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:

Combinators over Discrimated Unions in ML

Discriminated unions or sum types are a natural way to model logical OR. Often you have a property that distributes over OR. Say, in F# (used throughout the article, though the ideas should apply equally well to any ML), you can write a combinator of the type:

P<'T1> → P<'T2> → P<Choice<'T1,'T2>>

How to go from here to a nice set of combinators that would handle an arbitrary union? This question has been on my mind for a while, and finally I have an acceptable solution.

As a disclaimer, at the level of theory the question is completely trivial. The interest is in how to find a user-friendly ML interface. Even this, I suspect, has been solved before, and at a much more general level. I am aware for instance of Vesa Karvonen's Generics for the working ML'er, and a more recent Oleg Kiselyov's presentation of generics in OCaml. I am looking here at a much simpler setting, hopefully taking it to a more accessible, for-dummies-like-myself level.

Suppose you are designing binary format combinators to let users of your library construct values of the form:

type Format<'T> =
    {
        Read : BinaryReader → 'T
        Write : BinaryWriter → 'T → unit
    }

Given an arbitrary union type and some primitive Format values, the user should be able to compose them. This involves projecting from sub-types to the parent union type, and matching backwards. After some experimentation, the design I have looks like this:

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

let UFormat =
    Union (fun a b c x →
        match x with
        | A x → a x
        | B x → b x
        | C x → c x)
    << Case A IntFormat
    << Case B FloatFormat
    << Case C StringFormat
    <| End

That's pretty much it. The magic is in the type of Case combinator that accumulates types so that the Union combinator can present N-way pattern matching as a single reasonably convenient to write function. Full code: