Patience diff (#17)

.envrc

@@ -1 +1,23 @@
 use flake
+DOTNET_PATH=$(readlink "$(which dotnet)")
+SETTINGS_FILE=$(find . -maxdepth 1 -type f -name '*.sln.DotSettings.user')
+MSBUILD=$(realpath "$(find "$(dirname "$DOTNET_PATH")/../share/dotnet/sdk" -maxdepth 2 -type f -name MSBuild.dll)")
+if [ -f "$SETTINGS_FILE" ] ; then
+    xmlstarlet ed --inplace \
+    -N wpf="http://schemas.microsoft.com/winfx/2006/xaml/presentation" \
+    -N x="http://schemas.microsoft.com/winfx/2006/xaml" \
+    -N s="clr-namespace:System;assembly=mscorlib" \
+    -N ss="urn:shemas-jetbrains-com:settings-storage-xaml" \
+    --update "//s:String[@x:Key='/Default/Environment/Hierarchy/Build/BuildTool/DotNetCliExePath/@EntryValue']" \
+    --value "$(realpath "$(dirname "$DOTNET_PATH")/../share/dotnet/dotnet")" \
+    "$SETTINGS_FILE"
+
+    xmlstarlet ed --inplace \
+    -N wpf="http://schemas.microsoft.com/winfx/2006/xaml/presentation" \
+    -N x="http://schemas.microsoft.com/winfx/2006/xaml" \
+    -N s="clr-namespace:System;assembly=mscorlib" \
+    -N ss="urn:shemas-jetbrains-com:settings-storage-xaml" \
+    --update "//s:String[@x:Key='/Default/Environment/Hierarchy/Build/BuildTool/CustomBuildToolPath/@EntryValue']" \
+    --value "$MSBUILD" \
+    "$SETTINGS_FILE"
+fi

README.md

@@ -17,7 +17,7 @@ An [expect-testing](https://blog.janestreet.com/the-joy-of-expect-tests/) librar
 
 The basic mechanism works.
 Snapshot updating is vibe-coded with Opus 4 and is purely text-based; I didn't want to use the F# compiler services because that's a pretty heavyweight dependency which should be confined to a separate test runner entity.
-It's not very well tested, and I expect it to be kind of brittle.
+It's fairly well tested, but you will certainly be able to find ways to break it; try not to be too fancy with your syntax around the `snapshot` statement.
 
 # How to use
 
@@ -44,6 +44,13 @@ let ``This test fails: plain text comparison of ToString`` () =
         snapshot "  123  "
         return 123
     }
+
+[<Test>]
+let ``With return! and snapshotThrows, you can see exceptions too`` () =
+    expect {
+        snapshotThrows @"System.Exception: oh no"
+        return! (fun () -> failwith<int> "oh no")
+    }
 ```
 
 You can adjust the formatting:

WoofWare.Expect.Test/SimpleTest.fs

@@ -19,12 +19,9 @@ module SimpleTest =
     let ``Example of a failing test`` () =
         expect {
             snapshot
-                @"snapshot mismatch! snapshot at filepath.fs:99 (Example of a failing test) was:
+                @"snapshot mismatch! snapshot at filepath.fs:99 (Example of a failing test) diff:
 
 - 123
-
-actual was:
-
 + 124"
 
             return
@@ -64,26 +61,21 @@ actual was:
         // Out of the box, comments in snapshots cause the JSON parser to throw, so the snapshot fails to match...
         expect {
             snapshot
-                @"snapshot mismatch! snapshot at file.fs:99 (Custom JSON output) was:
+                @"snapshot mismatch! snapshot at file.fs:99 (Custom JSON output) diff:
 
 - [JSON failed to parse:] {
--     // a key here
+-   // a key here
--     ""a"":3
-- }
-
-actual was:
-
 + {
-+   ""a"": 3
+    ""a"": 3
-+ }"
+  }"
 
             return
                 Assert.Throws<ExpectException> (fun () ->
                     expectWithMockedFilePath ("file.fs", 99) {
                         snapshotJson
                             @"{
-    // a key here
+  // a key here
-    ""a"":3
+  ""a"": 3
 }"
 
                         return Map.ofList [ "a", 3 ]

WoofWare.Expect.Test/TestDiff.fs

@@ -0,0 +1,110 @@
+namespace WoofWare.Expect.Test
+
+open WoofWare.Expect
+open NUnit.Framework
+
+[<TestFixture>]
+module TestDiff =
+
+    [<Test>]
+    let ``Basic diff`` () =
+        let textA =
+            [|
+                "The quick brown fox"
+                "jumps over"
+                "the lazy dog"
+                "Some unique line here"
+                "The end"
+            |]
+
+        let textB =
+            [|
+                "The quick brown fox"
+                "Some unique line here"
+                "jumps over"
+                "the lazy dog"
+                "Another line"
+                "The end"
+            |]
+
+        let diff = Diff.patienceLines textA textB
+
+        expect {
+            snapshot
+                @"    0   0  The quick brown fox
++       1  Some unique line here
+    1   2  jumps over
+    2   3  the lazy dog
+-   3      Some unique line here
++       4  Another line
+    4   5  The end"
+
+            withFormat Diff.formatWithLineNumbers
+            return diff
+        }
+
+        expect {
+            snapshot
+                @"  The quick brown fox
++ Some unique line here
+  jumps over
+  the lazy dog
+- Some unique line here
++ Another line
+  The end"
+
+            withFormat Diff.format
+            return diff
+        }
+
+    [<Test>]
+    let ``An example from Incremental`` () =
+        let textA =
