[Haskell-cafe] Defining show for a function type.

Donald Bruce Stewart dons at cse.unsw.edu.au
Mon Jul 10 22:01:26 EDT 2006

> I am a haskell-beginner and I wish to write a Forth-like interpreter. 
> (Only for practice, no usefulness.)
> I would like use a list (as stack) that can contain several kinds of values.
> data Element = Int Int | Float Float | Func : Machine -> Machine  | ...
> Now I would like to have this type be an instance of the class Show, so 
> that I can see what the stack contains in ghci.

Here's an interesting, I think, show for functions that we use in
lambdabot's Haskell interpreter environment:

    module ShowQ where

    import Language.Haskell.TH
    import System.IO.Unsafe
    import Data.Dynamic

    instance (Typeable a, Typeable b) => Show (a -> b) where
        show e = '<' : (show . typeOf) e ++ ">"

    instance Ppr a => Show (Q a) where
        show e = unsafePerformIO $ runQ e >>= return . pprint

which generates results like:

     dons:: > toUpper
     lambdabot::  <Char -> Char>

     dons:: > \x -> x+1::Int
     lambdabot::  <Int -> Int>

     dons:: > map
     lambdabot::  Add a type signature

Note that also the standard libraries come with Text.Show.Functions

-- Don

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