | Uri ->
+            SynExpr.createLambda
+                "x"
+                (SynExpr.applyFunction (SynExpr.createLongIdent [ "System" ; "Uri" ]) (SynExpr.createIdent "x")),
+            Accumulation.Required,
+            ty
         | TimeSpan ->
             let parseExact =
                 attrs
@@ -308,7 +371,8 @@ module internal ArgParserGenerator =
             Accumulation.Required,
             ty
         | OptionType eltTy ->
-            let parseElt, acc, childTy = createParseFunction fieldName attrs eltTy
+            let parseElt, acc, childTy =
+                createParseFunction choice flagDus fieldName attrs eltTy
 
             match acc with
             | Accumulation.Optional ->
@@ -317,7 +381,7 @@ module internal ArgParserGenerator =
             | Accumulation.Choice _ ->
                 failwith
                     $"ArgParser does not support optionals containing choices at field %s{fieldName.idText}: %O{ty}"
-            | Accumulation.List ->
+            | Accumulation.List _ ->
                 failwith $"ArgParser does not support optional lists at field %s{fieldName.idText}: %O{ty}"
             | Accumulation.Required -> parseElt, Accumulation.Optional, childTy
         | ChoiceType elts ->
@@ -327,13 +391,13 @@ module internal ArgParserGenerator =
                     failwith
                         $"ArgParser was unable to prove types %O{elt1} and %O{elt2} to be equal in a Choice. We require them to be equal."
 
-                let parseElt, acc, childTy = createParseFunction fieldName attrs elt1
+                let parseElt, acc, childTy = createParseFunction choice flagDus fieldName attrs elt1
 
                 match acc with
                 | Accumulation.Optional ->
                     failwith
                         $"ArgParser does not support choices containing options at field %s{fieldName.idText}: %O{ty}"
-                | Accumulation.List ->
+                | Accumulation.List _ ->
                     failwith
                         $"ArgParser does not support choices containing lists at field %s{fieldName.idText}: %O{ty}"
                 | Accumulation.Choice _ ->
@@ -365,41 +429,46 @@ module internal ArgParserGenerator =
                         | [ "Myriad" ; "Plugins" ; "ArgumentDefaultEnvironmentVariableAttribute" ]
                         | [ "WoofWare" ; "Myriad" ; "Plugins" ; "ArgumentDefaultEnvironmentVariable" ]
                         | [ "WoofWare" ; "Myriad" ; "Plugins" ; "ArgumentDefaultEnvironmentVariableAttribute" ] ->
+
                             ArgumentDefaultSpec.EnvironmentVariable attr.ArgExpr |> Some
                         | _ -> None
                     )
 
                 let relevantAttr =
                     match relevantAttrs with
-                    | [] ->
-                        failwith
-                            $"Expected Choice to be annotated with ArgumentDefaultFunction or similar, but it was not. Field: %s{fieldName.idText}"
-                    | [ x ] -> x
+                    | [] -> None
+                    | [ x ] -> Some x
                     | _ ->
                         failwith
-                            $"Expected Choice to be annotated with exactly one ArgumentDefaultFunction or similar, but it was annotated with multiple. Field: %s{fieldName.idText}"
+                            $"Expected Choice to be annotated with at most one ArgumentDefaultFunction or similar, but it was annotated with multiple. Field: %s{fieldName.idText}"
 
-                parseElt, Accumulation.Choice relevantAttr, childTy
+                parseElt, Accumulation.Choice (choice relevantAttr), childTy
             | elts ->
                 let elts = elts |> List.map string<SynType> |> String.concat ", "
 
                 failwith
                     $"ArgParser requires Choice to be of the form Choice<'a, 'a>; that is, two arguments, both the same. For field %s{fieldName.idText}, got: %s{elts}"
         | ListType eltTy ->
-            let parseElt, acc, childTy = createParseFunction fieldName attrs eltTy
+            let parseElt, acc, childTy =
+                createParseFunction choice flagDus fieldName attrs eltTy
 
-            match acc with
-            | Accumulation.List ->
-                failwith $"ArgParser does not support nested lists at field %s{fieldName.idText}: %O{ty}"
-            | Accumulation.Choice _ ->
-                failwith $"ArgParser does not support lists containing choices at field %s{fieldName.idText}: %O{ty}"
-            | Accumulation.Optional ->
-                failwith $"ArgParser does not support lists of options at field %s{fieldName.idText}: %O{ty}"
-            | Accumulation.Required -> parseElt, Accumulation.List, childTy
-        | _ -> failwith $"Could not decide how to parse arguments for field %s{fieldName.idText} of type %O{ty}"
+            parseElt, Accumulation.List acc, childTy
+        | ty ->
+            match identifyAsFlag flagDus ty with
+            | None -> failwith $"Could not decide how to parse arguments for field %s{fieldName.idText} of type %O{ty}"
+            | Some flagDu ->
+                // Parse as a bool, and then do the `if-then` dance.
+                let parser =
+                    SynExpr.createIdent "x"
+                    |> SynExpr.applyFunction (SynExpr.createLongIdent [ "System" ; "Boolean" ; "Parse" ])
+                    |> FlagDu.FromBoolean flagDu
+                    |> SynExpr.createLambda "x"
+
+                parser, Accumulation.Required, ty
 
     let rec private toParseSpec
         (counter : int)
+        (flagDus : FlagDu list)
         (ambientRecords : RecordType list)
         (finalRecord : RecordType)
         : ParseTreeCrate * int
@@ -472,29 +541,79 @@ module internal ArgParserGenerator =
                 match ambientRecordMatch with
                 | Some ambient ->
                     // This field has a type we need to obtain from parsing another record.
-                    let spec, counter = toParseSpec counter ambientRecords ambient
+                    let spec, counter = toParseSpec counter flagDus ambientRecords ambient
                     counter, (ident, spec) :: acc
                 | None ->
 
-                let parser, accumulation, parseTy = createParseFunction ident attrs fieldType
-
                 match positionalArgAttr with
                 | Some _ ->
+                    let getChoice (spec : ArgumentDefaultSpec option) : unit =
+                        match spec with
+                        | Some _ ->
+                            failwith
+                                "Positional Choice args cannot have default values. Remove [<ArgumentDefault*>] from the positional arg."
+                        | None -> ()
+
+                    let parser, accumulation, parseTy =
+                        createParseFunction<unit> getChoice flagDus ident attrs fieldType
+
+                    let isBoolLike =
+                        match parseTy with
+                        | PrimitiveType ident when ident |> List.map _.idText = [ "System" ; "Boolean" ] ->
+                            Some (Choice2Of2 ())
+                        | parseTy -> identifyAsFlag flagDus parseTy |> Option.map Choice1Of2
+
                     match accumulation with
-                    | Accumulation.List ->
+                    | Accumulation.List (Accumulation.List _) ->
+                        failwith "A list of positional args cannot contain lists."
+                    | Accumulation.List Accumulation.Optional ->
+                        failwith "A list of positional args cannot contain optionals. What would that even mean?"
+                    | Accumulation.List (Accumulation.Choice ()) ->
                         {
                             FieldName = ident
                             Parser = parser
                             TargetVariable = Ident.create $"arg_%i{counter}"
-                            Accumulation = accumulation
+                            Accumulation = ChoicePositional.Choice
                             TargetType = parseTy
                             ArgForm = argify ident
                             Help = helpText
+                            BoolCases = isBoolLike
                         }
                         |> fun t -> ParseTree.PositionalLeaf (t, Teq.refl)
-                        |> ParseTreeCrate.make
-                    | _ -> failwith $"Expected positional arg accumulation type to be List, but it was %O{fieldType}"
+                    | Accumulation.List Accumulation.Required ->
+                        {
+                            FieldName = ident
+                            Parser = parser
+                            TargetVariable = Ident.create $"arg_%i{counter}"
+                            Accumulation = ChoicePositional.Normal
+                            TargetType = parseTy
+                            ArgForm = argify ident
+                            Help = helpText
+                            BoolCases = isBoolLike
+                        }
+                        |> fun t -> ParseTree.PositionalLeaf (t, Teq.refl)
+                    | Accumulation.Choice _
+                    | Accumulation.Optional
+                    | Accumulation.Required ->
+                        failwith $"Expected positional arg accumulation type to be List, but it was %O{fieldType}"
+                    |> ParseTreeCrate.make
                 | None ->
+                    let getChoice (spec : ArgumentDefaultSpec option) : ArgumentDefaultSpec =
+                        match spec with
+                        | None ->
+                            failwith
+                                "Non-positional Choice args must have an `[<ArgumentDefault*>]` attribute on them."
+                        | Some spec -> spec
+
+                    let parser, accumulation, parseTy =
+                        createParseFunction getChoice flagDus ident attrs fieldType
+
+                    let isBoolLike =
+                        match parseTy with
+                        | PrimitiveType ident when ident |> List.map _.idText = [ "System" ; "Boolean" ] ->
+                            Some (Choice2Of2 ())
+                        | parseTy -> identifyAsFlag flagDus parseTy |> Option.map Choice1Of2
+
                     {
                         FieldName = ident
                         Parser = parser
@@ -503,6 +622,7 @@ module internal ArgParserGenerator =
                         TargetType = parseTy
                         ArgForm = argify ident
                         Help = helpText
+                        BoolCases = isBoolLike
                     }
                     |> fun t -> ParseTree.NonPositionalLeaf (t, Teq.refl)
                     |> ParseTreeCrate.make
@@ -524,12 +644,66 @@ module internal ArgParserGenerator =
     /// let helpText : string = ...
     let private helpText
         (typeName : Ident)
-        (positional : ParseFunction option)
-        (args : ParseFunction list)
+        (positional : ParseFunctionPositional option)
+        (args : ParseFunctionNonPositional list)
         : SynBinding
         =
-        let toPrintable (prefix : string) (arg : ParseFunction) : SynExpr =
-            let ty = arg.TargetType |> SynType.toHumanReadableString
+        let describeNonPositional
+            (acc : Accumulation<ArgumentDefaultSpec>)
+            (flagCases : Choice<FlagDu, unit> option)
+            : SynExpr
+            =
+            match acc with
+            | Accumulation.Required -> SynExpr.CreateConst ""
+            | Accumulation.Optional -> SynExpr.CreateConst " (optional)"
+            | Accumulation.Choice (ArgumentDefaultSpec.EnvironmentVariable var) ->
+                // We don't print out the default value in case it's a secret. People often pass secrets
+                // through env vars!
+                var
+                |> SynExpr.pipeThroughFunction (
+                    SynExpr.applyFunction
+                        (SynExpr.createIdent "sprintf")
+                        (SynExpr.CreateConst " (default value populated from env var %s)")
+                )
+                |> SynExpr.paren
+            | Accumulation.Choice (ArgumentDefaultSpec.FunctionCall var) ->
+                match flagCases with
+                | None -> SynExpr.callMethod var.idText (SynExpr.createIdent' typeName)
+                | Some (Choice2Of2 ()) -> SynExpr.callMethod var.idText (SynExpr.createIdent' typeName)
+                | Some (Choice1Of2 flagDu) ->
+                    // Care required here. The return value from the Default call is not a bool,
+                    // but we should display it as such to the user!
+                    [
+                        SynMatchClause.create
+                            (SynPat.identWithArgs [ flagDu.Name ; flagDu.Case1Name ] (SynArgPats.create []))
+                            (SynExpr.ifThenElse
+                                (SynExpr.equals flagDu.Case1Arg (SynExpr.CreateConst true))
+                                (SynExpr.CreateConst "false")
+                                (SynExpr.CreateConst "true"))
+                        SynMatchClause.create
+                            (SynPat.identWithArgs [ flagDu.Name ; flagDu.Case2Name ] (SynArgPats.create []))
+                            (SynExpr.ifThenElse
+                                (SynExpr.equals flagDu.Case2Arg (SynExpr.CreateConst true))
+                                (SynExpr.CreateConst "false")
+                                (SynExpr.CreateConst "true"))
+                    ]
+                    |> SynExpr.createMatch (SynExpr.callMethod var.idText (SynExpr.createIdent' typeName))
+                |> SynExpr.pipeThroughFunction (
+                    SynExpr.applyFunction (SynExpr.createIdent "sprintf") (SynExpr.CreateConst " (default value: %O)")
+                )
+                |> SynExpr.paren
+            | Accumulation.List _ -> SynExpr.CreateConst " (can be repeated)"
+
+        let describePositional _ _ =
+            SynExpr.CreateConst " (positional args) (can be repeated)"
+
+        /// We may sometimes lie about the type name, if e.g. this is a flag DU which we're pretending is a boolean.
+        /// So the `renderTypeName` takes the Accumulation which tells us whether we're lying.
+        let toPrintable (describe : 'a -> Choice<FlagDu, unit> option -> SynExpr) (arg : ParseFunction<'a>) : SynExpr =
+            let ty =
+                match arg.BoolCases with
+                | None -> SynType.toHumanReadableString arg.TargetType
+                | Some _ -> "bool"
 
             let helpText =
                 match arg.Help with
@@ -539,31 +713,9 @@ module internal ArgParserGenerator =
                     |> SynExpr.applyTo (SynExpr.paren helpText)
                     |> SynExpr.paren
 
-            let descriptor =
-                match arg.Accumulation with
-                | Accumulation.Required -> SynExpr.CreateConst ""
-                | Accumulation.Optional -> SynExpr.CreateConst " (optional)"
-                | Accumulation.Choice (ArgumentDefaultSpec.EnvironmentVariable var) ->
-                    // We don't print out the default value in case it's a secret. People often pass secrets
-                    // through env vars!
-                    var
-                    |> SynExpr.pipeThroughFunction (
-                        SynExpr.applyFunction
-                            (SynExpr.createIdent "sprintf")
-                            (SynExpr.CreateConst " (default value populated from env var %s)")
-                    )
-                    |> SynExpr.paren
-                | Accumulation.Choice (ArgumentDefaultSpec.FunctionCall var) ->
-                    SynExpr.callMethod var.idText (SynExpr.createIdent' typeName)
-                    |> SynExpr.pipeThroughFunction (
-                        SynExpr.applyFunction
-                            (SynExpr.createIdent "sprintf")
-                            (SynExpr.CreateConst " (default value: %O)")
-                    )
-                    |> SynExpr.paren
-                | Accumulation.List -> SynExpr.CreateConst " (can be repeated)"
+            let descriptor = describe arg.Accumulation arg.BoolCases
 
-            let prefix = $"%s{arg.ArgForm}  %s{ty}%s{prefix}"
+            let prefix = $"%s{arg.ArgForm}  %s{ty}"
 
             SynExpr.applyFunction (SynExpr.createIdent "sprintf") (SynExpr.CreateConst (prefix + "%s%s"))
             |> SynExpr.applyTo descriptor
@@ -571,11 +723,11 @@ module internal ArgParserGenerator =
             |> SynExpr.paren
 
         args
-        |> List.map (toPrintable "")
+        |> List.map (toPrintable describeNonPositional)
         |> fun l ->
             match positional with
             | None -> l
-            | Some pos -> l @ [ toPrintable " (positional args)" pos ]
+            | Some pos -> l @ [ toPrintable describePositional pos ]
         |> SynExpr.listLiteral
         |> SynExpr.pipeThroughFunction (
             SynExpr.applyFunction (SynExpr.createLongIdent [ "String" ; "concat" ]) (SynExpr.CreateConst @"\n")
@@ -586,69 +738,106 @@ module internal ArgParserGenerator =
     /// Returns a possible error.
     /// A parse failure might not be fatal (e.g. maybe the input was optionally of arity 0, and we failed to do
     /// the parse because in fact the key decided not to take this argument); in that case we return Error None.
-    let private processKeyValue (argParseErrors : Ident) (args : ParseFunction list) : SynBinding =
-        (SynExpr.applyFunction (SynExpr.createIdent "Error") (SynExpr.createIdent "None"), args)
-        ||> List.fold (fun finalBranch arg ->
-            match arg.Accumulation with
-            | Accumulation.Required
-            | Accumulation.Choice _
-            | Accumulation.Optional ->
-                let multipleErrorMessage =
-                    SynExpr.createIdent "sprintf"
-                    |> SynExpr.applyTo (SynExpr.CreateConst "Argument '%s' was supplied multiple times: %O and %O")
-                    |> SynExpr.applyTo (SynExpr.CreateConst arg.ArgForm)
-                    |> SynExpr.applyTo (SynExpr.createIdent "x")
-                    |> SynExpr.applyTo (SynExpr.createIdent "value")
+    let private processKeyValue
+        (argParseErrors : Ident)
+        (pos : ParseFunctionPositional option)
+        (args : ParseFunctionNonPositional list)
+        : SynBinding
+        =
+        let args =
+            args
+            |> List.map (fun arg ->
+                match arg.Accumulation with
+                | Accumulation.Required
+                | Accumulation.Choice _
+                | Accumulation.Optional ->
+                    let multipleErrorMessage =
+                        SynExpr.createIdent "sprintf"
+                        |> SynExpr.applyTo (SynExpr.CreateConst "Argument '%s' was supplied multiple times: %O and %O")
+                        |> SynExpr.applyTo (SynExpr.CreateConst arg.ArgForm)
+                        |> SynExpr.applyTo (SynExpr.createIdent "x")
+                        |> SynExpr.applyTo (SynExpr.createIdent "value")
 
-                let performAssignment =
+                    let performAssignment =
+                        [
+                            SynExpr.createIdent "value"
+                            |> SynExpr.pipeThroughFunction arg.Parser
+                            |> SynExpr.pipeThroughFunction (SynExpr.createIdent "Some")
+                            |> SynExpr.assign (SynLongIdent.createI arg.TargetVariable)
+
+                            SynExpr.applyFunction (SynExpr.createIdent "Ok") (SynExpr.CreateConst ())
+                        ]
+                        |> SynExpr.sequential
+
+                    [
+                        SynMatchClause.create
+                            (SynPat.nameWithArgs "Some" [ SynPat.named "x" ])
+                            (SynExpr.sequential
+                                [
+                                    multipleErrorMessage
+                                    |> SynExpr.pipeThroughFunction (
+                                        SynExpr.dotGet "Add" (SynExpr.createIdent' argParseErrors)
+                                    )
+                                    SynExpr.applyFunction (SynExpr.createIdent "Ok") (SynExpr.CreateConst ())
+                                ])
+                        SynMatchClause.create
+                            (SynPat.named "None")
+                            (SynExpr.pipeThroughTryWith
+                                SynPat.anon
+                                (SynExpr.createLongIdent [ "exc" ; "Message" ]
+                                 |> SynExpr.pipeThroughFunction (SynExpr.createIdent "Some")
+                                 |> SynExpr.pipeThroughFunction (SynExpr.createIdent "Error"))
+                                performAssignment)
+                    ]
+                    |> SynExpr.createMatch (SynExpr.createIdent' arg.TargetVariable)
+                | Accumulation.List (Accumulation.List _)
+                | Accumulation.List Accumulation.Optional
+                | Accumulation.List (Accumulation.Choice _) ->
+                    failwith
+                        "WoofWare.Myriad invariant violated: expected a list to contain only a Required accumulation. Non-positional lists cannot be optional or Choice, nor can they themselves contain lists."
+                | Accumulation.List Accumulation.Required ->
                     [
                         SynExpr.createIdent "value"
                         |> SynExpr.pipeThroughFunction arg.Parser
-                        |> SynExpr.pipeThroughFunction (SynExpr.createIdent "Some")
-                        |> SynExpr.assign (SynLongIdent.createI arg.TargetVariable)
-
-                        SynExpr.applyFunction (SynExpr.createIdent "Ok") (SynExpr.CreateConst ())
+                        |> SynExpr.pipeThroughFunction (
+                            SynExpr.createLongIdent' [ arg.TargetVariable ; Ident.create "Add" ]
+                        )
+                        SynExpr.CreateConst () |> SynExpr.pipeThroughFunction (SynExpr.createIdent "Ok")
                     ]
                     |> SynExpr.sequential
+                |> fun expr -> arg.ArgForm, expr
+            )
 