+            """digraph G {
+  rankdir = TB
+  bgcolor = transparent
+    n4 [shape=Mrecord label="{{n4|BindMain|height=2}}"  "fontname"="Sans Serif"]
+    n3 [shape=Mrecord label="{{n3|BindLhsChange|height=1}}"  "fontname"="Sans Serif"]
+    n1 [shape=Mrecord label="{{n1|Const|height=0}}"  "fontname"="Sans Serif"]
+    n2 [shape=Mrecord label="{{n2|Const|height=0}}"  "fontname"="Sans Serif"]
+  n3 -> n4
+  n2 -> n4
+  n1 -> n3
+}"""
+
+        let textB =
+            """digraph G {
+  rankdir = TB
+  bgcolor = transparent
+    n4 [shape=box label="{{n4|BindMain|height=2}}" ]
+    n3 [shape=box label="{{n3|BindLhsChange|height=1}}" ]
+    n1 [shape=box label="{{n1|Const|height=0}}" ]
+    n2 [shape=box label="{{n2|Const|height=0}}" ]
+  n3 -> n4
+  n2 -> n4
+  n1 -> n3
+}"""
+
+        let diff = Diff.patience textA textB
+
+        expect {
+            snapshot
+                @"  digraph G {
+    rankdir = TB
+    bgcolor = transparent
+-     n4 [shape=Mrecord label=""{{n4|BindMain|height=2}}""  ""fontname""=""Sans Serif""]
+-     n3 [shape=Mrecord label=""{{n3|BindLhsChange|height=1}}""  ""fontname""=""Sans Serif""]
+-     n1 [shape=Mrecord label=""{{n1|Const|height=0}}""  ""fontname""=""Sans Serif""]
+-     n2 [shape=Mrecord label=""{{n2|Const|height=0}}""  ""fontname""=""Sans Serif""]
++     n4 [shape=box label=""{{n4|BindMain|height=2}}"" ]
++     n3 [shape=box label=""{{n3|BindLhsChange|height=1}}"" ]
++     n1 [shape=box label=""{{n1|Const|height=0}}"" ]
++     n2 [shape=box label=""{{n2|Const|height=0}}"" ]
+    n3 -> n4
+    n2 -> n4
+    n1 -> n3
+  }"
+
+            withFormat Diff.format
+            return diff
+        }

WoofWare.Expect.Test/TestExceptionThrowing.fs

@@ -0,0 +1,14 @@
+namespace WoofWare.Expect.Test
+
+open NUnit.Framework
+open WoofWare.Expect
+
+[<TestFixture>]
+module TestExceptionThrowing =
+
+    [<Test>]
+    let ``Can throw an exception`` () =
+        expect {
+            snapshotThrows @"System.Exception: oh no"
+            return! (fun () -> failwith<int> "oh no")
+        }

WoofWare.Expect.Test/WoofWare.Expect.Test.fsproj

@@ -13,6 +13,8 @@
       <Compile Include="Assembly.fs" />
       <Compile Include="BulkUpdateExample.fs" />
       <Compile Include="SimpleTest.fs" />
+      <Compile Include="TestDiff.fs" />
+      <Compile Include="TestExceptionThrowing.fs" />
       <Compile Include="TestSurface.fs" />
       <Compile Include="TestSnapshotFinding\TestSnapshotFinding.fs" />
       <Compile Include="TestSnapshotFinding\TestUnicodeCharacters.fs" />

WoofWare.Expect/Builder.fs

@@ -23,8 +23,6 @@ type Mode =
 /// <param name="applyChanges">When running the tests, instead of throwing an exception on failure, update the snapshot.</param>
 /// <param name="sourceOverride">Override the file path and line numbers reported in snapshots, so that your tests can be fully stable even on failure. (You almost certainly don't want to set this.)</param>
 type ExpectBuilder (mode : Mode) =
-    member private this.Mode = Unchecked.defaultof<Mode>
-
     new (sourceOverride : string * int) = ExpectBuilder (Mode.AssertMockingSource sourceOverride)
 
     new (update : bool)
@@ -144,6 +142,51 @@ type ExpectBuilder (mode : Mode) =
                 Actual = None
             }
 
+    /// <summary>
+    /// Expresses that the given expression throws during evaluation.
+    /// </summary>
+    /// <example>
+    /// <code>
+    /// expect {
+    ///     snapshotThrows @"System.Exception: oh no"
+    ///     return! (fun () -> failwith "oh no")
+    /// }
+    /// </code>
+    /// </example>
+    [<CustomOperation("snapshotThrows", MaintainsVariableSpaceUsingBind = true)>]
+    member _.SnapshotThrows<'a>
+        (
+            state : ExpectState<'a>,
+            snapshot : string,
+            [<CallerMemberName>] ?memberName : string,
+            [<CallerLineNumber>] ?callerLine : int,
+            [<CallerFilePath>] ?filePath : string
+        )
+        : ExpectState<'a>
+        =
+        match state.Snapshot with
+        | Some _ -> failwith "snapshot can only be specified once"
+        | None ->
+
+            let memberName = defaultArg memberName "<unknown method>"
+            let filePath = defaultArg filePath "<unknown file>"
+            let lineNumber = defaultArg callerLine -1
+
+            let callerInfo =
+                {
+                    MemberName = memberName
+                    FilePath = filePath
+                    LineNumber = lineNumber
+                }
+
+            {
+                Formatter = None
+                JsonSerialiserOptions = state.JsonSerialiserOptions
+                JsonDocOptions = state.JsonDocOptions
+                Snapshot = Some (SnapshotValue.ThrowsException snapshot, callerInfo)
+                Actual = None
+            }
+
     /// <summary>
     /// Express that the <c>return</c> value of this builder should be formatted using this function, before
     /// comparing to the snapshot.
