WoofWare.Myriad.Plugins.Attributes 2.0.5

WoofWare.Myriad.Plugins

Some helpers in 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-style HTTP client).
• GenerateMock (to stamp out a record type corresponding to an interface).

JsonParse

Takes records like this:

[<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:

/// 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#.

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 and tests.

JsonSerialize

Takes records like this:

[<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:

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

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 and tests.

RemoveOptions

Takes a record like this:

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

and stamps out a record like this:

[<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:

[<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:

/// 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.

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:

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

and stamps out a type like this:

/// 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 in my experience are a rich source of flaky tests. The Grug-brained developer 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.

Limitations

• We make the resulting record type at most internal (never public), since this is intended only to be used in tests. You will therefore need an AssemblyInfo.fs file like the one in WoofWare.Myriad's own tests.

Detailed examples

See the tests. For example, 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:

  <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:

  <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:

  <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:

  <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:

<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, the F# source formatter which powers Myriad, is customisable with editorconfig, but it does not easily expose 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 if you use Fantomas to format your repo; the alternative is to manually reformat every time Myriad changes the generated files.