patch applied (ghc): Make rebindable do-notation behave as advertised

[Simon Peyton Jones]

Wed Nov 21 09:49:14 PST 2007  simonpj at microsoft.com
  * Make rebindable do-notation behave as advertised
  
  Adopt Trac #1537.  The patch ended up a bit bigger than I expected,
  so I suggest we do not merge this into the 6.8 branch.  But there
  is no funadamental reason why not.
  
  With this patch, rebindable do-notation really does type as if you
  had written the original (>>) and (>>=) operations in desguared form.
  
  I ended up refactoring some of the (rather complicated) error-context
  stuff in TcUnify, by pushing an InstOrigin into tcSubExp and its
  various calls. That means we could get rid of tcFunResTy, and the
  SubCtxt type.  This should improve error messages slightly
  in complicated situations, because we have an Origin to hand
  to instCall (in the (isSigmaTy actual_ty) case of tc_sub1).
  
  Thanks to Pepe for the first draft of the patch.
  

    M ./compiler/typecheck/TcBinds.lhs -2 +3
    M ./compiler/typecheck/TcExpr.lhs -11 +13
    M ./compiler/typecheck/TcMatches.lhs -25 +24
    M ./compiler/typecheck/TcPat.lhs -1 +3
    M ./compiler/typecheck/TcRnDriver.lhs -2 +1
    M ./compiler/typecheck/TcRnTypes.lhs -2 +10
    M ./compiler/typecheck/TcUnify.lhs -77 +50