[Haskell] Read Instances for Data.Map and Data.Set

[Georg Martius]

Am Freitag, 21. Oktober 2005 11:22 schrieb Simon Marlow:
> On 21 October 2005 08:43, Johannes Waldmann wrote:
> > Georg made the point that if a type is an instance of Show,
> > then it should also be an instance of Read, and  read . show == id
>
> I rather think that this should be an explicit design requirement for
> Read/Show instances in the libraries, along with the requirement that
> the Show instance outputs valid Haskell syntax that can be included in a
> program (assuming appropriate imports, and possibly an appropriate type
> signature).  Show is not for pretty-printing, it is for serialising.
>
> Are there any Show instances that violate these requirements in the
> base/haskell98 packages currently?

AFAIK not, but the "new" Show instances of Data.Map and such like do not 
follow this requirement, since they introduce new syntax like: 
 { a := b,..}  and < 1,2,3 > ...

Therefore I would suggest to change these instances to something like:
 show $ Data.Map.fromList [(1,"Foo"),(2,"Bar")] = 
 	"Data.Map.fromList [(1,\"Foo\"),(2,\"Bar\")]"

It uses the ordinary List syntax  and the constructing function is given as 
well.

Cheers,
	Georg

>
> Cheers,
> 	Simon
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