Combinators over Records and Unions

In the previous post, I discussed designing combinator libraries that compose some property over unions. It is only fitting to throw records in the mix.

type U =
    | A of int
    | B of float
    | C of string

let UFormat =
    (
        UnionCase A IntFormat <<
        UnionCase B FloatFormat <<
        UnionCase C StringFormat
    )
    |> Union (fun a b c x ->
        match x with
        | A x -> a x
        | B x -> b x
        | C x -> c x)

type R =
    {
        A : int
        B : float
        C : string
    }

let RFormat : Format<R> =
    (
        RecordField (fun r -> r.A) IntFormat <<
        RecordField (fun r -> r.B) FloatFormat <<
        RecordField (fun r -> r.C) StringFormat
    )
    |> Record (fun a b c -> { A = a; B = b; C = c })

With some simplifications, here is the code:

	open System.IO
	type Format<'T> =
	{
	Read : BinaryReader -> 'T
	Write : BinaryWriter -> 'T -> unit
	}
	(* Unions *)
	type UnionPart<'X,'U> =
	{
	R : list<BinaryReader -> 'U>
	W : 'X -> 'U -> BinaryWriter -> unit
	}
	let UnionCase (ctor: 'C -> 'U) (fmt: Format<'C>)
	(part: UnionPart<'X,'U>) : UnionPart<('C -> unit) -> 'X,'U> =
	let tag = char part.R.Length
	let write (w: BinaryWriter) x =
	w.Write(tag)
	fmt.Write w x
	{
	R = (fmt.Read >> ctor) :: part.R
	W = fun f u w -> part.W (f (write w)) u w
	}
	let private unionEnd<'U> : UnionPart<('U -> unit),'U> =
	{
	R = []
	W = fun f u w -> f u
	}
	let Union proof f =
	let part = f unionEnd
	let rs = Array.rev (List.toArray part.R)
	{
	Read = fun r ->
	let tag = int (r.ReadChar())
	rs.[tag] r
	Write = fun w x -> part.W proof x w
	}
	(* Records *)
	type RecordPart<'X,'R> =
	{
	R : 'X -> BinaryReader -> 'R
	W : list<BinaryWriter -> 'R -> unit>
	}
	let RecordField<'F,'U,'X> (proj: 'U -> 'F) (fmt: Format<'F>) (part: RecordPart<'X,'U>) : RecordPart<'F -> 'X, 'U> =
	{
	R = fun x r -> part.R (x (fmt.Read(r))) r
	W = (fun w x -> fmt.Write w (proj x)) :: part.W
	}
	let private recordEnd<'T> : RecordPart<'T,'T> =
	{
	R = fun r _ -> r
	W = []
	}
	let Record proof f =
	let x = f recordEnd
	let ws = List.toArray x.W
	{
	Read = x.R proof
	Write = fun w x -> for f in ws do f w x
	}
	(* Scalars *)
	let IntFormat : Format<int> =
	{
	Read = fun r -> r.ReadInt32()
	Write = fun w x -> w.Write(x)
	}
	let FloatFormat : Format<float> =
	{
	Read = fun r -> r.ReadDouble()
	Write = fun w x -> w.Write(x)
	}
	let StringFormat : Format<string> =
	{
	Read = fun r -> r.ReadString()
	Write = fun w x -> w.Write(x)
	}

view raw RecordUnionSerializers.fs hosted with ❤ by GitHub