Instance Show for function

5
votes

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 function for show looks like that:

  showOp :: (Int -> Int -> Int) -> String
  showOp op
    | op 3 3 == 6 = "plus"
    | op 3 3 == 0 = "minus"
    | op 3 3 == 9 = "times"
    | op 3 3 == 1 = "divide"
    | otherwise = "undefined"

But I can't get an instance of Show for (Int -> Int -> Int). I tried it like that:

    instance Show (Int -> Int -> Int) where
    show op = show "asdf"

But it doesn't work. WinHugs just returns the Error 

    Syntax error in instance head (variable expected)

Is it even possible to define Show for functions? If it is, how could I tackle that problem?

3
haskell

3 Answers

6
votes

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 #-}

3
votes

(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"
1
votes

You can also use Hugs.

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