patch applied (ghc): Add left-to-right impredicative instantiation

Wed Dec 13 08:29:15 PST 2006  [email protected]
  * Add left-to-right impredicative instantiation
  
  People keep complaining, with some justification, that
  
  	runST $ foo
  
  doesn't work.  So I've finally caved in.  The difficulty with the above
  is that we need to decide how to instantiate ($)'s type arguments based 
  on the first argument (runST), and then use that info to check the second
  argumnent.  There is a left-to-right flow of information.
  
  It's not hard to implement this, and it's clearly useful.  The main 
  change is in TcExpr.tcArgs, with some knock-on effects elsewhere.
  
  I was finally provoked into this by Trac #981, which turned out, after some
  head-scratching, to be another instance of the same problem.
  
  (There was some bug-fixing too; a type like ((?x::Int) => ...) is a polytype
  even though it has no leading for-alls, but the new TcUnify code was not 
  treating it right.)
  
  Test for this is tc222
  
  

    M ./compiler/typecheck/TcExpr.lhs -37 +60
    M ./compiler/typecheck/TcMType.lhs -2 +1
    M ./compiler/typecheck/TcUnify.lhs -38 +75