Show and function types in 4.03

[Conal Elliott]

Did something change in 4.03 w.r.t. Show and functions?  Mag stopped
compiling:

                ghc-4.03 -c -O2 -fvia-C -O2-for-C -H20M UU_Pretty.hs

                UU_Pretty.hs:9:
                    No instance for `Show (Int -> String -> String)'
                    When deriving classes for `Format'

Here's the code:

                data Format  = Elem { height  :: Int
                                    , last_w  :: Int
                                    , total_w :: Int
                                    , txtstr  :: Int -> String -> String
                                    }
                            deriving Show

- Conal