WoofWare.Myriad/README.md

# WoofWare.Myriad.Plugins

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![Project logo: the face of a cartoon Shiba Inu, staring with powerful cyborg eyes directly at the viewer, with a background of stylised plugs.](./WoofWare.Myriad.Plugins/logo.png)

Some helpers in [Myriad](https://github.com/MoiraeSoftware/myriad/) which might be useful.

These are currently somewhat experimental, and I personally am their primary customer.
The `RemoveOptions` generator in particular is extremely half-baked.

If you would like to ensure that your particular use-case remains unbroken, please do contribute tests to this repository.
The `ConsumePlugin` assembly contains a number of invocations of these source generators,
so you just need to add copies of your types to that assembly to ensure that I will at least notice if I break the build;
and if you add tests to `WoofWare.Myriad.Plugins.Test` then I will also notice if I break the runtime semantics of the generated code.

Currently implemented:

* `JsonParse` (to stamp out `jsonParse : JsonNode -> 'T` methods);
* `JsonSerialize` (to stamp out `toJsonNode : 'T -> JsonNode` methods);
* `RemoveOptions` (to strip `option` modifiers from a type).
* `HttpClient` (to stamp out a [RestEase](https://github.com/canton7/RestEase)-style HTTP client).
* `GenerateMock` (to stamp out a record type corresponding to an interface).
* `CreateCatamorphism` (to stamp out a non-stack-overflowing [catamorphism](https://fsharpforfunandprofit.com/posts/recursive-types-and-folds/) for a discriminated union).

## `JsonParse`

Takes records like this:

```fsharp
[<WoofWare.Myriad.Plugins.JsonParse>]
type InnerType =
    {
        [<JsonPropertyName "something">]
        Thing : string
    }

/// My whatnot
[<WoofWare.Myriad.Plugins.JsonParse>]
type JsonRecordType =
    {
        /// A thing!
        A : int
        /// Another thing!
        B : string
        [<System.Text.Json.Serialization.JsonPropertyName "hi">]
        C : int list
        D : InnerType
    }

```

and stamps out parsing methods like this:

```fsharp
/// Module containing JSON parsing methods for the InnerType type
[<RequireQualifiedAccess>]
[<CompilationRepresentation(CompilationRepresentationFlags.ModuleSuffix)>]
module InnerType =
    /// Parse from a JSON node.
    let jsonParse (node: System.Text.Json.Nodes.JsonNode) : InnerType =
        let Thing = node.["something"].AsValue().GetValue<string>()
        { Thing = Thing }
namespace UsePlugin

/// Module containing JSON parsing methods for the JsonRecordType type
[<RequireQualifiedAccess>]
[<CompilationRepresentation(CompilationRepresentationFlags.ModuleSuffix)>]
module JsonRecordType =
    /// Parse from a JSON node.
    let jsonParse (node: System.Text.Json.Nodes.JsonNode) : JsonRecordType =
        let D = InnerType.jsonParse node.["d"]

        let C =
            node.["hi"].AsArray() |> Seq.map (fun elt -> elt.GetValue<int>()) |> List.ofSeq

        let B = node.["b"].AsValue().GetValue<string>()
        let A = node.["a"].AsValue().GetValue<int>()
        { A = A; B = B; C = C; D = D }
```

You can optionally supply the boolean `true` to the attribute,
which will cause Myriad to stamp out an extension method rather than a module with the same name as the type.
This is useful if you want to reuse the type name as a module name yourself,
or if you want to apply multiple source generators which each want to use the module name.

### What's the point?

`System.Text.Json`, in a `PublishAot` context, relies on C# source generators.
The default reflection-heavy implementations have the necessary code trimmed away, and result in a runtime exception.
But C# source generators [are entirely unsupported in F#](https://github.com/dotnet/fsharp/issues/14300).

This Myriad generator expects you to use `System.Text.Json` to construct a `JsonNode`,
and then the generator takes over to construct a strongly-typed object.

