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

21 Jun 2013

      #8002: Type family causing GHC to hang on recompilation
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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
GHC http://www.haskell.org/ghc/
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