What's the constraint kinds syntax for GHC 7.4.1?
-
13-11-2019 - |
문제
I'm getting an error that Constraint
is not in scope, when I try to write a simple example,
{-# LANGUAGE UndecidableInstances,
MultiParamTypeClasses,
KindSignatures,
Rank2Types,
ConstraintKinds,
FlexibleInstances,
OverlappingInstances,
TypeFamilies #-}
type family A a :: Constraint
The error is,
> ghci test.hs
[1 of 1] Compiling Main ( test.hs, interpreted )
test.hs:10:20: Not in scope: type constructor or class `Constraint'
Failed, modules loaded: none.
Constraint synonyms seem to work as expected,
type ShowOrd a = (Ord a, Show a)
Thanks in advance.
해결책
Your error appears to be caused by Constraint
being, unsurprisingly, not in scope. You'll want to import it from GHC.Prim
or from GHC.Exts
.
It seems a bit odd that enabling ConstraintKinds
doesn't bring it into scope automatically, as it does in a 7.3.2 snapshot build I had lying around, but there you go.
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