[GHC] #5642: Deriving Generic of a big type takes a long time and lots of space

[GHC]

#5642: Deriving Generic of a big type takes a long time and lots of space
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    Reporter:  basvandijk        |       Owner:  dreixel                     
        Type:  bug               |      Status:  new                         
    Priority:  normal            |   Component:  Compiler                    
     Version:  7.2.1             |    Keywords:                              
    Testcase:                    |   Blockedby:                              
          Os:  Unknown/Multiple  |    Blocking:                              
Architecture:  Unknown/Multiple  |     Failure:  Compile-time performance bug
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Comment(by basvandijk):

 Replying to [comment:2 clintm]:
 > Came to report this sort of thing too.  I can't actually get this code
 to completely run as my machine runs out of memory (12g) almost
 immediately.
 > ...

 This doesn't have anything to do with generics. You just forgot to give a
 definition for either `show` or `showsPrec`. Since these methods have
 default implementations which are defined in terms of each other you get
 an infinite loop which apparently overflows the stack.

 Also note that the `Show` type class does not use `DefaultSignatures` and
 so does not have default generic implementations.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5642#comment:5>
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