See the accepted proposal:  https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0005-bidir-constr-sigs.rst



David Feuer
Well-Typed, LLP

-------- Original message --------
From: Matthew Pickering <matthewtpickering@gmail.com>
Date: 1/15/18 3:27 PM (GMT-05:00)
To: David Feuer <david@well-typed.com>
Cc: Simon Peyton Jones <simonpj@microsoft.com>, GHC developers <ghc-devs@haskell.org>
Subject: Re: Implementing pattern synonym constructor signatures

What is a constructor signature? Where is this specified?

On Mon, Jan 15, 2018 at 7:10 PM, David Feuer <david@well-typed.com> wrote:
> 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 Feuer
> Well-Typed, LLP
>
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