### Limitations

This source generator is enough for what I first wanted to use it for.
However, there is *far* more that could be done.

* Make it possible to give an exact format and cultural info in date and time parsing.
* Make it possible to reject parsing if extra fields are present.
* Generally support all the `System.Text.Json` attributes.

For an example of using both `JsonParse` and `JsonSerialize` together with complex types, see [the type definitions](./ConsumePlugin/SerializationAndDeserialization.fs) and [tests](./WoofWare.Myriad.Plugins.Test/TestJsonSerialize/TestJsonSerde.fs).

## `JsonSerialize`

Takes records like this:
```fsharp
[<WoofWare.Myriad.Plugins.JsonSerialize true>]
type InnerTypeWithBoth =
    {
        [<JsonPropertyName("it's-a-me")>]
        Thing : string
        ReadOnlyDict : IReadOnlyDictionary<string, Uri list>
    }
```

and stamps out modules like this:
```fsharp
module InnerTypeWithBoth =
    let toJsonNode (input : InnerTypeWithBoth) : System.Text.Json.Nodes.JsonNode =
        let node = System.Text.Json.Nodes.JsonObject ()

        do
            node.Add (("it's-a-me"), System.Text.Json.Nodes.JsonValue.Create<string> input.Thing)

            node.Add (
                "ReadOnlyDict",
                (fun field ->
                    let ret = System.Text.Json.Nodes.JsonObject ()

                    for (KeyValue (key, value)) in field do
                        ret.Add (key.ToString (), System.Text.Json.Nodes.JsonValue.Create<Uri> value)

                    ret
                ) input.Map
            )

        node
```

Also includes an *opinionated* serializer for discriminated unions.
(Any such serializer must be opinionated, because JSON does not natively model DUs.)

As in `JsonParse`, you can optionally supply the boolean `true` to the attribute,
which will cause Myriad to stamp out an extension method rather than a module with the same name as the type.

The same limitations generally apply to `JsonSerialize` as do to `JsonParse`.

For an example of using both `JsonParse` and `JsonSerialize` together with complex types, see [the type definitions](./ConsumePlugin/SerializationAndDeserialization.fs) and [tests](./WoofWare.Myriad.Plugins.Test/TestJsonSerialize/TestJsonSerde.fs).

## `RemoveOptions`

Takes a record like this:

```fsharp
type Foo =
    {
        A : int option
        B : string
        C : float list
    }
```

and stamps out a record like this:

```fsharp
[<RequireQualifiedAccess>]
module Foo =
    type Short =
        {
            A : int
            B : string
            C : float list
        }
```

### What's the point?

The motivating example is argument parsing.
An argument parser naturally wants to express "the user did not supply this, so I will provide a default".
But it's not a very ergonomic experience for the programmer to deal with all these options,
so this Myriad generator stamps out a type *without* any options,
and also stamps out an appropriate constructor function.

### Limitations

This generator is *far* from where I want it, because I haven't really spent any time on it.

* It really wants to be able to recurse into the types within the record, to strip options from them.
* It needs some sort of attribute to mark a field as *not* receiving this treatment.
* What do we do about discriminated unions?

## `HttpClient`

Takes a type like this:

```fsharp
[<WoofWare.Myriad.Plugins.HttpClient>]
type IPureGymApi =
    [<Get "v1/gyms/">]
    abstract GetGyms : ?ct : CancellationToken -> Task<Gym list>

    [<Get "v1/gyms/{gym_id}/attendance">]
    abstract GetGymAttendance : [<Path "gym_id">] gymId : int * ?ct : CancellationToken -> Task<GymAttendance>

    [<Get "v1/member">]
    abstract GetMember : ?ct : CancellationToken -> Task<Member>

    [<Get "v1/gyms/{gym_id}">]
    abstract GetGym : [<Path "gym_id">] gymId : int * ?ct : CancellationToken -> Task<Gym>

    [<Get "v1/member/activity">]
    abstract GetMemberActivity : ?ct : CancellationToken -> Task<MemberActivityDto>

    [<Get "v2/gymSessions/member">]
    abstract GetSessions :
        [<Query>] fromDate : DateTime * [<Query>] toDate : DateTime * ?ct : CancellationToken -> Task<Sessions>
```

and stamps out a type like this:

```fsharp
/// Module for constructing a REST client.
[<CompilationRepresentation(CompilationRepresentationFlags.ModuleSuffix)>]
[<RequireQualifiedAccess>]
module PureGymApi =
    /// Create a REST client.
    let make (client : System.Net.Http.HttpClient) : IPureGymApi =
        { new IPureGymApi with
            member _.GetGyms (ct : CancellationToken option) =
                async {
                    let! ct = Async.CancellationToken

                    let httpMessage =
                        new System.Net.Http.HttpRequestMessage (
                            Method = System.Net.Http.HttpMethod.Get,
                            RequestUri = System.Uri (client.BaseAddress.ToString () + "v1/gyms/")
                        )

                    let! response = client.SendAsync (httpMessage, ct) |> Async.AwaitTask
                    let response = response.EnsureSuccessStatusCode ()
                    let! stream = response.Content.ReadAsStreamAsync ct |> Async.AwaitTask

                    let! node =
                        System.Text.Json.Nodes.JsonNode.ParseAsync (stream, cancellationToken = ct)
                        |> Async.AwaitTask

                    return node.AsArray () |> Seq.map (fun elt -> Gym.jsonParse elt) |> List.ofSeq
                }
                |> (fun a -> Async.StartAsTask (a, ?cancellationToken = ct))

            // (more methods here)
        }
```

### What's the point?

The motivating example is again ahead-of-time compilation: we wish to avoid the reflection which RestEase does.

### Features

* Variable and constant header values are supported:
  see [the definition of `IApiWithHeaders`](./ConsumePlugin/RestApiExample.fs).

### Limitations

RestEase is complex, and handles a lot of different stuff.

* If you set the `BaseAddress` on your input `HttpClient`, make sure to end with a trailing slash
  on any trailing directories (so `"blah/foo/"` rather than `"blah/foo"`).
  We combine URIs using `UriKind.Relative`, so without a trailing slash, the last component may be chopped off.
* Parameters are serialised naively with `toJsonNode` as though the `JsonSerialize` generator were applied,
  and you can't control the serialisation. You can't yet serialise e.g. a primitive type this way (other than `String`);
  all body parameters must be types which have a suitable `toJsonNode : 'a -> JsonNode` method.
* Deserialisation follows the same logic as the `JsonParse` generator,
  and it generally assumes you're using types which `JsonParse` is applied to.
* Anonymous parameters are currently forbidden.

There are also some design decisions:

* Every function must take an optional `CancellationToken` (which is good practice anyway);
  so arguments are forced to be tupled.
* The `[<Optional>]` attribute is not supported and will probably not be supported, because I consider it to be cursed.

## `GenerateMock`

Takes a type like this:

```fsharp
[<GenerateMock>]
type IPublicType =
    abstract Mem1 : string * int -> string list
    abstract Mem2 : string -> int
```

and stamps out a type like this:

```fsharp
/// Mock record type for an interface
type internal PublicTypeMock =
    {
        Mem1 : string * int -> string list
        Mem2 : string -> int
    }

    static member Empty : PublicTypeMock =
        {
            Mem1 = (fun x -> raise (System.NotImplementedException "Unimplemented mock function"))
            Mem2 = (fun x -> raise (System.NotImplementedException "Unimplemented mock function"))
        }

    interface IPublicType with
        member this.Mem1 (arg0, arg1) = this.Mem1 (arg0, arg1)
        member this.Mem2 (arg0) = this.Mem2 (arg0)
```

### What's the point?

Reflective mocking libraries like [Foq](https://github.com/fsprojects/Foq) in my experience are a rich source of flaky tests.
The [Grug-brained developer](https://grugbrain.dev/) would prefer to do this without reflection, and this reduces the rate of strange one-in-ten-thousand "failed to generate IL" errors.
But since F# does not let you partially update an interface definition, we instead stamp out a record,
thereby allowing the programmer to use F#'s record-update syntax.

