Re: [Haskell-cafe] type/class question: toString

If you're willing to have an extra Typeable constraint, this does what you want:

import Data.Typeable (Typeable, cast) import Data.Maybe (fromMaybe)

toString :: (Show a, Typeable a) => a -> String toString x = fromMaybe (show x) (cast x)

*Main> toString "blah" "blah" *Main> toString 1 "1" *Main> toString (Just 0.5) "Just 0.5" So Strings are just cast into the result. Non-strings become Nothing, which fromMaybe turns into (show x). -- Nicholas Messenger nmessenger@gmail.com On Nov 6, 2007 4:23 PM, Graham Fawcett wrote:

On Nov 6, 2007 3:29 PM, Graham Fawcett wrote:

...

On Nov 6, 2007 2:21 PM, Jeff Polakow wrote:

...

Have you tried using -fglasgow-exts? That should enable all ghc extensions.

If anyone's interested, I had best results when I added the flag -fallow-incoherent-instances. Without it, I could not handle numbers without declaring their types, e.g. 'toString (33 :: Int)' would work, but 'toString 33' would lead to:

Ambiguous type variable `t' in the constraints: `ToString t' arising from use of `toString' at /home/graham/tmp/ToString.hs:13:15-25 `Num t' arising from the literal `33' at /home/graham/tmp/ToString.hs:13:24-25 Probable fix: add a type signature that fixes these type variable(s)

Here's the code I ended up with.

{-# OPTIONS -fglasgow-exts -fallow-overlapping-instances #-} {-# OPTIONS -fallow-incoherent-instances -fallow-undecidable-instances #-}

module ToString (ToString(..)) where

class Show a => ToString a where toString :: a -> String instance ToString String where toString s = s instance (Show a) => ToString a where toString s = show s

Thanks to all who responded; I learned a lot from this.

Graham

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