The constraint solver can certainly simplify this to get b ~ y.   Perhaps all that is needed is to call it?

David: a small reproducer would be helpful.

Mathias, yell if you need help.

Simon

From: ghc-devs <ghc-devs-bounces@haskell.org> On Behalf Of David Feuer
Sent: 07 June 2019 18:37
To: GHC users <glasgow-haskell-users@haskell.org>; ghc-devs <ghc-devs@haskell.org>
Subject: Typed hole improvements

I've been playing around with 8.6.3, and I've really been appreciating the improvements in typed hole messages. Both the information about constraints and the suggestions for filling the holes have proven valuable in heavily typish programming. Thanks! One thing that's still not where I'd like it to be is the equality constraint display. I recognize this is a Hard Problem. But digging through something like

  x ~ C y z

  x ~ C y1 z2

  D p b ~ q z y1

to get b ~ y gets tiresome. Is there room for improvement here? If not, are there any programs around using the GHC API to offer an interactive system for such calculations? It'd be quite helpful when seeing such constraints to be able to express special interest in a particular type variable, for example.