patch applied (ghc): Make the type-defaulting in GHCi use () as the first default type

[Simon Peyton Jones]

Thu Mar 15 07:28:12 PDT 2007  simonpj at microsoft.com
  * Make the type-defaulting in GHCi use () as the first default type
  
  See Trac #1200
  
  This is a somewhat experimental fix.  I'm not sure we want it in 6.6.1
  
  The idea is explained in Note [Default unitTy] in TcSimplify.  In
  interative mode (or with -fextended-default-rules) we add () as the
  first type we try when defaulting.  This has very little real impact,
  except in the following case.  Consider:
  	Text.Printf.printf "hello"
  This has type (forall a. IO a); it prints "hello", and returns
  'undefined'.  We don't want the GHCi repl loop to try to print that
  'undefined'.  The neatest thing is to default the 'a' to (), rather
  than to Integer (which is what would otherwise happen; and then GHCi
  doesn't attempt to print the ().  So in interactive mode, we add () to
  the list of defaulting types.  
  
  

    M ./compiler/typecheck/TcDefaults.lhs -1 +1
    M ./compiler/typecheck/TcRnMonad.lhs -2 +2
    M ./compiler/typecheck/TcSimplify.lhs -11 +36