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Github WIP: Prototyping Erased Union types by Swoorup · Pull Request #10566 · do...

 3 years ago
source link: https://github.com/dotnet/fsharp/pull/10566
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Contributor

Swoorup commented on Nov 29, 2020 •

edited by dsyme

RFC https://github.com/fsharp/fslang-design/blob/master/RFCs/FS-1092-anonymous-type-tagged-unions.md

I am currently experimenting hacking the compiler to support union types. I'll admit I don't have much knowledge of the F# compiler and this is a merely an exploration to see if the features would fit well within F#.

What currently works:

  • Type Declaration (Commutativity and Associativity)
    type A = (int | (int64 | int16) is same as ((int | int64) | int16)
    type C = (int | string) same as (string | int)
  • Subtyping rules:
type I = interface end
type IA = interface end
type IB = interface end

type AA = AA of int interface IA
type BB = BB of int interface IB
let aa = AA(10)
let aabb: (AA|BB) = aa
aabb :> (IA|IB) // should pass
aabb :> (IA|IB|string) // should pass
aabb :> I // should fail
aabb :> IA // should fail
aabb :> (IA|int) // should fail

What doesn't:

  • Some issues appear to exist when interacting with anonymous records.
  • No exhaustive match checks, no union types within match cases.
  • Would be nice not to have :> _ every time to convert to union type.
  • Naked Generics, following doesn't work (Should likely be disallowed) 
     let formUnion<'a, 'b when 'a :> ('a |'b) and 'b :> ('a | 'b)> (a: 'a) (b: 'b) = a :> ('a| 'b)
 let formUnion2<'a, 'b> (a: 'a) (b: 'b) = a :> ('a| 'b) // yields constraint issue

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