@@ -158,7 +201,7 @@ type ExpectBuilder (mode : Mode) =
         | Some _ -> failwith "Please don't supply withFormat more than once"
         | None ->
             { state with
-                Formatter = Some formatter
+                Formatter = Some (fun f -> f () |> formatter)
             }
 
     /// <summary>
@@ -226,6 +269,17 @@ type ExpectBuilder (mode : Mode) =
 
     /// Expresses the "actual value" component of the assertion "expected snapshot = actual value".
     member _.Return (value : 'T) : ExpectState<'T> =
+        {
+            Snapshot = None
+            Formatter = None
+            JsonDocOptions = None
+            JsonSerialiserOptions = None
+            Actual = Some (fun () -> value)
+        }
+
+    /// Expresses the "actual value" component of the assertion "expected snapshot = actual value", but delayed behind
+    /// a function (by contrast with `Return`).
+    member _.ReturnFrom (value : unit -> 'T) : ExpectState<'T> =
         {
             Snapshot = None
             Formatter = None
@@ -244,40 +298,42 @@ type ExpectBuilder (mode : Mode) =
         let raiseError (snapshot : string) (actual : string) : unit =
             match mode with
             | Mode.AssertMockingSource (mockSource, line) ->
+                let diff = Diff.patience snapshot actual
+
                 sprintf
-                    "snapshot mismatch! snapshot at %s:%i (%s) was:\n\n%s\n\nactual was:\n\n%s"
+                    "snapshot mismatch! snapshot at %s:%i (%s) diff:\n\n%s"
                     mockSource
                     line
                     state.Caller.MemberName
-                    (snapshot |> Text.predent '-')
+                    (Diff.format diff)
-                    (actual |> Text.predent '+')
                 |> ExpectException
                 |> raise
             | Mode.Assert ->
-                if GlobalBuilderConfig.bulkUpdate.Value > 0 then
+                if GlobalBuilderConfig.isBulkUpdateMode () then
                     GlobalBuilderConfig.registerTest state
                 else
+                    let diff = Diff.patience snapshot actual
+
                     sprintf
-                        "snapshot mismatch! snapshot at %s:%i (%s) was:\n\n%s\n\nactual was:\n\n%s"
+                        "snapshot mismatch! snapshot at %s:%i (%s) diff:\n\n%s"
                         state.Caller.FilePath
                         state.Caller.LineNumber
                         state.Caller.MemberName
-                        (snapshot |> Text.predent '-')
+                        (Diff.format diff)
-                        (actual |> Text.predent '+')
                     |> ExpectException
                     |> raise
             | Mode.Update ->
                 let lines = File.ReadAllLines state.Caller.FilePath
                 let oldContents = String.concat "\n" lines
                 let lines = SnapshotUpdate.updateSnapshotAtLine lines state.Caller.LineNumber actual
-                File.WriteAllLines (state.Caller.FilePath, lines)
+                File.writeAllLines lines state.Caller.FilePath
                 failwith ("Snapshot successfully updated. Previous contents:\n" + oldContents)
 
         match CompletedSnapshotGeneric.passesAssertion state with
         | None ->
-            match mode, GlobalBuilderConfig.bulkUpdate.Value with
+            match mode, GlobalBuilderConfig.isBulkUpdateMode () with
             | Mode.Update, _
-            | _, 1 ->
+            | _, true ->
                 failwith
                     "Snapshot assertion passed, but we are in snapshot-updating mode. Use the `expect` builder instead of `expect'` to assert the contents of a single snapshot; disable `GlobalBuilderConfig.bulkUpdate` to move back to assertion-checking mode."
             | _ -> ()

WoofWare.Expect/Config.fs

@@ -1,12 +1,19 @@
 namespace WoofWare.Expect
 
-open System.Threading
-
 /// Module holding global mutable state controlling the behaviour of WoofWare.Expect
 /// when running in bulk-update mode.
 [<RequireQualifiedAccess>]
 module GlobalBuilderConfig =
-    let internal bulkUpdate = ref 0
+    /// All access to the global mutable state locks on this.
+    let private locker = obj ()
+
+    // Global mutable state ensuring there is at most one `enterBulkUpdateMode`/`updateAllSnapshots` pair running at once.
+    let private bulkUpdate = ref 0
+
+    let private allTests : ResizeArray<CompletedSnapshot> = ResizeArray ()
+
+    let internal isBulkUpdateMode () : bool =
+        lock locker (fun () -> bulkUpdate.Value > 0)
 
     /// <summary>
     /// Call this to make the <c>expect</c> builder register all tests for bulk update as it runs.
@@ -16,11 +23,15 @@ module GlobalBuilderConfig =
     /// The implied global mutable state is liable to interfere with other expect builders in other fixtures otherwise.
     /// </remarks>
     let enterBulkUpdateMode () =
-        if Interlocked.Increment bulkUpdate <> 1 then
+        lock
-            failwith
+            locker
-                "WoofWare.Expect requires bulk updates to happen serially: for example, make the test fixture `[<NonParallelizable>]` if you're using NUnit."
+            (fun () ->
+                if bulkUpdate.Value <> 0 then
+                    failwith
+                        "WoofWare.Expect requires bulk updates to happen serially: for example, make the test fixture `[<NonParallelizable>]` if you're using NUnit."
 
-    let private allTests : ResizeArray<CompletedSnapshot> = ResizeArray ()
+                bulkUpdate.Value <- bulkUpdate.Value + 1
+            )
 
     /// <summary>
     /// Clear the set of failing tests registered by any previous bulk-update runs.
@@ -30,23 +41,31 @@ module GlobalBuilderConfig =
     /// You probably don't need to do this, because your test runner is probably tearing down
     /// anyway after the tests have failed; this is mainly here for WoofWare.Expect's own internal testing.
