How to define an operator to show a sequence of polymorphic things

[Malcolm Wallace]

Bernd Holzmüller <holzmueller@ics-ag.de> writes:

> I'd like to have a better way for "showing" a list of things, e.g.
> 
> show (f1 t) ++ show (f2 t) ++ show (f3 t) ++ ....
> 
> I would love to just write: (f1 # f2 # f3) t but I do not see how to 
> define an operator like # in Haskell without applying show over again on 
> the first arguments (i.e. f1 and f2). Because polymorphic lists are not 
> possible, I can not define # as to write [f1,f2,f3] # t either.

How about the following?

    infixr #
    (#) :: (Show b) => (a->b) -> (a->String) -> (a->String)
    f # g = \t-> show (f t) ++ g t

    newline, empty :: a -> String
    newline _ = "\n"
    empty   _ = ""

which you could use like

    (f1 # f2 # f3 # newline) t

Regards,
    Malcolm