Re: [Haskell-cafe] Alternate instance Show (Maybe a)?
G'day all. Quoting Sergey Zaharchenko [EMAIL PROTECTED]: Yes, I think another Show-like class will probably be a better solution... This is the one that I use. Very simple. import Text.PrettyPrint.HughesPJ class Pretty a where -- Equivalent of showsPrec prettyP :: Int - a - Doc prettyP _ x = pretty x -- Equivalent of shows pretty :: a - Doc pretty x = prettyP 0 x Cheers, Andrew Bromage ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Alternate instance Show (Maybe a)?
On Feb 2, 2007, at 21:10 , Sergey Zaharchenko wrote:
Hello list,
Suppose I want show Nothing to return , and show (Just foo) return
show foo. I don't seem to be able to. Looks like I either have to use
some other function name, like `mShow', or have to import Prelude
hiding
Maybe, reimplement Maybe, write all the other useful instances
(Functor,
Monad) for it, etc. Not particularly hard, but looks ugly. Isn't
there a
better solution? I recall some discussion about this, but can't
find it
in the archives...
With GHC you can at least avoid rewriting all the instances: make a
newtype, use newtype deriving to get all the instances except Show,
and write your own Show instance.
{-# OPTIONS_GHC -fglasgow-exts #-}
newtype MyMaybe a = MyMaybe (Maybe a)
deriving (Eq,Ord,Monad,Functor)
instance Show a = Show (MyMaybe a) where
showsPrec _ (MyMaybe Nothing) = id
showsPrec n (MyMaybe (Just x)) = showsPrec n x
/Björn___
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Re: [Haskell-cafe] Alternate instance Show (Maybe a)?
Hi Sergey, You wrote: Suppose I want show Nothing to return , and show (Just foo) return show foo. I don't seem to be able to. Looks like I either have to use some other function name, like `mShow' That is correct. Show instances are supposed to follow the convention that show x is a Haskell expression that recreates x. In other words, Show is mainly used for debugging, for simple serialization, and for interactive use in a Haskell shell like ghci or hugs. If you need to create strings from a datatype for some other reason, use a different function name. If you need to do it for several datatypes, create your own class. Of course, Show does turn out to be more generally useful for numeric types. Hope this helps. Regards, Yitz ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Alternate instance Show (Maybe a)?
Hello Yitzchak! Sat, Feb 03, 2007 at 07:54:17PM +0200 you wrote: Show instances are supposed to follow the convention that show x is a Haskell expression that recreates x. In other words, Show is mainly used for debugging, for simple serialization, and for interactive use in a Haskell shell like ghci or hugs. If you need to create strings from a datatype for some other reason, use a different function name. If you need to do it for several datatypes, create your own class. Yes, I think another Show-like class will probably be a better solution... Thanks Yitzchak and Bjorn, -- DoubleF No virus detected in this message. Ehrm, wait a minute... /kernel: pid 56921 (antivirus), uid 32000: exited on signal 9 Oh yes, no virus:) pgpKrflZnT8EB.pgp Description: PGP signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Alternate instance Show (Maybe a)?
Hello list, Suppose I want show Nothing to return , and show (Just foo) return show foo. I don't seem to be able to. Looks like I either have to use some other function name, like `mShow', or have to import Prelude hiding Maybe, reimplement Maybe, write all the other useful instances (Functor, Monad) for it, etc. Not particularly hard, but looks ugly. Isn't there a better solution? I recall some discussion about this, but can't find it in the archives... TIA, -- DoubleF No virus detected in this message. Ehrm, wait a minute... /kernel: pid 56921 (antivirus), uid 32000: exited on signal 9 Oh yes, no virus:) pgpK6irUBjFEF.pgp Description: PGP signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
