Type families causing the compiler to hang on recompilation

21 Jun 2013

      Hello,

I am having a problem with (re)compiling some code I have. I have two
modules A and B. In A I have some classes and instances and B uses this.
When I try to compile B (with cabal or ghc --make) the first time
everything works. When I now modify B (add a space) B is recompiled but the
compiler hangs and doesn't seems to be doing anything.

I have tested the problem with GHC (x86_64) 7.6.2. 7.6.3 and HEAD.

It seems to be very similar to a problem I had earlier:
http://hackage.haskell.org/trac/ghc/ticket/7321
but this time there are no GADTs involved.

The code of module A (clutter that doesn't contribute to the problem has
been removed):
...
{-# LANGUAGE FlexibleInstances, UndecidableInstances     #-}
{-# LANGUAGE GADTs                 #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE TypeFamilies, TypeOperators, ScopedTypeVariables #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE DataKinds, PolyKinds #-}
module A where
import GHC.Generics
class QA a where
  type QRep a
 type QRep a = QRep (GRep (Rep a))
instance QA () where
  type QRep () = ()
-- Kind-polymorphic proxies;
data Pr (a :: k) = Pr
class (QA (GRep f)) => CaseOf (f :: * -> *) where
    type Alg f r k :: *
    type GRep f :: *
-- Only used for the product structure
class QA (ProdRep f) => CaseOfProd (f :: * -> *) where
  type ProdAlg f r :: *
  type ProdRep f :: *
The code of module B:
...
module B where
import qualified A
Given that the code type checks (and if I do not recompile and make an
executable directly it actually works) I think this is a bug that might be
similar to the bug mentioned in ticket 7321.

Does anybody recognise the problem? Should I create a ticket?

Cheers,

Jeroen Weijers