[ghc-steering-committee] Proposal: Or patterns #43 (Round 2)

[Manuel M T Chakravarty]

We now have a revised version of the ”Or pattern” proposal #43 to consider:

 (formatted) https://github.com/osa1/ghc-proposals/blob/or_patterns/proposals/0000-or-patterns.rst
 (PR thread) https://github.com/ghc-proposals/ghc-proposals/pull/43

You may remember that we discuss this a while back

  https://mail.haskell.org/pipermail/ghc-steering-committee/2017-November/000231.html

and finally decided to bounce it back to the authors with the following recommendation:

  https://github.com/ghc-proposals/ghc-proposals/pull/43#issuecomment-358189327

We previously decided that we are inclined to accept this proposal if the authors change the points that we highlighted. Judging from the conversation of SimonPJ and Richard with the author on the GitHub thread, it appears to me that most of the issues have been addressed. However, Simon’s point

	• "Patterns that bind existentials, dictionaries, or equalities are rejected by the type checker". I disagree. Earlier I suggested "no variable bound by an or-pattern can have a type that mentions an existential variable bound by the or-pattern". That is much more friendly because you can still match against an existential constructor; you just can’t bind an existential variable.

from <https://github.com/ghc-proposals/ghc-proposals/pull/43#issuecomment-368933401 <https://github.com/ghc-proposals/ghc-proposals/pull/43#issuecomment-368933401>> still doesn’t seem to be addressed, or am I misunderstanding?

Overall, I propose to accept the proposal, possibly requiring that the above point be addressed.

What do you all think?

Cheers,
Manuel

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