WoofWare.Myriad/README.md

# WoofWare.Myriad.Plugins

![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.

Currently implemented:
* `JsonParse` (to stamp out `jsonParse : JsonNode -> 'T` methods);
* `RemoveOptions` (to strip `option` modifiers from a type).

## `JsonParse`

Takes records like this:

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

/// My whatnot
[<MyriadPlugin.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 }
```

### 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.
* Rather than just throwing `NullReferenceException`, print out the field name that failed.
* Generally support all the `System.Text.Json` attributes.

## `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?

# Detailed examples

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