-                [
-                    SynMatchClause.create
-                        (SynPat.nameWithArgs "Some" [ SynPat.named "x" ])
-                        (SynExpr.sequential
-                            [
-                                multipleErrorMessage
-                                |> SynExpr.pipeThroughFunction (
-                                    SynExpr.dotGet "Add" (SynExpr.createIdent' argParseErrors)
-                                )
-                                SynExpr.applyFunction (SynExpr.createIdent "Ok") (SynExpr.CreateConst ())
-                            ])
-                    SynMatchClause.create
-                        (SynPat.named "None")
-                        (SynExpr.pipeThroughTryWith
-                            SynPat.anon
-                            (SynExpr.createLongIdent [ "exc" ; "Message" ]
-                             |> SynExpr.pipeThroughFunction (SynExpr.createIdent "Some")
-                             |> SynExpr.pipeThroughFunction (SynExpr.createIdent "Error"))
-                            performAssignment)
-                ]
-                |> SynExpr.createMatch (SynExpr.createIdent' arg.TargetVariable)
-            | Accumulation.List ->
+        let posArg =
+            match pos with
+            | None -> []
+            | Some pos ->
                 [
                     SynExpr.createIdent "value"
-                    |> SynExpr.pipeThroughFunction arg.Parser
+                    |> SynExpr.pipeThroughFunction pos.Parser
+                    |> fun p ->
+                        match pos.Accumulation with
+                        | ChoicePositional.Choice -> p |> SynExpr.pipeThroughFunction (SynExpr.createIdent "Choice1Of2")
+                        | ChoicePositional.Normal -> p
                     |> SynExpr.pipeThroughFunction (
-                        SynExpr.createLongIdent' [ arg.TargetVariable ; Ident.create "Add" ]
+                        SynExpr.createLongIdent' [ pos.TargetVariable ; Ident.create "Add" ]
                     )
                     SynExpr.CreateConst () |> SynExpr.pipeThroughFunction (SynExpr.createIdent "Ok")
                 ]
                 |> SynExpr.sequential
+                |> fun expr -> pos.ArgForm, expr
+                |> List.singleton
+
+        (SynExpr.applyFunction (SynExpr.createIdent "Error") (SynExpr.createIdent "None"), posArg @ args)
+        ||> List.fold (fun finalBranch (argForm, arg) ->
+            arg
             |> SynExpr.ifThenElse
                 (SynExpr.applyFunction
                     (SynExpr.createLongIdent [ "System" ; "String" ; "Equals" ])
                     (SynExpr.tuple
                         [
                             SynExpr.createIdent "key"
-                            SynExpr.CreateConst arg.ArgForm
+                            SynExpr.CreateConst argForm
                             SynExpr.createLongIdent [ "System" ; "StringComparison" ; "OrdinalIgnoreCase" ]
                         ]))
                 finalBranch
@@ -672,9 +861,12 @@ module internal ArgParserGenerator =
         )
 
     /// `let setFlagValue (key : string) : bool = ...`
-    let private setFlagValue (argParseErrors : Ident) (flags : ParseFunction list) : SynBinding =
+    /// The second member of the `flags` list tuple is the constant "true" with which we will interpret the
+    /// arity-0 `--foo`. So in the case of a boolean-typed field, this is `true`; in the case of a Flag-typed field,
+    /// this is `FlagType.WhicheverCaseHadTrue`.
+    let private setFlagValue (argParseErrors : Ident) (flags : (ParseFunction<'a> * SynExpr) list) : SynBinding =
         (SynExpr.CreateConst false, flags)
-        ||> List.fold (fun finalExpr flag ->
+        ||> List.fold (fun finalExpr (flag, trueCase) ->
             let multipleErrorMessage =
                 SynExpr.createIdent "sprintf"
                 |> SynExpr.applyTo (SynExpr.CreateConst "Flag '%s' was supplied multiple times")
@@ -696,7 +888,7 @@ module internal ArgParserGenerator =
                     ([
                         SynExpr.assign
                             (SynLongIdent.createI flag.TargetVariable)
-                            (SynExpr.applyFunction (SynExpr.createIdent "Some") (SynExpr.CreateConst true))
+                            (SynExpr.pipeThroughFunction (SynExpr.createIdent "Some") trueCase)
                         SynExpr.CreateConst true
                      ]
                      |> SynExpr.sequential)
@@ -721,6 +913,7 @@ module internal ArgParserGenerator =
     let private mainLoop
         (parseState : Ident)
         (errorAcc : Ident)
+        (leftoverArgAcc : ChoicePositional)
         (leftoverArgs : Ident)
         (leftoverArgParser : SynExpr)
         : SynBinding
@@ -767,6 +960,11 @@ module internal ArgParserGenerator =
                     [
                         SynExpr.createIdent "arg"
                         |> SynExpr.pipeThroughFunction leftoverArgParser
+                        |> fun p ->
+                            match leftoverArgAcc with
+                            | ChoicePositional.Normal -> p
+                            | ChoicePositional.Choice ->
+                                p |> SynExpr.pipeThroughFunction (SynExpr.createIdent "Choice1Of2")
                         |> SynExpr.pipeThroughFunction (SynExpr.createLongIdent' [ leftoverArgs ; Ident.create "Add" ])
 
                         recurseKey
@@ -932,6 +1130,16 @@ module internal ArgParserGenerator =
                             |> SynExpr.pipeThroughFunction (
                                 SynExpr.applyFunction (SynExpr.createLongIdent [ "Seq" ; "map" ]) leftoverArgParser
                             )
+                            |> fun p ->
+                                match leftoverArgAcc with
+                                | ChoicePositional.Normal -> p
+                                | ChoicePositional.Choice ->
+                                    p
+                                    |> SynExpr.pipeThroughFunction (
+                                        SynExpr.applyFunction
+                                            (SynExpr.createLongIdent [ "Seq" ; "map" ])
+                                            (SynExpr.createIdent "Choice2Of2")
+                                    )
                         ))
                         (SynExpr.createIdent' leftoverArgs))
                 SynMatchClause.create (SynPat.listCons (SynPat.named "arg") (SynPat.named "args")) argBody
@@ -950,8 +1158,14 @@ module internal ArgParserGenerator =
         |> SynBinding.withRecursion true
 
     /// Takes a single argument, `args : string list`, and returns something of the type indicated by `recordType`.
-    let createRecordParse (parseState : Ident) (ambientRecords : RecordType list) (recordType : RecordType) : SynExpr =
-        let spec, _ = toParseSpec 0 ambientRecords recordType
+    let createRecordParse
+        (parseState : Ident)
+        (flagDus : FlagDu list)
+        (ambientRecords : RecordType list)
+        (recordType : RecordType)
+        : SynExpr
+        =
+        let spec, _ = toParseSpec 0 flagDus ambientRecords recordType
         // For each argument (positional and non-positional), create an accumulator for it.
         let nonPos, pos =
             { new ParseTreeEval<_> with
@@ -970,7 +1184,12 @@ module internal ArgParserGenerator =
                     |> SynBinding.basic [ pf.TargetVariable ] []
                     |> SynBinding.withMutability true
                     |> SynBinding.withReturnAnnotation (SynType.appPostfix "option" pf.TargetType)
-                | Accumulation.List ->
+                | Accumulation.List (Accumulation.List _)
+                | Accumulation.List Accumulation.Optional
+                | Accumulation.List (Accumulation.Choice _) ->
+                    failwith
+                        "WoofWare.Myriad invariant violated: expected a list to contain only a Required accumulation. Non-positional lists cannot be optional or Choice, nor can they themselves contain lists."
+                | Accumulation.List Accumulation.Required ->
                     SynExpr.createIdent "ResizeArray"
                     |> SynExpr.applyTo (SynExpr.CreateConst ())
                     |> SynBinding.basic [ pf.TargetVariable ] []
@@ -984,7 +1203,11 @@ module internal ArgParserGenerator =
                     Ident.create "parser_LeftoverArgs",
                     (SynExpr.createLambda "x" (SynExpr.createIdent "x")),
                     SynType.string
-                | Some pf -> pf.TargetVariable, pf.Parser, pf.TargetType
+                | Some pf ->
+                    match pf.Accumulation with
+                    | ChoicePositional.Choice ->
+                        pf.TargetVariable, pf.Parser, SynType.app "Choice" [ pf.TargetType ; pf.TargetType ]
+                    | ChoicePositional.Normal -> pf.TargetVariable, pf.Parser, pf.TargetType
 
             let bindings =
                 SynExpr.createIdent "ResizeArray"
@@ -1023,6 +1246,43 @@ module internal ArgParserGenerator =
                                 name
                                 |> SynExpr.pipeThroughFunction (SynExpr.createIdent "getEnvironmentVariable")
 
