Proposal: Add Data.String, containing IsString(fromString), to base

[David Roundy]

On Tue, Jan 30, 2007 at 01:54:02PM +0000, Ian Lynagh wrote:
> +-- | Class for string-like datastructures; used by the overloaded string
> +--   extension (-foverloaded-strings in GHC).
> +class IsString a where
> +    fromString :: String -> a
> +
> +instance IsString [Char] where
> +    fromString xs = xs

Wouldn't it make sense to have IsString require an Eq instance?

class Eq a => IsString a where

so that string literals could be used in pattern matching? The
only reason I could see not to do this would be if you wanted to make
something like (Ptr CChar) be an IsString, but that seems pretty crazy to
me.  I guess you could make (ForeignPtr CChar) be an IsString, and that
wouldn't be particularly crazy, but it still seems ugly, and reasonably
pointless.

I'd rather have the inferred type of

foo "bar" = True

be

foo :: IsString a => a -> Bool

than

foo :: (Eq a, IsString a) => a -> Bool

or even worse, to have pattern matching not work as expected with
-foverloaded-strings.
-- 
David Roundy
http://www.darcs.net