Weird issue with ConstraintKinds
Consider the following (minimal, for illustration purposes) code: {-# LANGUAGE ConstraintKinds #-} module Weird where class A a where class B b where class C c where data X a = X a data Y a = Y a -- works fine, but can be verbose when things multiply instance (A a, B a) => C (X a) where -- So use ConstraintKinds: type D a = (A a, B a) instance D a => C (Y a) where ===== and now I get Variable `a' occurs more often than in the instance head in the constraint: D a (Use -XUndecidableInstances to permit this) In the instance declaration for `C (Y a)' Why?? Since D is an abbreviation, why would it behave differently than when I hand expand it? [This is with GHC 7.6.3] Jacques
This is a bug fixed in HEAD http://ghc.haskell.org/trac/ghc/ticket/8359.
On Sat, Oct 26, 2013 at 3:49 PM, Jacques Carette
Consider the following (minimal, for illustration purposes) code:
{-# LANGUAGE ConstraintKinds #-} module Weird where
class A a where class B b where class C c where
data X a = X a data Y a = Y a
-- works fine, but can be verbose when things multiply instance (A a, B a) => C (X a) where
-- So use ConstraintKinds: type D a = (A a, B a) instance D a => C (Y a) where
===== and now I get Variable `a' occurs more often than in the instance head in the constraint: D a (Use -XUndecidableInstances to permit this) In the instance declaration for `C (Y a)'
Why?? Since D is an abbreviation, why would it behave differently than when I hand expand it? [This is with GHC 7.6.3]
Jacques ______________________________**_________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/**mailman/listinfo/haskell-cafehttp://www.haskell.org/mailman/listinfo/haskell-cafe
Great. So I'll go ahead and use this, and remove UndecidableInstances when 7.8 ships. Jacques On 2013-10-26 10:10 , Dag Odenhall wrote:
This is a bug fixed in HEAD http://ghc.haskell.org/trac/ghc/ticket/8359.
participants (2)
-
Dag Odenhall -
Jacques Carette