patch applied (ghc): Refactor the simplifier' s treatment of case expressions

[Simon Peyton Jones]

Fri Feb  9 09:29:38 PST 2007  simonpj at microsoft.com
  * Refactor the simplifier's treatment of case expressions
  
  (NB: this patch could conceivably require some bits of the 
  following SpecConstr patch to compile cleanly.  It's conceptually
  independent, but I'm not 100% certain that I've included all
  the necessary bits here.)
  
  This patch cleans up the simplifier's handling of various
  otimisations for case expressions, notably
    - case elimination (discarding the case altogether)
    - merging identical alternatives
    - discarding impossible alternative
    - merging nested cases
  
  Previously this was partly handled before, and partly after,
  simplifying the case alternatives. The trouble with that is
  that the dead-ness information on the case binders gets munged
  during simplification, and that turned out to mean that 
  case elmination essentially never happened -- stupid.
  
  Now I've moved it all to before simplifying the alterntives.
  In fact this reduces the amount of code, I think, and it's
  certainly tidier.  I don't think there is any loss.
  
  

    M ./compiler/simplCore/SimplUtils.lhs -213 +135
    M ./compiler/simplCore/Simplify.lhs -101 +162