Why is it impossible to declare a type alias to an F# struct tuple?

Question

It is impossible to define a type alias to a struct tuple in F#. Only with a workaround, it works.

let x = struct (1, 2)
> val x : struct (int * int) = struct (1, 2)

let y : struct (int * int) = struct (4, 5) // explicit type
> val y : struct (int * int) = struct (4, 5)

type S = struct (int * int) // straight definition
> error FS0010: Unexpected symbol '(' in member definition

type S = ValueTuple<int, int> // workaround
> [<Struct>]
  type S = struct (int * int)

Is the error for "type S = struct (int * int)" a compiler bug?

no this is a language feature. you can however try `type S = int * int` then `let s = S(1,2)`. — marmeladze, Mar 06 '21 at 12:13
I would say `ValueTuple` is the straight definition but the friendlier name `struct (int * int)` should work. Best to file in dotnet/fsharp . — Charles Roddie, Mar 06 '21 at 12:46

score 5 · Accepted Answer · answered Mar 06 '21 at 15:01

5

Tried to file a bug and found the answer here: https://github.com/dotnet/fsharp/issues/7014

parenthesis around the type are required:

type Alias = (struct(int * int))

answered Mar 06 '21 at 15:01

citykid

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Why is it impossible to declare a type alias to an F# struct tuple?

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