Instance Show for function

Question

I\'m currently trying to write a little Show instance for primitive arithmetic functions.

Goal ist to make a list of functions showable.

The very simple fun

Answer 1

Don't use WinHugs. Use GHC.

In fact, in recent Haskell Platform versions there is already an instance of functions for Show.

Prelude Text.Show.Functions> show (+1)
"<function>"
Prelude Text.Show.Functions> show (\x -> x ++ "foo")
"<function>"

Now, in your case, however, you need -XFlexibleInstances on, since your instance isn't of the form (Constructor a1 .. an) where a1 .. an are distinct type variables.

Turn it on with {-# LANGUAGE FlexibleInstances #-}

Answer 2

You can also use Hugs.

Start Hugs with hugs -98 +o or runhugs -X-98 +o and use {-# LANGUAGE FlexibleInstances #-} in your source file.

Answer 3

(This isn't an answer (Don's covers it), but it is far too long for a comment)

The code has a lot of repeated logic (specifically op 3 3 == occurs a lot), but there is away to make this cleaner: case expressions. This allows us to compute op 3 3 once, and then handle the various cases (exactly the same as pattern matching in function definitions).

showOp op = case op 3 3 of
              6 -> "plus"
              0 -> "minus"
              9 -> "times"
              1 -> "divide"
              _ -> "undefined"