     /// </remarks>
-    let clearTests () = lock allTests allTests.Clear
+    let clearTests () = lock locker allTests.Clear
 
     let internal registerTest (s : CompletedSnapshotGeneric<'T>) : unit =
         let toAdd = s |> CompletedSnapshot.make
-        lock allTests (fun () -> allTests.Add toAdd)
+        lock locker (fun () -> allTests.Add toAdd)
 
     /// <summary>
     /// For all tests whose failures have already been registered,
     /// transform the files on disk so that the failing snapshots now pass.
     /// </summary>
     let updateAllSnapshots () =
-        let bulkUpdate' = Interlocked.Decrement bulkUpdate
+        // It's OK for this to be called when `enterBulkUpdateMode` has not been called, i.e. when `bulkUpdate` has
+        // value 0. That just means we aren't in bulk-update mode, so we expect the following simply to do nothing.
+        // (This is an expected workflow: we expect users to run `updateAllSnapshots` unconditionally in a
+        // one-time tear-down of the test suite, and they use the one-time setup to control whether any work is actually
+        // performed here.)
+        lock
+            locker
+            (fun () ->
+                let allTests = Seq.toArray allTests
 
-        try
+                try
-            if bulkUpdate' = 0 then
+                    SnapshotUpdate.updateAll allTests
-                let allTests = lock allTests (fun () -> Seq.toArray allTests)
+                finally
-                SnapshotUpdate.updateAll allTests
+                    // double acquiring of reentrant lock is OK, we're not switching threads
-
+                    clearTests ()
-        finally
+                    bulkUpdate.Value <- 0
-            clearTests ()
+            )

WoofWare.Expect/Diff.fs

@@ -0,0 +1,296 @@
+namespace WoofWare.Expect
+
+open System.Collections.Generic
+
+/// A unit of measure tagging "positions in a sequence".
+[<Measure>]
+type pos
+
+/// Position in a sequence
+type Position = int<pos>
+
+/// A Patience diff is composed of a sequence of transformations to get from one string to another.
+/// This represents those transformations.
+type DiffOperation =
+    /// This line is the same on both sides of the diff.
+    /// On the left, it appears at position posA. On the right, at position posB.
+    | Match of posA : Position * posB : Position * line : string
+    /// Delete this line, which is at this position.
+    | Delete of posA : Position * line : string
+    /// Insert this line at the given position.
+    | Insert of posB : Position * line : string
+
+/// The diff between two line-oriented pieces of text.
+type Diff = private | Diff of DiffOperation list
+
+/// A match between positions in two sequences
+type internal LineMatch =
+    {
+        PosA : Position
+        PosB : Position
+        Line : string
+    }
+
+/// Result of finding unique lines in a sequence
+type internal UniqueLines =
+    {
+        /// Map from line content to its position (only for unique lines)
+        LinePositions : Map<string, Position>
+        /// All line counts (for verification)
+        LineCounts : Map<string, int>
+    }
+
+[<RequireQualifiedAccess>]
+module Diff =
+    /// Find lines that appear exactly once in a sequence
+    let private findUniqueLines (lines : string array) : UniqueLines =
+        let positions = Dictionary<string, Position> ()
+        let counts = Dictionary<string, int> ()
+
+        lines
+        |> Array.iteri (fun i line ->
+            if counts.ContainsKey (line) then
+                counts.[line] <- counts.[line] + 1
+            else
+                counts.[line] <- 1
+                positions.[line] <- i * 1<pos>
+        )
+
+        let uniquePositions =
+            positions
+            |> Seq.filter (fun kvp -> counts.[kvp.Key] = 1)
+            |> Seq.map (fun kvp -> (kvp.Key, kvp.Value))
+            |> Map.ofSeq
+
+        let allCounts = counts |> Seq.map (fun kvp -> (kvp.Key, kvp.Value)) |> Map.ofSeq
+
+        {
+            LinePositions = uniquePositions
+            LineCounts = allCounts
+        }
+
+    /// Find longest increasing subsequence based on B positions
+    let private longestIncreasingSubsequence (matches : LineMatch array) : LineMatch list =
+        let n = matches.Length
+
+        if n = 0 then
+            []
+        else
+            // Dynamic programming arrays
+            let lengths = Array.create n 1
+            let parents = Array.create n -1
+
+            // Build LIS
+            for i in 1 .. n - 1 do
+                for j in 0 .. i - 1 do
+                    let bj = matches.[j].PosB
+                    let bi = matches.[i].PosB
+
+                    if bj < bi && lengths.[j] + 1 > lengths.[i] then
+                        lengths.[i] <- lengths.[j] + 1
+                        parents.[i] <- j
+
+            // Find longest sequence
+            let maxLength = Array.max lengths
+            let endIndex = Array.findIndex ((=) maxLength) lengths
+
+            // Reconstruct sequence
+            let rec reconstruct idx acc =
+                if idx = -1 then
+                    acc
+                else
+                    reconstruct parents.[idx] (matches.[idx] :: acc)
+
+            reconstruct endIndex []
+
+    /// Simple Myers diff implementation. You probably want to use `patience` instead, for more human-readable diffs.
