Commit bbb8cb92 authored by Ben Gamari's avatar Ben Gamari Committed by Ben Gamari

users-guide: Mention changes necessary due to #13391

Some variant of this should also be added to the migration guide.

[skip ci]

Test Plan: Read it

Reviewers: goldfire, austin

Reviewed By: goldfire

Subscribers: rwbarton, thomie

GHC Trac Issues: #13391

Differential Revision:
parent 022455ff
......@@ -59,6 +59,24 @@ Language
data StrictJust a <- Just !a where
StrictJust !a = Just a
- GADTs with kind-polymorphic type arguments now require :ghc-flag:`TypeInType`.
For instance, consider the following, ::
data G :: k -> * where
GInt :: G Int
GMaybe :: G Maybe
In previous releases this would compile with :ghc-flag:`PolyKinds` alone due
to bug :ghc-ticket:`13391`. As of GHC 8.4, however, this requires
:ghc-flag:`TypeInType`. Note that since GADT kind signatures aren't generalized,
this will also require that you provide a :ref:`CUSK
<complete-kind-signatures>` by explicitly quantifying over the kind argument,
``k``, ::
data G :: forall k. k -> * where
GInt :: G Int
GMaybe :: G Maybe
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