+                            /// Assumes access to a non-null variable `x` containing the string value.
+                            let parser =
+                                match pf.BoolCases with
+                                | Some boolLike ->
+                                    let trueCase, falseCase =
+                                        match boolLike with
+                                        | Choice2Of2 () -> SynExpr.CreateConst true, SynExpr.CreateConst false
+                                        | Choice1Of2 flag ->
+                                            FlagDu.FromBoolean flag (SynExpr.CreateConst true),
+                                            FlagDu.FromBoolean flag (SynExpr.CreateConst false)
+
+                                    // We permit environment variables to be populated with 0 and 1 as well.
+                                    SynExpr.ifThenElse
+                                        (SynExpr.applyFunction
+                                            (SynExpr.createLongIdent [ "System" ; "String" ; "Equals" ])
+                                            (SynExpr.tuple
+                                                [
+                                                    SynExpr.createIdent "x"
+                                                    SynExpr.CreateConst "1"
+                                                    SynExpr.createLongIdent
+                                                        [ "System" ; "StringComparison" ; "OrdinalIgnoreCase" ]
+                                                ]))
+                                        (SynExpr.ifThenElse
+                                            (SynExpr.applyFunction
+                                                (SynExpr.createLongIdent [ "System" ; "String" ; "Equals" ])
+                                                (SynExpr.tuple
+                                                    [
+                                                        SynExpr.createIdent "x"
+                                                        SynExpr.CreateConst "0"
+                                                        SynExpr.createLongIdent
+                                                            [ "System" ; "StringComparison" ; "OrdinalIgnoreCase" ]
+                                                    ]))
+                                            (SynExpr.createIdent "x" |> SynExpr.pipeThroughFunction pf.Parser)
+                                            falseCase)
+                                        trueCase
+                                | None -> (SynExpr.createIdent "x" |> SynExpr.pipeThroughFunction pf.Parser)
+
                             let errorMessage =
                                 SynExpr.createIdent "sprintf"
                                 |> SynExpr.applyTo (
@@ -1044,9 +1304,7 @@ module internal ArgParserGenerator =
                                             unchecked
                                         ])
 
-                                SynMatchClause.create
-                                    (SynPat.named "x")
-                                    (SynExpr.createIdent "x" |> SynExpr.pipeThroughFunction pf.Parser)
+                                SynMatchClause.create (SynPat.named "x") parser
                             ]
                             |> SynExpr.createMatch result
                         | ArgumentDefaultSpec.FunctionCall name ->
@@ -1065,7 +1323,12 @@ module internal ArgParserGenerator =
                     |> SynBinding.basic [ pf.TargetVariable ] []
                 | Accumulation.Optional ->
                     SynBinding.basic [ pf.TargetVariable ] [] (SynExpr.createIdent' pf.TargetVariable)
-                | Accumulation.List ->
+                | Accumulation.List (Accumulation.List _)
+                | Accumulation.List Accumulation.Optional
+                | Accumulation.List (Accumulation.Choice _) ->
+                    failwith
+                        "WoofWare.Myriad invariant violated: expected a list to contain only a Required accumulation. Non-positional lists cannot be optional or Choice, nor can they themselves contain lists."
+                | Accumulation.List Accumulation.Required ->
                     SynBinding.basic
                         [ pf.TargetVariable ]
                         []
@@ -1157,12 +1420,24 @@ module internal ArgParserGenerator =
 
         let flags =
             nonPos
-            |> List.filter (fun pf ->
+            |> List.choose (fun pf ->
                 match pf.TargetType with
-                | PrimitiveType pt -> (pt |> List.map _.idText) = [ "System" ; "Boolean" ]
-                | _ -> false
+                | PrimitiveType pt ->
+                    if (pt |> List.map _.idText) = [ "System" ; "Boolean" ] then
+                        Some (pf, SynExpr.CreateConst true)
+                    else
+                        None
+                | ty ->
+                    match identifyAsFlag flagDus ty with
+                    | Some flag -> (pf, FlagDu.FromBoolean flag (SynExpr.CreateConst true)) |> Some
+                    | _ -> None
             )
 
+        let leftoverArgAcc =
+            match pos with
+            | None -> ChoicePositional.Normal
+            | Some pos -> pos.Accumulation
+
         [
             SynExpr.createIdent "go"
             |> SynExpr.applyTo (SynExpr.createLongIdent' [ parseState ; Ident.create "AwaitingKey" ])
@@ -1174,24 +1449,87 @@ module internal ArgParserGenerator =
         |> SynExpr.createLet (
             bindings
             @ [
-                processKeyValue argParseErrors (Option.toList pos @ nonPos)
+                processKeyValue argParseErrors pos nonPos
                 setFlagValue argParseErrors flags
-                mainLoop parseState argParseErrors leftoverArgsName leftoverArgsParser
+                mainLoop parseState argParseErrors leftoverArgAcc leftoverArgsName leftoverArgsParser
             ]
         )
 
+    // The type for which we're generating args may refer to any of the supplied records/unions.
     let createModule
         (opens : SynOpenDeclTarget list)
         (ns : LongIdent)
         ((taggedType : SynTypeDefn, spec : ArgParserOutputSpec))
-        (allUnionTypes : SynTypeDefn list)
-        (allRecordTypes : SynTypeDefn list)
+        (allUnionTypes : UnionType list)
+        (allRecordTypes : RecordType list)
         : SynModuleOrNamespace
         =
-        // The type for which we're generating args may refer to any of these records/unions.
-        let allRecordTypes = allRecordTypes |> List.map RecordType.OfRecord
+        let flagDus =
+            allUnionTypes
+            |> List.choose (fun ty ->
+                match ty.Cases with
+                | [ c1 ; c2 ] ->
+                    match c1.Fields, c2.Fields with
+                    | [], [] ->
+                        let c1Attr =
+                            c1.Attributes
+                            |> List.tryPick (fun attr ->
+                                match attr.TypeName with
+                                | SynLongIdent.SynLongIdent (id, _, _) ->
+                                    match id |> List.last |> _.idText with
+                                    | "ArgumentFlagAttribute"
+                                    | "ArgumentFlag" -> Some (SynExpr.stripOptionalParen attr.ArgExpr)
+                                    | _ -> None
+                            )
 