+    let myers (a : string array) (b : string array) : Diff =
+        let rec diffHelper (i : int) (j : int) (acc : DiffOperation list) =
+            match i < a.Length, j < b.Length with
+            | false, false -> List.rev acc
+            | true, false ->
+                let deletes =
+                    [ i .. a.Length - 1 ] |> List.map (fun idx -> Delete (idx * 1<pos>, a.[idx]))
+
+                (List.rev acc) @ deletes
+            | false, true ->
+                let inserts =
+                    [ j .. b.Length - 1 ] |> List.map (fun idx -> Insert (idx * 1<pos>, b.[idx]))
+
+                (List.rev acc) @ inserts
+            | true, true ->
+                if a.[i] = b.[j] then
+                    diffHelper (i + 1) (j + 1) (Match (i * 1<pos>, j * 1<pos>, a.[i]) :: acc)
+                else
+                    // Look ahead for matches (simple heuristic)
+                    let lookAhead = 3
+
+                    let aheadMatch =
+                        [ 1 .. min lookAhead (min (a.Length - i) (b.Length - j)) ]
+                        |> List.tryFind (fun k -> a.[i + k - 1] = b.[j + k - 1])
+
+                    match aheadMatch with
+                    | Some k when k <= 2 ->
+                        // Delete/insert to get to the match
+                        let ops =
+                            [ 0 .. k - 2 ]
+                            |> List.collect (fun offset ->
+                                [
+                                    Delete ((i + offset) * 1<pos>, a.[i + offset])
+                                    Insert ((j + offset) * 1<pos>, b.[j + offset])
+                                ]
+                            )
+
+                        diffHelper (i + k - 1) (j + k - 1) (List.rev ops @ acc)
+                    | _ ->
+                        // No close match, just delete and insert
+                        diffHelper (i + 1) j (Delete (i * 1<pos>, a.[i]) :: acc)
+
+        diffHelper 0 0 [] |> Diff
+
+    /// Patience diff: a human-readable line-based diff.
+    /// Operates on lines of string; the function `patience` will split on lines for you.
+    let rec patienceLines (a : string array) (b : string array) : Diff =
+        // Handle empty sequences
+        match a.Length, b.Length with
+        | 0, 0 -> [] |> Diff
+        | 0, _ ->
+            b
+            |> Array.mapi (fun i line -> Insert (i * 1<pos>, line))
+            |> Array.toList
+            |> Diff
+        | _, 0 ->
+            a
+            |> Array.mapi (fun i line -> Delete (i * 1<pos>, line))
+            |> Array.toList
+            |> Diff
+        | _, _ ->
+            // Find unique lines
+            let uniqueA = findUniqueLines a
+            let uniqueB = findUniqueLines b
+
+            // Find common unique lines
+            let commonUniques =
+                Set.intersect
+                    (uniqueA.LinePositions |> Map.toSeq |> Seq.map fst |> Set.ofSeq)
+                    (uniqueB.LinePositions |> Map.toSeq |> Seq.map fst |> Set.ofSeq)
+
+            if Set.isEmpty commonUniques then
+                // No unique common lines, fall back to Myers
+                myers a b
+            else
+                // Build matches for unique common lines
+                let matches =
+                    commonUniques
+                    |> Set.toArray
+                    |> Array.map (fun line ->
+                        {
+                            PosA = uniqueA.LinePositions.[line]
+                            PosB = uniqueB.LinePositions.[line]
+                            Line = line
+                        }
+                    )
+                    |> Array.sortBy (fun m -> m.PosA)
+
+                // Find LIS
+                let anchorMatches = longestIncreasingSubsequence matches |> List.toArray
+
+                // Build diff imperatively
+                let result = ResizeArray<DiffOperation> ()
+                let mutable prevA = 0<pos>
+                let mutable prevB = 0<pos>
+
+                // Process each anchor
+                for anchor in anchorMatches do
+                    let anchorA = anchor.PosA
+                    let anchorB = anchor.PosB
+
+                    // Add diff for section before this anchor
+                    if prevA < anchorA || prevB < anchorB then
+                        let sectionA = a.[prevA / 1<pos> .. anchorA / 1<pos> - 1]
+                        let sectionB = b.[prevB / 1<pos> .. anchorB / 1<pos> - 1]
+                        let (Diff sectionDiff) = patienceLines sectionA sectionB
+
+                        // Adjust positions and add to result
+                        for op in sectionDiff do
+                            match op with
+                            | Match (pa, pb, line) -> result.Add (Match ((pa + prevA), (pb + prevB), line))
+                            | Delete (pa, line) -> result.Add (Delete ((pa + prevA), line))
+                            | Insert (pb, line) -> result.Add (Insert ((pb + prevB), line))
+
+                    // Add the anchor match
+                    result.Add (Match (anchor.PosA, anchor.PosB, anchor.Line))
+
+                    // Update positions
+                    prevA <- anchorA + 1<pos>
+                    prevB <- anchorB + 1<pos>
+
+                // Handle remaining elements after last anchor
+                if prevA / 1<pos> < a.Length || prevB / 1<pos> < b.Length then
+                    let remainingA = a.[prevA / 1<pos> ..]
+                    let remainingB = b.[prevB / 1<pos> ..]
+                    let (Diff remainingDiff) = patienceLines remainingA remainingB
+
+                    for op in remainingDiff do
+                        match op with
+                        | Match (pa, pb, line) -> result.Add (Match ((pa + prevA), (pb + prevB), line))
+                        | Delete (pa, line) -> result.Add (Delete ((pa + prevA), line))
+                        | Insert (pb, line) -> result.Add (Insert ((pb + prevB), line))
+
+                result |> Seq.toList |> Diff
+
+    /// Patience diff: a human-readable line-based diff.
+    let patience (a : string) (b : string) =
+        patienceLines (a.Split '\n') (b.Split '\n')
+
+    /// Format the diff as a human-readable string, including line numbers at the left.
+    let formatWithLineNumbers (Diff ops) : string =
+        ops
+        |> List.map (fun op ->
+            match op with
+            | Match (a, b, line) -> sprintf "  %3d %3d  %s" a b line
+            | Delete (a, line) -> sprintf "- %3d      %s" a line
+            | Insert (b, line) -> sprintf "+     %3d  %s" b line
+        )
+        |> String.concat "\n"
+
+    /// Format the diff as a human-readable string.
