[GHC] #8002: Type family causing GHC to hang on recompilation

[GHC]

#8002: Type family causing GHC to hang on recompilation
-----------------------------+----------------------------------------------
Reporter:  jweijers          |          Owner:                  
    Type:  bug               |         Status:  new             
Priority:  normal            |      Component:  Compiler        
 Version:  7.6.3             |       Keywords:                  
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
 Failure:  None/Unknown      |      Blockedby:                  
Blocking:                    |        Related:                  
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 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 (build
 on 20/06/2013).

 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.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/8002>
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