forall in constraint

Simon Peyton Jones simonpj at
Mon Oct 23 21:04:36 UTC 2017

I’m lost. Could you give me a bit more context?

I’m deeply suspicious about that ForallXImplicitBndrs thing with strange higher kinded parameters.   Smells all wrong to me.


From: ghc-devs [mailto:ghc-devs-bounces at] On Behalf Of Alan & Kim Zimmerman
Sent: 23 October 2017 16:07
To: ghc-devs at
Subject: forall in constraint

I am working on the Trees that Grow stuff, and hit a small problem
I have

type family XIB               x thing
type family XNewImplicitBndrs x thing

type ForallXImplicitBndrs (c :: * -> Constraint) (x :: *) (thing :: *) =
       ( c (XIB               x thing)
       , c (XNewImplicitBndrs x thing)
and I want to extend the DataId constraint

type DataId p =
  ( Data p

  , ForallXImplicitBndrs Data p thing
But the problem is I do not have `thing` at this point, and to get it in the code will involve some hs-boot nastiness.
Is there any way to require "forall thing. Data thing" inside the DataId constraint?
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