-        let taggedType = RecordType.OfRecord taggedType
+                        let c2Attr =
+                            c2.Attributes
+                            |> List.tryPick (fun attr ->
+                                match attr.TypeName with
+                                | SynLongIdent.SynLongIdent (id, _, _) ->
+                                    match id |> List.last |> _.idText with
+                                    | "ArgumentFlagAttribute"
+                                    | "ArgumentFlag" -> Some (SynExpr.stripOptionalParen attr.ArgExpr)
+                                    | _ -> None
+                            )
+
+                        match c1Attr, c2Attr with
+                        | Some c1Attr, Some c2Attr ->
+                            // Sanity check where possible
+                            match c1Attr, c2Attr with
+                            | SynExpr.Const (SynConst.Bool b1, _), SynExpr.Const (SynConst.Bool b2, _) ->
+                                if b1 = b2 then
+                                    failwith
+                                        "[<ArgumentFlag>] must have opposite argument values on each case in a two-case discriminated union."
+                            | _, _ -> ()
+
+                            {
+                                Name = ty.Name
+                                Case1Name = c1.Name
+                                Case1Arg = c1Attr
+                                Case2Name = c2.Name
+                                Case2Arg = c2Attr
+                            }
+                            |> Some
+                        | Some _, None
+                        | None, Some _ ->
+                            failwith
+                                "[<ArgumentFlag>] must be placed on both cases of a two-case discriminated union, with opposite argument values on each case."
+                        | _, _ -> None
+                    | _, _ ->
+                        failwith "[<ArgumentFlag>] may only be placed on discriminated union members with no data."
+                | _ -> None
+            )
+
+        let taggedType =
+            match taggedType with
+            | SynTypeDefn.SynTypeDefn (sci,
+                                       SynTypeDefnRepr.Simple (SynTypeDefnSimpleRepr.Record (access, fields, _), _),
+                                       smd,
+                                       _,
+                                       _,
+                                       _) -> RecordType.OfRecord sci smd access fields
+            | _ -> failwith "[<ArgParser>] currently only supports being placed on records."
 
         let modAttrs, modName =
             if spec.ExtensionMethods then
@@ -1212,13 +1550,15 @@ module internal ArgParserGenerator =
             [
                 SynUnionCase.create
                     {
-                        Attrs = []
+                        Attributes = []
                         Fields = []
-                        Ident = Ident.create "AwaitingKey"
+                        Name = Ident.create "AwaitingKey"
+                        XmlDoc = Some (PreXmlDoc.create "Ready to consume a key or positional arg")
+                        Access = None
                     }
                 SynUnionCase.create
                     {
-                        Attrs = []
+                        Attributes = []
                         Fields =
                             [
                                 {
@@ -1227,7 +1567,9 @@ module internal ArgParserGenerator =
                                     Type = SynType.string
                                 }
                             ]
-                        Ident = Ident.create "AwaitingValue"
+                        Name = Ident.create "AwaitingValue"
+                        XmlDoc = Some (PreXmlDoc.create "Waiting to receive a value for the key we've already consumed")
+                        Access = None
                     }
             ]
             |> SynTypeDefnRepr.union
@@ -1244,7 +1586,7 @@ module internal ArgParserGenerator =
                 |> SynPat.annotateType (SynType.appPostfix "list" SynType.string)
 
             let parsePrime =
-                createRecordParse parseStateIdent allRecordTypes taggedType
+                createRecordParse parseStateIdent flagDus allRecordTypes taggedType
                 |> SynBinding.basic
                     [ Ident.create "parse'" ]
                     [
@@ -1342,12 +1684,12 @@ module internal ArgParserGenerator =
                         (([], [], []), types)
                         ||> List.fold (fun
                                            (unions, records, others)
-                                           (SynTypeDefn.SynTypeDefn (_, repr, _, _, _, _) as ty) ->
+                                           (SynTypeDefn.SynTypeDefn (sci, repr, smd, _, _, _) as ty) ->
                             match repr with
-                            | SynTypeDefnRepr.Simple (SynTypeDefnSimpleRepr.Union _, _) ->
-                                ty :: unions, records, others
-                            | SynTypeDefnRepr.Simple (SynTypeDefnSimpleRepr.Record _, _) ->
-                                unions, ty :: records, others
+                            | SynTypeDefnRepr.Simple (SynTypeDefnSimpleRepr.Union (access, cases, _), _) ->
+                                UnionType.OfUnion sci smd access cases :: unions, records, others
+                            | SynTypeDefnRepr.Simple (SynTypeDefnSimpleRepr.Record (access, fields, _), _) ->
+                                unions, RecordType.OfRecord sci smd access fields :: records, others
                             | _ -> unions, records, ty :: others
                         )
 

WoofWare.Myriad.Plugins/AstHelper.fs

@@ -72,25 +72,17 @@ type internal RecordType =
     }
 
     /// Parse from the AST.
-    static member OfRecord (record : SynTypeDefn) : RecordType =
-        let sci, sdr, smd, smdo =
-            match record with
-            | SynTypeDefn.SynTypeDefn (sci, sdr, smd, smdo, _, _) -> sci, sdr, smd, smdo
-
-        let synAccessOption, recordFields =
-            match sdr with
-            | SynTypeDefnRepr.Simple (SynTypeDefnSimpleRepr.Record (sa, fields, _), _) -> sa, fields
-            | _ -> failwith $"expected a record; got: %+A{record}"
-
+    static member OfRecord
+        (sci : SynComponentInfo)
+        (smd : SynMemberDefns)
+        (access : SynAccess option)
+        (recordFields : SynField list)
+        : RecordType
+        =
         match sci with
-        | SynComponentInfo.SynComponentInfo (attrs, typars, _, longId, doc, _, access, _) ->
-            if access <> synAccessOption then
-                failwith
-                    $"TODO what's happened, two different accessibility modifiers: %O{access} and %O{synAccessOption}"
-
-            match smdo with
-            | Some v -> failwith $"TODO what's happened, got a synMemberDefn of %O{v}"
-            | None -> ()
+        | SynComponentInfo.SynComponentInfo (attrs, typars, _, longId, doc, _, access2, _) ->
+            if access <> access2 then
+                failwith $"TODO what's happened, two different accessibility modifiers: %O{access} and %O{access2}"
 
             {
                 Name = List.last longId
@@ -98,10 +90,87 @@ type internal RecordType =
                 Members = if smd.IsEmpty then None else Some smd
                 XmlDoc = if doc.IsEmpty then None else Some doc
                 Generics = typars
-                Accessibility = synAccessOption
+                Accessibility = access
                 Attributes = attrs |> List.collect (fun l -> l.Attributes)
             }
 
