Allow type declaration with `sig` on code/script files

[noralambda]

I propose we ... (describe your suggestion here)

sig could be used for type declaration in script and code files.

Examples:

sig foo : int -> float -> string in
let foo a b = 
    //...

sig x : Person in
let x = {Name=10; Age=5}

type Foo() = 
    sig member A : () -> 'X in
    member _.A () = // ...

    sig member B : 'X in
    member val B = // ...

in should be optional on Recursive Modules.

I feel this syntax for class member signature is weird. Help?

Multiple declaration should be considered error:

sig bar : float -> float -> int in
let bar a (b:float) = // Error
    // ...

Pros and Cons

The advantages of making this adjustment to F# are

Code becomes easier to read and reason about

Extra information

Estimated cost (XS, S, M, L, XL, XXL):

Related suggestions: (put links to related suggestions here)

341

Affidavit (please submit!)

Please tick these items by placing a cross in the box:

• [x] This is not a question (e.g. like one you might ask on StackOverflow) and I have searched StackOverflow for discussions of this issue
• [x] This is a language change and not purely a tooling change (e.g. compiler bug, editor support, warning/error messages, new warning, non-breaking optimisation) belonging to the compiler and tooling repository
• [x] This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it
• [x] I have searched both open and closed suggestions on this site and believe this is not a duplicate

Please tick all that apply:

• [x] This is not a breaking change to the F# language design
• [ ] I or my company would be willing to help implement and/or test this

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[T-Gro]

Duplicate of https://github.com/fsharp/fslang-suggestions/issues/341 , which was closed as "probably not".

[T-Gro]

Closing comment: https://github.com/fsharp/fslang-suggestions/issues/341#issuecomment-1506752584

[vzarytovskii]

Yeah, I think it's essentially the same. Also, personally don't like adding another keyword (which is quite unfamiliar), val would've been better (which was also rejected). I am going to close this one.

@dsyme please reopen if your opinion on this changed.