README.md

FSharp.HotChocolate

Support for F# types and nullability in HotChocolate.

Quick start

1. Remove any existing calls to AddFSharpTypeConverters or AddTypeConverter<OptionTypeConverter>.
2. Call AddFSharpSupport:

   .AddGraphQLServer().AddFSharpSupport()

Features

FSharp.HotChocolate supports the following:

• Idiomatic F# nullability through Option<_>
• Async<_> fields
• F# collection types on input
• F# unions as GraphQL enums
• F# unions as GraphQL unions

Idiomatic F# nullability through Option<_>

All fields defined in F# (including HotChocolate type extensions for types not defined in F#) will have idiomatic F# nullability applied. This means that everything except Option-wrapped values will be non-nullable (!) in the GraphQL schema. Any usages of [<GraphQLNonNullType>], or NonNullType<_> in [<GraphQLType>], will be ignored.

Opting out of F# nullability

Due to limitations (see below) or other reasons, you may want to opt out of F# nullability for a certain scope. You can apply the SkipFSharpNullability attribute to parameters, members, types (including HotChocolate type extensions), or whole assemblies to disable F# nullability processing for that scope.

Limitations in F# nullability

• When using global object identification, Option-wrapped node resolvers are not supported (#5).
• When using global object identification, Option-wrapped ID values inside lists are not supported (#6).
• Support for ValueOption<_> is not yet added (#10).
• When using UsePaging, the nullability of the first, last, before, and after parameters is controlled by HotChocolate. These are always nullable. Therefore, if these parameters are explicitly present in your method (e.g. if doing custom pagination), make sure you wrap them in Option<_>. The only exception is if you use RequirePagingBoundaries = true with AllowBackwardPagination = false; in that case, HotChocolate will effectively enforce that these (only) two parameters are non-null on input (even though they are nullable in the schema), and it's safe to not wrap them in Option<_> in code.

Async<_> fields

Fields can now be Async<_>.

The computations are automatically wired up to the RequestAborted cancellation token. If you do not want that, please convert the Async<_> to Task<_> yourself as you see fit.

Limitations in Async<_> fields

• When using some built-in middleware such as [<UsePaging>], Async<_> is not supported for that field. (#8).
• When using global object identification, Async<_> is not supported for node resolvers (#9).

F# collection types on input

Parameters and input types can now use List<_> or Set<_>.

F# unions as GraphQL enums

You can use F# fieldless unions as enum types in GraphQL, similar to how normal enums work:

type MyUnion =
    | A
    | B
    | [<GraphQLName("custom_name")>] C
    | [<GraphQLIgnore>] D

Add the type to GraphQL using FSharpUnionAsEnumDescriptor:

AddGraphQLServer().AddType<FSharpUnionAsEnumDescriptor<MyUnion>>()

It will give this schema:

enum MyUnion {
  A
  B
  custom_name
}

Customizing enums

You can inherit from FSharpUnionAsEnumDescriptor to customize the type as usual. Remember to call base.Configure in your override.

type MyEnumDescriptor() =
    inherit FSharpUnionAsUnionDescriptor<MyUnion>()

    override this.Configure(descriptor: IEnumTypeDescriptor<MyUnion>) =
        base.Configure(descriptor)
        descriptor.Name("CustomName") |> ignore

Then, use your subtype in AddType:

AddType<MyEnumDescriptor>()

F# unions as GraphQL unions

You can define an F# union type to be used as a union in the GraphQL schema:

type MyUnion =
    | A of MyTypeA
    | B of MyTypeB

(The case names are not used.)

Add the type to GraphQL using FSharpUnionAsUnionDescriptor:

AddGraphQLServer().AddType<FSharpUnionAsUnionDescriptor<MyUnion>>()

You can then return MyUnion directly through fields:

type Query() =

    member _.MyUnion : MyUnion = ...

It will give this schema:

type MyTypeA { ... }
type MyTypeB { ... }
type Query { myUnion: MyUnion! }
union MyUnion = MyTypeA | MyTypeB

Customizing unions

You can inherit from FSharpUnionAsUnionDescriptor to customize the type as usual. Remember to call base.Configure in your override.

type MyUnionDescriptor() =
    inherit FSharpUnionAsUnionDescriptor<MyUnion>()

    override this.Configure(descriptor) =
        base.Configure(descriptor)
        descriptor.Name("CustomName") |> ignore

Then, use your subtype in AddType:

AddType<MyUnionDescriptor>()

Overriding the union case types

If the default inferred types for the union cases are not correct (for example, if you have multiple GraphQL schema types for the same runtime type), you can use GraphQLTypeAttribute on individual cases to specify the correct type:

type MyUnion =
    | [<GraphQLType(typeof<MyTypeA2Descriptor>)>] A of MyTypeA
    | B of B

Acknowledgements

Many thanks to @Stock44 for creating this gist that sparked this project. Without that, I'd have no idea where to even begin.

Deployment checklist

For maintainers.

• Make necessary changes to the code
• Update the changelog
• Update the version and release notes in the fsproj files
• Commit and tag the commit in the format v/x.y.z (this is what triggers deployment)
• Push the changes and the tag to the repo. If the build succeeds, the package is automatically published to NuGet.