+/// Methods for manipulating UnionCase.
+[<RequireQualifiedAccess>]
+module UnionCase =
+    /// Construct our structured `UnionCase` from an FCS `SynUnionCase`: extract everything
+    /// we care about from the AST representation.
+    let ofSynUnionCase (case : SynUnionCase) : UnionCase<Ident option> =
+        match case with
+        | SynUnionCase.SynUnionCase (attributes, ident, caseType, xmlDoc, access, _, _) ->
+
+        let ident =
+            match ident with
+            | SynIdent.SynIdent (ident, _) -> ident
+
+        let fields =
+            match caseType with
+            | SynUnionCaseKind.Fields cases -> cases
+            | SynUnionCaseKind.FullType _ -> failwith "unexpected FullType union"
+
+        {
+            Name = ident
+            XmlDoc = if xmlDoc.IsEmpty then None else Some xmlDoc
+            Access = access
+            Attributes = attributes |> List.collect (fun t -> t.Attributes)
+            Fields = fields |> List.map SynField.extract
+        }
+
+    /// Functorial `map`.
+    let mapIdentFields<'a, 'b> (f : 'a -> 'b) (unionCase : UnionCase<'a>) : UnionCase<'b> =
+        {
+            Attributes = unionCase.Attributes
+            Name = unionCase.Name
+            Access = unionCase.Access
+            XmlDoc = unionCase.XmlDoc
+            Fields = unionCase.Fields |> List.map (SynField.mapIdent f)
+        }
+
+/// Everything you need to know about a discriminated union definition.
+type internal UnionType =
+    {
+        /// The name of the DU: for example, `type Foo = | Blah` has this being `Foo`.
+        Name : Ident
+        /// Any additional members which are not union cases.
+        Members : SynMemberDefns option
+        /// Any docstring associated with the DU itself (not its cases).
+        XmlDoc : PreXmlDoc option
+        /// Generic type parameters this DU takes: `type Foo<'a> = | ...`.
+        Generics : SynTyparDecls option
+        /// Attributes of the DU (not its cases): `[<Attr>] type Foo = | ...`
+        Attributes : SynAttribute list
+        /// Accessibility modifier of the DU: `type private Foo = ...`
+        Accessibility : SynAccess option
+        /// The actual DU cases themselves.
+        Cases : UnionCase<Ident option> list
+    }
+
+    static member OfUnion
+        (sci : SynComponentInfo)
+        (smd : SynMemberDefns)
+        (access : SynAccess option)
+        (cases : SynUnionCase list)
+        : UnionType
+        =
+        match sci with
+        | SynComponentInfo.SynComponentInfo (attrs, typars, _, longId, doc, _, access2, _) ->
+            if access <> access2 then
+                failwith $"TODO what's happened, two different accessibility modifiers: %O{access} and %O{access2}"
+
+            {
+                Name = List.last longId
+                Members = if smd.IsEmpty then None else Some smd
+                XmlDoc = if doc.IsEmpty then None else Some doc
+                Generics = typars
+                Attributes = attrs |> List.collect (fun l -> l.Attributes)
+                Accessibility = access
+                Cases = cases |> List.map UnionCase.ofSynUnionCase
+            }
+
 /// Anything that is part of an ADT.
 /// A record is a product of stuff; this type represents one of those stuffs.
 type internal AdtNode =

WoofWare.Myriad.Plugins/JsonParseGenerator.fs

@@ -416,11 +416,11 @@ module internal JsonParseGenerator =
     let createUnionMaker (spec : JsonParseOutputSpec) (typeName : LongIdent) (fields : UnionCase<Ident> list) =
         fields
         |> List.map (fun case ->
-            let propertyName = JsonSerializeGenerator.getPropertyName case.Ident case.Attrs
+            let propertyName = JsonSerializeGenerator.getPropertyName case.Name case.Attributes
 
             let body =
                 if case.Fields.IsEmpty then
-                    SynExpr.createLongIdent' (typeName @ [ case.Ident ])
+                    SynExpr.createLongIdent' (typeName @ [ case.Name ])
                 else
                     case.Fields
                     |> List.map (fun field ->
@@ -429,7 +429,7 @@ module internal JsonParseGenerator =
                         createParseRhs options propertyName field.Type
                     )
                     |> SynExpr.tuple
-                    |> SynExpr.applyFunction (SynExpr.createLongIdent' (typeName @ [ case.Ident ]))
+                    |> SynExpr.applyFunction (SynExpr.createLongIdent' (typeName @ [ case.Name ]))
                     |> SynExpr.createLet
                         [
                             SynExpr.index (SynExpr.CreateConst "data") (SynExpr.createIdent "node")
@@ -600,7 +600,7 @@ module internal JsonParseGenerator =
                     | Some i -> i
 
                 cases
-                |> List.map SynUnionCase.extract
+                |> List.map UnionCase.ofSynUnionCase
                 |> List.map (UnionCase.mapIdentFields optionGet)
                 |> createUnionMaker spec ident
             | SynTypeDefnRepr.Simple (SynTypeDefnSimpleRepr.Enum (cases, _range), _) ->

WoofWare.Myriad.Plugins/JsonSerializeGenerator.fs

@@ -263,11 +263,11 @@ module internal JsonSerializeGenerator =
 
     let unionModule (spec : JsonSerializeOutputSpec) (typeName : LongIdent) (cases : SynUnionCase list) =
         let inputArg = Ident.create "input"
-        let fields = cases |> List.map SynUnionCase.extract
+        let fields = cases |> List.map UnionCase.ofSynUnionCase
 
         fields
         |> List.map (fun unionCase ->
-            let propertyName = getPropertyName unionCase.Ident unionCase.Attrs
+            let propertyName = getPropertyName unionCase.Name unionCase.Attributes
 
             let caseNames = unionCase.Fields |> List.mapi (fun i _ -> $"arg%i{i}")
 
@@ -275,7 +275,7 @@ module internal JsonSerializeGenerator =
 
             let pattern =
                 SynPat.LongIdent (
-                    SynLongIdent.create (typeName @ [ unionCase.Ident ]),
+                    SynLongIdent.create (typeName @ [ unionCase.Name ]),
                     None,
                     None,
                     argPats,

WoofWare.Myriad.Plugins/SurfaceBaseline.txt

@@ -11,4 +11,27 @@ WoofWare.Myriad.Plugins.JsonParseGenerator..ctor [constructor]: unit
 WoofWare.Myriad.Plugins.JsonSerializeGenerator inherit obj, implements Myriad.Core.IMyriadGenerator
 WoofWare.Myriad.Plugins.JsonSerializeGenerator..ctor [constructor]: unit
 WoofWare.Myriad.Plugins.RemoveOptionsGenerator inherit obj, implements Myriad.Core.IMyriadGenerator
-WoofWare.Myriad.Plugins.RemoveOptionsGenerator..ctor [constructor]: unit
+WoofWare.Myriad.Plugins.RemoveOptionsGenerator..ctor [constructor]: unit
+WoofWare.Myriad.Plugins.SynFieldData`1 inherit obj
+WoofWare.Myriad.Plugins.SynFieldData`1..ctor [constructor]: (Fantomas.FCS.Syntax.SynAttribute list, 'Ident, Fantomas.FCS.Syntax.SynType)
+WoofWare.Myriad.Plugins.SynFieldData`1.Attrs [property]: [read-only] Fantomas.FCS.Syntax.SynAttribute list
+WoofWare.Myriad.Plugins.SynFieldData`1.get_Attrs [method]: unit -> Fantomas.FCS.Syntax.SynAttribute list
+WoofWare.Myriad.Plugins.SynFieldData`1.get_Ident [method]: unit -> 'Ident
+WoofWare.Myriad.Plugins.SynFieldData`1.get_Type [method]: unit -> Fantomas.FCS.Syntax.SynType
+WoofWare.Myriad.Plugins.SynFieldData`1.Ident [property]: [read-only] 'Ident
+WoofWare.Myriad.Plugins.SynFieldData`1.Type [property]: [read-only] Fantomas.FCS.Syntax.SynType
+WoofWare.Myriad.Plugins.UnionCase inherit obj
+WoofWare.Myriad.Plugins.UnionCase.mapIdentFields [static method]: ('a -> 'b) -> 'a WoofWare.Myriad.Plugins.UnionCase -> 'b WoofWare.Myriad.Plugins.UnionCase
+WoofWare.Myriad.Plugins.UnionCase.ofSynUnionCase [static method]: Fantomas.FCS.Syntax.SynUnionCase -> Fantomas.FCS.Syntax.Ident option WoofWare.Myriad.Plugins.UnionCase
+WoofWare.Myriad.Plugins.UnionCase`1 inherit obj
+WoofWare.Myriad.Plugins.UnionCase`1..ctor [constructor]: (Fantomas.FCS.Syntax.Ident, Fantomas.FCS.Xml.PreXmlDoc option, Fantomas.FCS.Syntax.SynAccess option, Fantomas.FCS.Syntax.SynAttribute list, 'ident WoofWare.Myriad.Plugins.SynFieldData list)
+WoofWare.Myriad.Plugins.UnionCase`1.Access [property]: [read-only] Fantomas.FCS.Syntax.SynAccess option
+WoofWare.Myriad.Plugins.UnionCase`1.Attributes [property]: [read-only] Fantomas.FCS.Syntax.SynAttribute list
+WoofWare.Myriad.Plugins.UnionCase`1.Fields [property]: [read-only] 'ident WoofWare.Myriad.Plugins.SynFieldData list
+WoofWare.Myriad.Plugins.UnionCase`1.get_Access [method]: unit -> Fantomas.FCS.Syntax.SynAccess option
+WoofWare.Myriad.Plugins.UnionCase`1.get_Attributes [method]: unit -> Fantomas.FCS.Syntax.SynAttribute list
+WoofWare.Myriad.Plugins.UnionCase`1.get_Fields [method]: unit -> 'ident WoofWare.Myriad.Plugins.SynFieldData list
+WoofWare.Myriad.Plugins.UnionCase`1.get_Name [method]: unit -> Fantomas.FCS.Syntax.Ident
+WoofWare.Myriad.Plugins.UnionCase`1.get_XmlDoc [method]: unit -> Fantomas.FCS.Xml.PreXmlDoc option
+WoofWare.Myriad.Plugins.UnionCase`1.Name [property]: [read-only] Fantomas.FCS.Syntax.Ident
+WoofWare.Myriad.Plugins.UnionCase`1.XmlDoc [property]: [read-only] Fantomas.FCS.Xml.PreXmlDoc option

