-1. A ShowS instance would interfere with any other anonymous reader-monad style IsString instance. -Edward Kmett On Mon, Oct 5, 2009 at 5:23 AM, Nicolas Pouillard < nicolas.pouillard@gmail.com> wrote:

Excerpts from Bas van Dijk's message of Mon Oct 05 09:27:07 +0200 2009:

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On Sun, Sep 27, 2009 at 10:51 PM, Bas van Dijk wrote:

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Hello,

It would be really nice to use OverloadedStrings when constructing a ShowS, for example in a definition of showsPrec in an instance for Show.

The patch attached in the ticket[3544] adds the necessary instance to base/GHC/Show.lhs:

instance IsString ShowS where fromString = showString

Note that this does require TypeSynonymInstances.

Discussion period 2 weeks.

regards,

Bas

[3544] http://hackage.haskell.org/trac/ghc/ticket/3544

Just a reminder for this proposal. One week has passed and one to go.

Any comments, +1 or -1?

+1

-- Nicolas Pouillard http://nicolaspouillard.fr _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

On Mon, 2009-10-05 at 10:42 +0100, Sittampalam, Ganesh wrote:

Bas van Dijk wrote:

...

Hello,

It would be really nice to use OverloadedStrings when constructing a ShowS, for example in a definition of showsPrec in an instance for Show.

The patch attached in the ticket[3544] adds the necessary instance to base/GHC/Show.lhs:

instance IsString ShowS where fromString = showString

Note that this does require TypeSynonymInstances.

-1 : I don't think it's appropriate to use a type synonym instance in a standard library.

Strictly speaking it's not the type synonym that is worrying, it's the need for flexible instances which then leads on to needing overlapping and incoherent instances. http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extension...

A concrete example of the problems this would cause is that it would interfere with any generic function instance for IsString, e.g.

instance IsString b => IsString (a -> b) where fromString = const . fromString

Right, then uses of fromString would require -XOverlappingInstances and possibly -XIncoherentInstances. This needs -XOverlappingInstances foo :: String -> String foo = fromString "blah" This needs -XOverlappingInstances -XIncoherentInstances foo :: a -> String foo = fromString "blah"

I am not suggesting that such an instance is generally useful, but people who wanted to use it in their own private code would be obstructed from doing so by an instance for (String -> String).

Right. It makes me nervous too. I think what this show us is that ShowS should have been a newtype. Duncan

Because of the required language extensions, I'm not in favor of this ticket[1] anymore. So I'm resolving it as WONTFIX. regards, Bas [1] http://hackage.haskell.org/trac/ghc/ticket/3544

-1 What Ganesh said. On Mon, Oct 5, 2009 at 1:32 PM, Ross Paterson wrote:

On Sun, Sep 27, 2009 at 10:51:04PM +0200, Bas van Dijk wrote:

...

It would be really nice to use OverloadedStrings when constructing a ShowS, for example in a definition of showsPrec in an instance for Show.

The patch attached in the ticket[3544] adds the necessary instance to base/GHC/Show.lhs:

instance IsString ShowS where     fromString = showString

Note that this does require TypeSynonymInstances.

and also FlexibleInstances.  I think that's a problem, as Ganesh outlined. _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries