Implementing pattern synonym constructor signatures

David Feuer david at
Tue Jan 16 00:04:20 UTC 2018

Never mind about parsing. It looks like the parser is already doing what it needs to do and I need to look to RdrHsSyn.hs.

David FeuerWell-Typed, LLP
-------- Original message --------From: David Feuer <david at> Date: 1/15/18  2:10 PM  (GMT-05:00) To: Simon Peyton Jones <simonpj at> Cc: ghc-devs at Subject: Implementing pattern synonym constructor signatures 
Over the past week I've started digging into the code that implements pattern synonyms with an eye toward implementing the pattern synonym construction function signature proposal. I think I understand a decent amount of what's going on most places. However, I don't understand enough about type checking to have any idea about what needs to change where or how. There are several things that need to be addressed:
0. Parsing. I wasn't actually able to find the code that parses pattern synonyms. Can someone point me in the right direction?
1. When there is a constructor signature, it needs to be used for the construction function instead of the pattern signature. Can someone give point me in the right direction about how to do this?
2. When there is a constructor signature but no pattern signature, what should we do? I think "give up" sounds okay for now.
3. A pattern synonym without a constructor signature needs to be treated as it is today, so the machinery for matching things up needs to remain available.
David FeuerWell-Typed, LLP
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