+    let format (Diff ops) : string =
+        ops
+        |> List.map (fun op ->
+            match op with
+            | Match (_, _, line) -> sprintf "  %s" line
+            | Delete (_, line) -> sprintf "- %s" line
+            | Insert (_, line) -> sprintf "+ %s" line
+        )
+        |> String.concat "\n"
+
+    /// Compute diff statistics
+    type internal DiffStats =
+        {
+            Matches : int
+            Deletions : int
+            Insertions : int
+            TotalOperations : int
+        }
+
+    let internal computeStats (ops : DiffOperation list) : DiffStats =
+        let counts =
+            ops
+            |> List.fold
+                (fun (m, d, i) op ->
+                    match op with
+                    | Match _ -> (m + 1, d, i)
+                    | Delete _ -> (m, d + 1, i)
+                    | Insert _ -> (m, d, i + 1)
+                )
+                (0, 0, 0)
+
+        let matches, deletions, insertions = counts
+
+        {
+            Matches = matches
+            Deletions = deletions
+            Insertions = insertions
+            TotalOperations = matches + deletions + insertions
+        }

WoofWare.Expect/Domain.fs

@@ -17,20 +17,21 @@ type CallerInfo =
 type private SnapshotValue =
     | Json of expected : string
     | Formatted of expected : string
+    | ThrowsException of expected : string
 
 type private CompletedSnapshotValue<'T> =
     | Json of expected : string * JsonSerializerOptions * JsonDocumentOptions
-    | Formatted of expected : string * format : ('T -> string)
+    | Formatted of expected : string * format : ((unit -> 'T) -> string)
 
 /// The state accumulated by the `expect` builder. You should never find yourself interacting with this type.
 type ExpectState<'T> =
     private
         {
-            Formatter : ('T -> string) option
+            Formatter : ((unit -> 'T) -> string) option
             JsonSerialiserOptions : JsonSerializerOptions option
             JsonDocOptions : JsonDocumentOptions option
             Snapshot : (SnapshotValue * CallerInfo) option
-            Actual : 'T option
+            Actual : (unit -> 'T) option
         }
 
 /// The state accumulated by the `expect` builder. You should never find yourself interacting with this type.
@@ -39,7 +40,7 @@ type internal CompletedSnapshotGeneric<'T> =
         {
             SnapshotValue : CompletedSnapshotValue<'T>
             Caller : CallerInfo
-            Actual : 'T
+            Actual : unit -> 'T
         }
 
 [<RequireQualifiedAccess>]
@@ -68,11 +69,22 @@ module internal CompletedSnapshotGeneric =
                 | SnapshotValue.Formatted expected ->
                     let formatter =
                         match state.Formatter with
-                        | None -> fun x -> x.ToString ()
+                        | None -> fun x -> x().ToString ()
                         | Some f -> f
 
                     CompletedSnapshotValue.Formatted (expected, formatter)
 
+                | SnapshotValue.ThrowsException expected ->
+                    CompletedSnapshotValue.Formatted (
+                        expected,
+                        fun x ->
+                            try
+                                x () |> ignore
+                                "<no exception raised>"
+                            with e ->
+                                e.GetType().FullName + ": " + e.Message
+                    )
+
             {
                 SnapshotValue = snapshot
                 Caller = source
@@ -84,7 +96,7 @@ module internal CompletedSnapshotGeneric =
     let internal replacement (s : CompletedSnapshotGeneric<'T>) =
         match s.SnapshotValue with
         | CompletedSnapshotValue.Json (_existing, options, _) ->
-            JsonSerializer.Serialize (s.Actual, options)
+            JsonSerializer.Serialize (s.Actual (), options)
             |> JsonDocument.Parse
             |> _.RootElement
             |> _.ToString()
@@ -104,7 +116,7 @@ module internal CompletedSnapshotGeneric =
                     None
 
             let canonicalActual =
-                JsonSerializer.Serialize (state.Actual, jsonSerOptions) |> JsonDocument.Parse
+                JsonSerializer.Serialize (state.Actual (), jsonSerOptions) |> JsonDocument.Parse
 
             match canonicalSnapshot with
             | None -> Some ("[JSON failed to parse:] " + snapshot, canonicalActual.RootElement.ToString ())

WoofWare.Expect/File.fs

@@ -0,0 +1,32 @@
+namespace WoofWare.Expect
+
+open System
+open System.IO
+
+[<RequireQualifiedAccess>]
+module internal File =
+
+    /// Standard attempt at an atomic file write.
+    /// It may fail to be atomic if the working directory somehow spans multiple volumes,
+    /// and of course with external network storage all bets are off.
+    let writeAllLines (lines : string[]) (path : string) : unit =
+        let file = FileInfo path
+
+        let tempFile =
+            Path.Combine (file.Directory.FullName, file.Name + "." + Guid.NewGuid().ToString () + ".tmp")
+
+        try
+            File.WriteAllLines (tempFile, lines)
+            // Atomicity guarantees are undocumented, but on Unix this is an atomic `rename` call
+            // https://github.com/dotnet/runtime/blob/9a4be5b56d81aa04c7ea687c02b3f4e64c83761b/src/libraries/System.Private.CoreLib/src/System/IO/FileSystem.Unix.cs#L181
+            // and on Windows this is an atomic ReplaceFile:
+            // https://github.com/dotnet/runtime/blob/9a4be5b56d81aa04c7ea687c02b3f4e64c83761b/src/libraries/System.Private.CoreLib/src/System/IO/FileSystem.Windows.cs#L92
+            // calls https://github.com/dotnet/runtime/blob/9a4be5b56d81aa04c7ea687c02b3f4e64c83761b/src/libraries/Common/src/Interop/Windows/Kernel32/Interop.ReplaceFile.cs#L12
+            // which calls ReplaceFileW, whose atomicity guarantees are again apparently undocumented,
+            // but 4o-turbo, Opus 4, and Gemini 2.5 Flash all think it's atomic.
