[ghc-steering-committee] #475: New tuple and list syntax, rec: accept

[Vladislav Zavialov (int-index)]

Dear Committee,

Richard has proposed #475 "New tuple and list syntax”. Read it here:

https://github.com/goldfirere/ghc-proposals/blob/tuple-syntax/proposals/0000-tuple-syntax.rst

Here’s some background:
Earlier we accepted #281 "Visible 'forall' in types of terms”, which introduced, among other things, the -X(No)ListTupleTypeSyntax extension. During implementation, I discovered that this part of the proposal required further design considerations. Richard has kindly agreed to take a stab at this problem, and #475 is the result.

Short summary of the proposal:
* Introduce Tuple2, Tuple3, Tuple4, and so on, as alternative ways to write the types of tuples.
* Introduce List as the alternative way to write the type of a list.
* Do the same for unboxed tuples, unboxed sums, and constraint tuples.

This is the core part of the proposal, for which I strongly urge acceptance.

There are also other minor additions:
* Rename -XListTupleTypeSyntax to -XListTuplePuns.
* Introduce Tuple [a, b, c] as a more convenient way of writing Tuple3 a b c (likewise for n/=3)
* Introduce Constraints [a, b, c] as a more convenient way of writing CTuple3 a b c (likewise for n/=3)
* Introduce TupleN a b c as another more convenient way of writing Tuple3 a b c (likewise for n/=3)
* Introduce CTupleN a b c as another more convenient way of writing CTuple3 a b c (likewise for n/=3)

I foresee that if we don’t include Tuple/Constraints, users will end up defining their own, with different libraries exporting conflicting definitions. TupleN/ConstraintN, on the other hand, seem weakly motivated.

Personally I’d drop TupleN/ConstraintN and accept the rest of the proposal, but I’d love to hear more opinions. Let me know what you think!

- Vlad