Re: [Haskell-cafe] type/class question: toString
On Nov 7, 2007 4:34 PM, Nicholas Messenger [EMAIL PROTECTED] wrote: 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) Nice! Thank you for introducing me to the Typeable class (and by extension, Dynamic) -- I hadn't realized there was library support for dynamic types. G ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
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
[EMAIL PROTECTED]
On Nov 6, 2007 4:23 PM, Graham Fawcett [EMAIL PROTECTED] wrote:
On Nov 6, 2007 3:29 PM, Graham Fawcett [EMAIL PROTECTED] wrote:
On Nov 6, 2007 2:21 PM, Jeff Polakow [EMAIL PROTECTED] 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 Stringwhere 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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RE: [Haskell-cafe] type/class question: toString
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] 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 | x's type is an instance of Show = show x
Perhaps, in the type system, there's a way to declare a ToString
class, and somehow inherit all instances of Show as ToString
instances?
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
Alistair
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Re: [Haskell-cafe] type/class question: toString
On Nov 6, 2007 10:30 AM, Bayley, Alistair
[EMAIL PROTECTED] wrote:
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] 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 | x's type is an instance of Show = 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:
{-# 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
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Re: [Haskell-cafe] type/class question: toString
On 11/6/07, Graham Fawcett [EMAIL PROTECTED] wrote: 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' 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' foo.hs:6:0: Illegal instance declaration for `MyShow a' (All instance types must be of the form (T a1 ... an) where a1 ... an are distinct type *variables* Use -XFlexibleInstances if you want to disable this.) In the instance declaration for `MyShow a' When I run with -XTypeSynonymInstances -XFlexibleInstances it works. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] type/class question: toString
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
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Re: [Haskell-cafe] type/class question: toString
On Tue, 2007-11-06 at 09:18 -0500, Graham Fawcett wrote: Hi folks, Is there a way to declare a 'toString' function, such that toString x | x is a String = x toString x | x's type is an instance of Show = show x Perhaps, in the type system, there's a way to declare a ToString class, and somehow inherit all instances of Show as ToString instances? 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 generally try to keep the law read . show == id Thus, for anything that needs to be printed in a nicer way I use something like this: class PPrint a where pretty :: a - Doc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] type/class question: toString
Hello, Have you tried using -fglasgow-exts? That should enable all ghc extensions. -Jeff [EMAIL PROTECTED] wrote on 11/06/2007 02:02:11 PM: On Nov 6, 2007 12:15 PM, David Benbennick [EMAIL PROTECTED] 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] type/class question: toString
On Nov 6, 2007 12:03 PM, Thomas Schilling [EMAIL PROTECTED] wrote: On Tue, 2007-11-06 at 09:18 -0500, Graham Fawcett wrote: Hi folks, Is there a way to declare a 'toString' function, such that toString x | x is a String = x toString x | x's type is an instance of Show = 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] type/class question: toString
On Nov 6, 2007 2:21 PM, Jeff Polakow [EMAIL PROTECTED] wrote: Have you tried using -fglasgow-exts? That should enable all ghc extensions. Ah thanks, that does it. G ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] type/class question: toString
On Nov 6, 2007 3:29 PM, Graham Fawcett [EMAIL PROTECTED] wrote:
On Nov 6, 2007 2:21 PM, Jeff Polakow [EMAIL PROTECTED] 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 Stringwhere 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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