+            File.Replace (tempFile, path, null)
+        finally
+            try
+                File.Delete tempFile
+            with _ ->
+                ()

WoofWare.Expect/SnapshotUpdate.fs

@@ -15,7 +15,7 @@ type private StringLiteralInfo =
     override this.ToString () =
         sprintf "%i:%i to %i:%i: %s" this.StartLine this.StartColumn this.EndLine this.EndColumn this.Content
 
-type private Position =
+type private SnapshotPosition =
     {
         Line : int
         Column : int
@@ -28,8 +28,8 @@ module internal SnapshotUpdate =
     let tripleQuote = "\"\"\""
 
     /// Convert a string position to line/column
-    let private positionToLineColumn (text : string) (offset : int) : Position =
+    let private positionToLineColumn (text : string) (offset : int) : SnapshotPosition =
-        let rec loop (line : int) (col : int) (totalOffset : int) (i : int) : Position =
+        let rec loop (line : int) (col : int) (totalOffset : int) (i : int) : SnapshotPosition =
             if i >= text.Length || totalOffset = offset then
                 {
                     Line = line
@@ -147,7 +147,8 @@ module internal SnapshotUpdate =
             let searchText = String.concat "\n" relevantLines
 
             // Find snapshot keyword
-            let snapshotMatch = Regex.Match (searchText, @"\b(snapshot|snapshotJson)\b")
+            let snapshotMatch =
+                Regex.Match (searchText, @"\b(snapshot|snapshotJson|snapshotThrows)\b")
 
             if not snapshotMatch.Success then
                 None
@@ -300,5 +301,5 @@ module internal SnapshotUpdate =
 
             let newContents = updateAllLines contents sources
 
-            System.IO.File.WriteAllLines (callerFile, newContents)
+            File.writeAllLines newContents callerFile
         )

WoofWare.Expect/SurfaceBaseline.txt

@@ -8,6 +8,48 @@ WoofWare.Expect.CallerInfo inherit obj, implements WoofWare.Expect.CallerInfo Sy
 WoofWare.Expect.CallerInfo.Equals [method]: (WoofWare.Expect.CallerInfo, System.Collections.IEqualityComparer) -> bool
 WoofWare.Expect.CompletedSnapshot inherit obj, implements WoofWare.Expect.CompletedSnapshot System.IEquatable, System.Collections.IStructuralEquatable, WoofWare.Expect.CompletedSnapshot System.IComparable, System.IComparable, System.Collections.IStructuralComparable
 WoofWare.Expect.CompletedSnapshot.Equals [method]: (WoofWare.Expect.CompletedSnapshot, System.Collections.IEqualityComparer) -> bool
+WoofWare.Expect.Diff inherit obj, implements WoofWare.Expect.Diff System.IEquatable, System.Collections.IStructuralEquatable, WoofWare.Expect.Diff System.IComparable, System.IComparable, System.Collections.IStructuralComparable
+WoofWare.Expect.Diff.Equals [method]: (WoofWare.Expect.Diff, System.Collections.IEqualityComparer) -> bool
+WoofWare.Expect.DiffModule inherit obj
+WoofWare.Expect.DiffModule.format [static method]: WoofWare.Expect.Diff -> string
+WoofWare.Expect.DiffModule.formatWithLineNumbers [static method]: WoofWare.Expect.Diff -> string
+WoofWare.Expect.DiffModule.myers [static method]: string [] -> string [] -> WoofWare.Expect.Diff
+WoofWare.Expect.DiffModule.patience [static method]: string -> string -> WoofWare.Expect.Diff
+WoofWare.Expect.DiffModule.patienceLines [static method]: string [] -> string [] -> WoofWare.Expect.Diff
+WoofWare.Expect.DiffOperation inherit obj, implements WoofWare.Expect.DiffOperation System.IEquatable, System.Collections.IStructuralEquatable, WoofWare.Expect.DiffOperation System.IComparable, System.IComparable, System.Collections.IStructuralComparable - union type with 3 cases
+WoofWare.Expect.DiffOperation+Delete inherit WoofWare.Expect.DiffOperation
+WoofWare.Expect.DiffOperation+Delete.get_line [method]: unit -> string
+WoofWare.Expect.DiffOperation+Delete.get_posA [method]: unit -> int
+WoofWare.Expect.DiffOperation+Delete.line [property]: [read-only] string
+WoofWare.Expect.DiffOperation+Delete.posA [property]: [read-only] int
+WoofWare.Expect.DiffOperation+Insert inherit WoofWare.Expect.DiffOperation
+WoofWare.Expect.DiffOperation+Insert.get_line [method]: unit -> string
+WoofWare.Expect.DiffOperation+Insert.get_posB [method]: unit -> int
+WoofWare.Expect.DiffOperation+Insert.line [property]: [read-only] string
+WoofWare.Expect.DiffOperation+Insert.posB [property]: [read-only] int
+WoofWare.Expect.DiffOperation+Match inherit WoofWare.Expect.DiffOperation
+WoofWare.Expect.DiffOperation+Match.get_line [method]: unit -> string
+WoofWare.Expect.DiffOperation+Match.get_posA [method]: unit -> int
+WoofWare.Expect.DiffOperation+Match.get_posB [method]: unit -> int
+WoofWare.Expect.DiffOperation+Match.line [property]: [read-only] string