WoofWare.Myriad.Plugins/SynExpr/SynField.fs

@@ -5,10 +5,17 @@ open Fantomas.FCS.Syntax
 open Fantomas.FCS.SyntaxTrivia
 open Fantomas.FCS.Xml
 
-type internal SynFieldData<'Ident> =
+/// The data needed to reconstitute a single piece of data within a union field, or a single record field.
+/// This is generic on whether the field is identified. For example, in `type Foo = Blah of int`, the `int`
+/// field is not identified; whereas in `type Foo = Blah of baz : int`, it is identified.
+type SynFieldData<'Ident> =
     {
+        /// Attributes on this field. I think you can only get these if this is a *record* field.
         Attrs : SynAttribute list
+        /// The identifier of this field (see docstring for SynFieldData).
         Ident : 'Ident
+        /// The type of the data contained in this field. For example, `type Foo = { Blah : int }`
+        /// has this being `int`.
         Type : SynType
     }
 

WoofWare.Myriad.Plugins/SynExpr/SynType.fs

@@ -363,6 +363,7 @@ module internal SynType =
         | DateTimeOffset -> "DateTimeOffset"
         | DateOnly -> "DateOnly"
         | TimeSpan -> "TimeSpan"
+        | SynType.LongIdent (SynLongIdent.SynLongIdent (ident, _, _)) -> ident |> List.map _.idText |> String.concat "."
         | ty -> failwithf "could not compute human-readable string for type: %O" ty
 
     /// Guess whether the types are equal. We err on the side of saying "no, they're different".
@@ -454,4 +455,11 @@ module internal SynType =
             match ty2 with
             | DateOnly -> true
             | _ -> false
-        | _ -> false
+        | _ ->
+
+        match ty1, ty2 with
+        | SynType.LongIdent (SynLongIdent (ident1, _, _)), SynType.LongIdent (SynLongIdent (ident2, _, _)) ->
+            let ident1 = ident1 |> List.map _.idText
+            let ident2 = ident2 |> List.map _.idText
+            ident1 = ident2
+        | _, _ -> false

WoofWare.Myriad.Plugins/SynExpr/SynUnionCase.fs

@@ -5,44 +5,24 @@ open Fantomas.FCS.Text.Range
 open Fantomas.FCS.Xml
 open Fantomas.FCS.SyntaxTrivia
 
-type internal UnionCase<'Ident> =
+/// Represents everything you need to know about a union case.
+/// This is generic on whether each field of this case must be named.
+type UnionCase<'ident> =
     {
-        Fields : SynFieldData<'Ident> list
-        Attrs : SynAttribute list
-        Ident : Ident
+        /// The name of the case: e.g. `| Foo of blah` has this being `Foo`.
+        Name : Ident
+        /// Any docstring associated with this case.
+        XmlDoc : PreXmlDoc option
+        /// Any accessibility modifier: e.g. `type Foo = private | Blah`.
+        Access : SynAccess option
+        /// Attributes on the case: for example, `| [<Attr>] Foo of blah`.
+        Attributes : SynAttribute list
+        /// The data contained within the case: for example, `[blah]` in `| Foo of blah`.
+        Fields : SynFieldData<'ident> list
     }
 
-[<RequireQualifiedAccess>]
-module internal UnionCase =
-    let mapIdentFields<'a, 'b> (f : 'a -> 'b) (unionCase : UnionCase<'a>) : UnionCase<'b> =
-        {
-            Fields = unionCase.Fields |> List.map (SynField.mapIdent f)
-            Attrs = unionCase.Attrs
-            Ident = unionCase.Ident
-        }
-
 [<RequireQualifiedAccess>]
 module internal SynUnionCase =
-    let extract (SynUnionCase (attrs, id, caseType, _, _, _, _)) : UnionCase<Ident option> =
-        match caseType with
-        | SynUnionCaseKind.FullType _ -> failwith "WoofWare.Myriad does not support FullType union cases."
-        | SynUnionCaseKind.Fields fields ->
-
-        let fields = fields |> List.map SynField.extract
-
-        let id =
-            match id with
-            | SynIdent.SynIdent (ident, _) -> ident
-
-        // As far as I can tell, there's no way to get any attributes here? :shrug:
-        let attrs = attrs |> List.collect (fun l -> l.Attributes)
-
-        {
-            Fields = fields
-            Attrs = attrs
-            Ident = id
-        }
-
     let create (case : UnionCase<Ident>) : SynUnionCase =
         let fields =
             case.Fields
@@ -63,11 +43,11 @@ module internal SynUnionCase =
             )
 
         SynUnionCase.SynUnionCase (
-            SynAttributes.ofAttrs case.Attrs,
-            SynIdent.SynIdent (case.Ident, None),
+            SynAttributes.ofAttrs case.Attributes,
+            SynIdent.SynIdent (case.Name, None),
             SynUnionCaseKind.Fields fields,
-            PreXmlDoc.Empty,
-            None,
+            case.XmlDoc |> Option.defaultValue PreXmlDoc.Empty,
+            case.Access,
             range0,
             {
                 BarRange = Some range0

WoofWare.Myriad.Plugins/version.json

@@ -1,5 +1,5 @@
 {
-  "version": "2.2",
+  "version": "2.3",
   "publicReleaseRefSpec": [
     "^refs/heads/main$"
   ],

global.json

@@ -1,6 +1,6 @@
 {
   "sdk": {
     "version": "8.0.100",
-    "rollForward": "latestFeature"
+    "rollForward": "latestMajor"
   }
 }