FSTan

Exactly a full-featured and practical implementation typeclasses and higher kinded types in F#.

For manuals check Guide.md, where you'll be told how to use these concise typeclasses, higher kinded types and constraints.

Motivation and Features

There are also other similar implementations in FSharp like Higher and FSharpPlus, but they're not able to provide all the features listed below, which motivate me to create a better one:

• Support instance resolution.
• Support ad-hoc polymorphism.
• Support to create a typeclass and add constraints to it.
• Support subtypeclassing.
• Support to directly access type constructor.
• Support default implementations for typeclass.
• All above operations are quite lightweighted and not implemented in a magic way.

Yes, exactly, it's a what many and I have dreamed about for so long.

Limitation

1. The performance might hurt in some scenarios, for each the datatype works with higher kinded types have to be upcasted to an unique abstract class, for instance, maybe<'a> has to be casted to hkt<Maybe, 'a>.

2. For some builtin datatypes cannot be interfaced with hkt, an extra wrapper class is required to work with higher kined types.

   For instance, interface type listData<'a> is required for the builtin List<'a>.

   You can use wrap and unwrap to transform datatypes from List<'a> to hkt<mkList<ListSig>,'a>, vice versa.

    module List' = List
    type List'<'a> = List<'a>
   
   
    type mkList<'L>() =
        inherit monad<mkList<'L>>()
            static member wrap<'a> (x : List'<'a>): hkt<mkList<'L>, 'a> =
                {wrap = x} :> _
            static member unwrap<'a> (x : hkt<mkList<'L>, 'a>): List'<'a> =
                (x :?> _).wrap
   
            default si.bind<'a, 'b> (m: hkt<mkList<'L>, 'a>) (k: 'a -> hkt<mkList<'L>, 'b>): hkt<mkList<'L>, 'b> =
                wrap <| List'.collect (unwrap << k) (unwrap m)
   
            default si.pure'<'a> (a: 'a): hkt<mkList<'L>, 'a> = wrap <| [a]
            interface show<mkList<'L>> with
                member si.show (x: hkt<mkList<'L>, 'a>) =
                    let x = unwrap x
                    x.ToString()
   
    and listData<'L, 'a> =
        {wrap : List'<'a>}
        interface hkt<mkList<'L>, 'a>
   
   
   // create a concrete List type
   type ListSig() =
    // default implements following type classes:
    // - monad (functor and applicative are implemented simultaneously)
    // - show
   
    inherit mkList<ListSig>()
   type list<'a> = hkt<mkList<ListSig>, 'a>
   
   let test() =
   
        let listm: _ list = Do {
            let! x = wrap [1, 2, 3]
            wrap [x]
        }
        // listm : resolved to be list<int>
   
        let f (x: int) : string = ""
        fmap f listm
        // return value is resolved to be list<string>

3. Cannot implement instance for datatypes that are not constructed by a type constructor. For instance, you cannot implement any typeclass for all primitives types like integers, floats and so on, unless you wrap them with an Identity type constructor.