### Features

* You may supply an `isInternal : bool` argument to the attribute. By default, we make the resulting record type at most internal (never public), since this is intended only to be used in tests; but you can instead make it public with `[<GenerateMock false>]`.

## `CreateCatamorphism`

Takes a collection of mutually recursive discriminated unions:

```fsharp
[<CreateCatamorphism "MyCata">]
type Expr =
    | Const of Const
    | Pair of Expr * Expr * PairOpKind
    | Sequential of Expr list
    | Builder of Expr * ExprBuilder

and ExprBuilder =
    | Child of ExprBuilder
    | Parent of Expr
```

and stamps out a type like this:
```fsharp
type ExprCata<'Expr, 'ExprBuilder> =
    abstract Const : Const -> 'Expr
    abstract Pair : 'Expr -> 'Expr -> PairOpKind -> 'Expr
    abstract Sequential : 'Expr list -> 'Expr
    abstract Builder : 'Expr -> 'ExprBuilder -> 'Expr

type ExprBuilderCata<'Expr, 'ExprBuilder> =
    abstract Child : 'ExprBuilder -> 'ExprBuilder
    abstract Parent : 'Expr -> 'ExprBuilder

type MyCata<'Expr, 'ExprBuilder> =
    {
        Expr : ExprCata<'Expr, 'ExprBuilder>
        ExprBuilder : ExprBuilderCata<'Expr, 'ExprBuilder>
    }

[<RequireQualifiedAccess>]
module ExprCata =
    let runExpr (cata : MyCata<'ExprRet, 'ExprBuilderRet>) (x : Expr) : 'ExprRet =
        failwith "this is implemented"

    let runExprBuilder (cata : MyCata<'ExprRet, 'ExprBuilderRet>) (x : ExprBuilder) : 'ExprBuilderRet =
        failwith "this is implemented"
```

### What's the point?
Recursing over a tree is not easy to get right, especially if you want to avoid stack overflows.
Instead of writing the recursion many times, it's better to do it once,
and then each time you only plug in what you want to do.

### Features

* Mutually recursive DUs are supported (as in the example above).
  Every DU in a recursive `type Foo... and Bar...` knot will be given an appropriate cata, as long as any one of those DUs has the `[<CreateCatamorphism>]` attribute.
* There is *limited* support for records and for lists.
* There is *extremely brittle* support for generics in the DUs you are cata'ing over.
  It is based on the names of the generic parameters, so you must ensure that generic parameters with the same name have the same meaning across the various cases in your recursive knot of DUs.
  (If you overstep the bounds of what this generator can do, you will get compile-time errors, e.g. with generics being constrained to each other's values.)
  See the [List tests](./WoofWare.Myriad.Plugins.Test/TestCataGenerator/TestMyList2.fs) for an example, where we re-implement `FSharpList<'a>`.

### Limitations

**I am not at all convinced of the correctness of this generator**, and I know it is very incomplete (in the sense that there are many possible DUs you could write for which the generator will bail out).
I *strongly* recommend implementing the identity catamorphism for your type and using property-based tests ([as I do](./WoofWare.Myriad.Plugins.Test/TestCataGenerator/TestDirectory.fs)) to assert that the correct thing happens.
Feel free to raise GitHub issues with code I can copy-paste to reproduce a case where the wrong thing happens (though I can't promise to look at them).

