patch applied (ghc): Fix Trac #2334: validity checking for type families

[Simon Peyton Jones]

Fri Jun  6 05:17:30 PDT 2008  simonpj at microsoft.com
  * Fix Trac #2334: validity checking for type families
  
  When we deal with a family-instance declaration (TcTyClsDecls.tcFamInstDecl)
  we must check the TyCon for validity; for example, that a newtype has exactly
  one field.  That is done all-at-once for normal declarations, and had been
  forgotten altogether for families.
  
  I also refactored the interface to tcFamInstDecl1 slightly.
  
  
  A slightly separate matter: if there's an error in family instances
  (e.g. overlap) we get a confusing error message cascade if we attempt to
  deal with 'deriving' clauses too; this patch bales out earlier in that case.
  
  
  Another slightly separate matter: standalone deriving for family 
  instances can legitimately have more specific types, just like normal
  data decls. For example
     
     data instance F [a] = ...
     deriving instance (Eq a, Eq b) => Eq (F [(a,b)])
  
  So tcLookupFamInstExact can a bit more forgiving than it was.
   
  

    M ./compiler/typecheck/TcDeriv.lhs -22 +30
    M ./compiler/typecheck/TcInstDcls.lhs +3
    M ./compiler/typecheck/TcTyClsDecls.lhs -11 +9

View patch online:
http://darcs.haskell.org/ghc/_darcs/patches/20080606121730-1287e-fdedb9cd7550de00aba8d5b04f755523accafe68.gz