On Nov 6, 2007 10:30 AM, Bayley, Alistair wrote:

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From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Graham Fawcett Is there a way to declare a 'toString' function, such that toString x | <x is a String> = x toString x | = show x I'm assuming you're not fond of the way the print function handles Strings?

More a curiosity about the flexibility of the type system -- but yes, I could see cases where such a thing could be useful.

With GHC you can do this:

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{-# OPTIONS -fallow-overlapping-instances #-} {-# OPTIONS -fallow-undecidable-instances #-}

class Show a => MyShow a where show_ :: a -> String instance MyShow String where show_ s = s instance (Show a) => MyShow a where show_ s = show s

This doesn't want to compile for me: $ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.6.1 $ ghc ToString.hs # your code, verbatim ToString.hs:5:0: Illegal instance declaration for `MyShow String' (The instance type must be of form (T a b c) where T is not a synonym, and a,b,c are distinct type variables) In the instance declaration for `MyShow String' ToString.hs:6:0: Illegal instance declaration for `MyShow a' (The instance type must be of form (T a b c) where T is not a synonym, and a,b,c are distinct type variables) In the instance declaration for `MyShow a' I'll read up on those two GHC options, and try to figure it out myself (but hints are welcome). Thanks, Graham

On Nov 6, 2007 12:15 PM, David Benbennick wrote:

In ghc 6.8.1, the error messages are more helpful:

foo.hs:5:0: Illegal instance declaration for `MyShow String' (All instance types must be of the form (T t1 ... tn) where T is not a synonym. Use -XTypeSynonymInstances if you want to disable this.) In the instance declaration for `MyShow String'

Thanks for the tip. I might give 6.8.1 a try; I still cannot get it to work in 6.6.1. The problem may exist between the keyboard and the chair. G

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

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

Ah thanks, that does it. G

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:

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

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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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I'm assuming you're not fond of the way the print function handles Strings?

With GHC you can do this:

...

{-# OPTIONS -fallow-overlapping-instances #-} {-# OPTIONS -fallow-undecidable-instances #-}

class Show a => MyShow a where show_ :: a -> String instance MyShow String where show_ s = s instance (Show a) => MyShow a where show_ s = show s

I'm curious why this works. How does GHC know to pick the MyShow String instance instead of the one coming from Show String? I expect there's no way to do this without undecidable instances, is there? I try to stay away from that flag nowadays, since I've seen some strange unpredictable behavior from it in the past (the unpredictability of the behavior may come from the fact that I don't know how the inference algorithm works). Luke

On Nov 6, 2007 12:03 PM, Thomas Schilling wrote:

On Tue, 2007-11-06 at 09:18 -0500, Graham Fawcett wrote:

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Hi folks, Is there a way to declare a 'toString' function, such that toString x | <x is a String> = x toString x | = show x I think the simpler solution (for your particular problem) is to tag strings that should be printed as-is:

newtype Literal = Literal String instance Show Literal where show (Literal x) = x lit :: String -> Literal lit = Literal

I almost replied to ask, "doesn't that solve a different problem?" But I see that in practice, it leads to a similar result, and without type-system trickery. The type-system trickery is still devilishly interesting, though. ;-) Thanks, G