Allow comma and parentheses for tuple type signatures

Allow comma and parentheses for tuple type signatures

I propose we allow using a comma and parentheses in declaring tuples in type signatures. A type signature 't * 'u could be rewritten as ('t, 'u). For example:

type Demo =
    abstract member f : int * string            // current tuple declaration
    abstract member g : (int, string)           // would be equivalent to `g` with this suggestion

    abstract member h : seq<int * string>       // current tuple instantiation of type arg
    abstract member i : seq<int, string>        // should still fail - seq only takes one type arg
    abstract member j : seq<(int, string)>      // would be equivalent to `i` with this suggestion
    abstract member k : Map<int, string>        // should continue to work as before - no parens, not a tuple
    abstract member l : Map<int, int * string>  // current tuple instantiation of type arg
    abstract member l : Map<int, (int, string)> // 'TKey = int, 'TValue = int * string aka (int, string)

The existing way of approaching this problem in F# is using the existing 't * 'u syntax.

Pros and Cons

The advantage of making this adjustment to F# is bringing the type signature syntax inline with the implementation syntax. Defining a tuple as 't * 'u but implementing it as x, y is a common stumbling block for first time F#ers.

The disadvantages of making this adjustment to F# are:

• Having two ways to say the same thing
• Moving further from ML style
• Breaking change? I don't think that it is a breaking changes as type signature with this syntax does not compile currently, but I might be missing a corner case in my assumption on that.

Extra information

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

Related suggestions: (put links to related suggestions here)

Affidavit (please submit!)

Please tick this by placing a cross in the box:

• This is not a question (e.g. like one you might ask on stackoverflow) and I have searched stackoverflow for discussions of this issue
• I have searched both open and closed suggestions on this site and believe this is not a duplicate
• 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.

Please tick all that apply:

• This is not a breaking change to the F# language design
  See note in Disadvantages
• I or my company would be willing to help implement and/or test this

For Readers

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Start with the proposed tuple-signature examples in this issue, especially the seq and Map cases, and check whether the syntax distinctions are fully specified. Done would mean an agreed language-design decision and an implementation and test plan for accepting parenthesized comma syntax without changing existing generic type-argument behavior.