+WoofWare.Expect.DiffOperation+Match.posA [property]: [read-only] int
+WoofWare.Expect.DiffOperation+Match.posB [property]: [read-only] int
+WoofWare.Expect.DiffOperation+Tags inherit obj
+WoofWare.Expect.DiffOperation+Tags.Delete [static field]: int = 1
+WoofWare.Expect.DiffOperation+Tags.Insert [static field]: int = 2
+WoofWare.Expect.DiffOperation+Tags.Match [static field]: int = 0
+WoofWare.Expect.DiffOperation.Equals [method]: (WoofWare.Expect.DiffOperation, System.Collections.IEqualityComparer) -> bool
+WoofWare.Expect.DiffOperation.get_IsDelete [method]: unit -> bool
+WoofWare.Expect.DiffOperation.get_IsInsert [method]: unit -> bool
+WoofWare.Expect.DiffOperation.get_IsMatch [method]: unit -> bool
+WoofWare.Expect.DiffOperation.get_Tag [method]: unit -> int
+WoofWare.Expect.DiffOperation.IsDelete [property]: [read-only] bool
+WoofWare.Expect.DiffOperation.IsInsert [property]: [read-only] bool
+WoofWare.Expect.DiffOperation.IsMatch [property]: [read-only] bool
+WoofWare.Expect.DiffOperation.NewDelete [static method]: (int, string) -> WoofWare.Expect.DiffOperation
+WoofWare.Expect.DiffOperation.NewInsert [static method]: (int, string) -> WoofWare.Expect.DiffOperation
+WoofWare.Expect.DiffOperation.NewMatch [static method]: (int, int, string) -> WoofWare.Expect.DiffOperation
+WoofWare.Expect.DiffOperation.Tag [property]: [read-only] int
 WoofWare.Expect.ExpectBuilder inherit obj
 WoofWare.Expect.ExpectBuilder..ctor [constructor]: (string * int)
 WoofWare.Expect.ExpectBuilder..ctor [constructor]: bool
@@ -16,9 +58,11 @@ WoofWare.Expect.ExpectBuilder.Bind [method]: ('U WoofWare.Expect.ExpectState, un
 WoofWare.Expect.ExpectBuilder.Delay [method]: (unit -> 'T WoofWare.Expect.ExpectState) -> (unit -> 'T WoofWare.Expect.ExpectState)
 WoofWare.Expect.ExpectBuilder.Return [method]: 'T -> 'T WoofWare.Expect.ExpectState
 WoofWare.Expect.ExpectBuilder.Return [method]: unit -> 'T WoofWare.Expect.ExpectState
+WoofWare.Expect.ExpectBuilder.ReturnFrom [method]: (unit -> 'T) -> 'T WoofWare.Expect.ExpectState
 WoofWare.Expect.ExpectBuilder.Run [method]: (unit -> 'T WoofWare.Expect.ExpectState) -> unit
 WoofWare.Expect.ExpectBuilder.Snapshot [method]: ('a WoofWare.Expect.ExpectState, string, string option, int option, string option) -> 'a WoofWare.Expect.ExpectState
 WoofWare.Expect.ExpectBuilder.SnapshotJson [method]: (unit WoofWare.Expect.ExpectState, string, string option, int option, string option) -> 'a WoofWare.Expect.ExpectState
+WoofWare.Expect.ExpectBuilder.SnapshotThrows [method]: ('a WoofWare.Expect.ExpectState, string, string option, int option, string option) -> 'a WoofWare.Expect.ExpectState
 WoofWare.Expect.ExpectBuilder.WithFormat [method]: ('T WoofWare.Expect.ExpectState, 'T -> string) -> 'T WoofWare.Expect.ExpectState
 WoofWare.Expect.ExpectBuilder.WithJsonDocOptions [method]: ('T WoofWare.Expect.ExpectState, System.Text.Json.JsonDocumentOptions) -> 'T WoofWare.Expect.ExpectState
 WoofWare.Expect.ExpectBuilder.WithJsonSerializerOptions [method]: ('T WoofWare.Expect.ExpectState, System.Text.Json.JsonSerializerOptions) -> 'T WoofWare.Expect.ExpectState
@@ -34,4 +78,5 @@ WoofWare.Expect.GlobalBuilderConfig.clearTests [static method]: unit -> unit
 WoofWare.Expect.GlobalBuilderConfig.enterBulkUpdateMode [static method]: unit -> unit
 WoofWare.Expect.GlobalBuilderConfig.updateAllSnapshots [static method]: unit -> unit
 WoofWare.Expect.Mode inherit obj, implements WoofWare.Expect.Mode System.IEquatable, System.Collections.IStructuralEquatable, WoofWare.Expect.Mode System.IComparable, System.IComparable, System.Collections.IStructuralComparable
-WoofWare.Expect.Mode.Equals [method]: (WoofWare.Expect.Mode, System.Collections.IEqualityComparer) -> bool
+WoofWare.Expect.Mode.Equals [method]: (WoofWare.Expect.Mode, System.Collections.IEqualityComparer) -> bool
+WoofWare.Expect.pos inherit obj

WoofWare.Expect/WoofWare.Expect.fsproj

@@ -18,6 +18,8 @@
     <ItemGroup>
         <Compile Include="AssemblyInfo.fs" />
         <Compile Include="Text.fs" />
+        <Compile Include="File.fs" />
+        <Compile Include="Diff.fs" />
         <Compile Include="Domain.fs" />
         <Compile Include="SnapshotUpdate.fs" />
         <Compile Include="Config.fs" />

WoofWare.Expect/version.json

@@ -1,5 +1,5 @@
 {
-  "version": "0.4",
+  "version": "0.5",
   "publicReleaseRefSpec": [
     "^refs/heads/main$"
   ],
@@ -10,4 +10,4 @@
     ":/Directory.Build.props",
     ":/LICENSE"
   ]
-}
+}

flake.nix

@@ -65,6 +65,7 @@
           pkgs.alejandra
           pkgs.nodePackages.markdown-link-check
           pkgs.shellcheck
+          pkgs.xmlstarlet
         ];
       };
     });