[GHC] #16374: Cannot deduce constraint from itself with poly-kinded type family
GHC
ghc-devs at haskell.org
Fri Mar 1 13:59:30 UTC 2019
#16374: Cannot deduce constraint from itself with poly-kinded type family
-------------------------------------+-------------------------------------
Reporter: roland | Owner: (none)
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler (Type | Version: 8.6.3
checker) |
Resolution: | Keywords:
Operating System: Unknown/Multiple | Architecture:
| Unknown/Multiple
Type of failure: None/Unknown | Test Case:
Blocked By: | Blocking:
Related Tickets: | Differential Rev(s):
Wiki Page: |
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Comment (by RyanGlScott):
Enabling `-fprint-explicit-kinds` reveals in greater detail what is
happening here:
{{{
$ ~/Software/ghc-gitlab/inplace/bin/ghc-stage2 --interactive Bug.hs
-fprint-explicit-kinds
GHCi, version 8.9.20190227: https://www.haskell.org/ghc/ :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Eq ( Bug.hs, interpreted )
Bug.hs:7:11: error:
• Could not deduce: (F @{*} @k0 Int :: k0)
~ (F @{*} @k0 Bool :: k0)
from the context: (F @{*} @k Int :: k) ~ (F @{*} @k Bool :: k)
bound by the type signature for:
withEq :: forall k a.
((F @{*} @k Int :: k) ~ (F @{*} @k Bool ::
k)) =>
a
at Bug.hs:7:11-29
NB: ‘F’ is a non-injective type family
The type variable ‘k0’ is ambiguous
• In the ambiguity check for ‘withEq’
To defer the ambiguity check to use sites, enable
AllowAmbiguousTypes
In the type signature: withEq :: F Int ~ F Bool => a
|
7 | withEq :: F Int ~ F Bool => a
| ^^^^^^^^^^^^^^^^^^^
}}}
Here, we can see exactly where the ambiguous kind variable `k0` occurs.
It might be helpful to have GHC automatically enable `-fprint-explicit-
kinds` is an ambiguous type variable occurs in a kind position, such as in
this program. We already do this in a number of places, such as when
[https://gitlab.haskell.org/ghc/ghc/blob/2e8f664957dc3763dc4375894b8dc4d046d2e95b/compiler/typecheck/TcErrors.hs#L1931
a type mismatch occurs due to kind arguments].
--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16374#comment:1>
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