What's the constraint kinds syntax for GHC 7.4.1?

Question

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.

Hamachi

Answer 1

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.

Nausea

Is there an official place to import it from instead of GHC.Prim. One that might allow it to work on other compilers that support those extensions. – Geanticline