[GHC] #9747: Odd failure to deduce a constraint
GHC
ghc-devs at haskell.org
Fri Oct 31 00:36:24 UTC 2014
#9747: Odd failure to deduce a constraint
-------------------------------------+-------------------------------------
Reporter: acowley | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.8.3
Keywords: TypeFamilies | Operating System:
ConstraintKinds | Unknown/Multiple
Architecture: Unknown/Multiple | Type of failure:
Difficulty: Unknown | None/Unknown
Blocked By: | Test Case:
Related Tickets: | Blocking:
| Differential Revisions:
-------------------------------------+-------------------------------------
Using a type family to build a compound constraint that includes several
constraints produced by applying another type family results in a type
checker error like,
{{{
Could not deduce (ConFun l a, ConFun l b)
arising from a use of ‘bar’
from the context (ConFun l a, ConFun l b)
bound by the type signature for
baz :: (ConFun l a, ConFun l b) => Proxy l -> HList '[a,
b] -> ()
}}}
An example program is given at,
https://gist.github.com/acowley/a001bd092599b5914bad
--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9747>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
More information about the ghc-tickets
mailing list