* This is a particularly half-baked generator which has so far seen no real-world use.
  It likely has a bunch of [80/20](https://en.wikipedia.org/wiki/Pareto_principle) low-hanging fruit remaining, but it also likely has impossible problems to solve which I don't know about yet.
* Only a very few kinds of DU field are currently implemented.
  For example, this generator can't see through an interface (e.g. the kind of interface one would use to implement the [crate pattern](https://www.patrickstevens.co.uk/posts/2021-10-19-crates/) to represent a [GADT](https://en.wikipedia.org/wiki/Generalized_algebraic_data_type)),
  so the generated cata will simply grant you access to the interface (rather than attempting to descend into it to discover recursive references).
  You can't nest lists deeply. All sorts of other cases are unaddressed.
* This generator does not try to solve the "exponential diamond dependency" problem.
  If you have a case of the form `type Expr = | Branch of Expr * Expr`, the cata will walk into both `Expr`s separately.
  If the `Expr`s happen to be equal, the cata will nevertheless traverse them individually (that is, it will traverse the same `Expr` twice).
  Your type may represent a [DAG](https://en.wikipedia.org/wiki/Directed_acyclic_graph), but we will always effectively expand it into a tree of paths and operate on each of the exponentially-many paths.

# Detailed examples

See the tests.
For example, [PureGymDto.fs](./ConsumePlugin/PureGymDto.fs) is a real-world set of DTOs.

## How to use

* In your `.fsproj` file, define a helper variable so that subsequent steps don't all have to be kept in sync:
    ```xml
    <PropertyGroup>
      <WoofWareMyriadPluginVersion>2.0.1</WoofWareMyriadPluginVersion>
    </PropertyGroup>
    ```
* Take a reference on `WoofWare.Myriad.Plugins.Attributes` (which has no other dependencies), to obtain access to the attributes which the generator will recognise:
    ```xml
    <ItemGroup>
        <PackageReference Include="WoofWare.Myriad.Plugins.Attributes" Version="2.0.2" />
    </ItemGroup>
    ```
* Take a reference (with private assets, to prevent these from propagating to your own assembly) on `WoofWare.Myriad.Plugins`, to obtain the plugins which Myriad will run, and on `Myriad.Sdk`, to obtain the Myriad binary itself:
    ```xml
    <ItemGroup>
        <PackageReference Include="WoofWare.Myriad.Plugins" Version="$(WoofWareMyriadPluginVersion)" PrivateAssets="all" />
        <PackageReference Include="Myriad.Sdk" Version="0.8.3" PrivateAssets="all" />
    </ItemGroup>
    ```
* Point Myriad to the DLL within the NuGet package which is the source of the plugins:
    ```xml
    <ItemGroup>
      <MyriadSdkGenerator Include="$(NuGetPackageRoot)/woofware.myriad.plugins/$(WoofWareMyriadPluginVersion)/lib/net6.0/WoofWare.Myriad.Plugins.dll" />
    </ItemGroup>
    ```

Now you are ready to start using the generators.
For example, this specifies that Myriad is to use the contents of `Client.fs` to generate the file `GeneratedClient.fs`:

```xml
<ItemGroup>
    <Compile Include="Client.fs" />
    <Compile Include="GeneratedClient.fs">
        <MyriadFile>Client.fs</MyriadFile>
    </Compile>
</ItemGroup>
```

### Myriad Gotchas

* MsBuild doesn't always realise that it needs to invoke Myriad during rebuild.
  You can always save a whitespace change to the source file (e.g. `Client.fs` above),
  and MsBuild will then execute Myriad during the next build.
* [Fantomas](https://github.com/fsprojects/fantomas), the F# source formatter which powers Myriad,
  is customisable with [editorconfig](https://editorconfig.org/),
  but it [does not easily expose](https://github.com/fsprojects/fantomas/issues/3031) this customisation
  except through the standalone Fantomas client.
  So Myriad's output is formatted without respect to any conventions which may hold in the rest of your repository.
  You should probably add these files to your [fantomasignore](https://github.com/fsprojects/fantomas/blob/a999b77ca5a024fbc3409955faac797e29b39d27/docs/docs/end-users/IgnoreFiles.md)
  if you use Fantomas to format your repo;
  the alternative is to manually reformat every time Myriad changes the generated files.