Laws for Show and Read?

[Bertram Felgenhauer]

Sven Panne wrote:
> We already have the law
>   read . show = id
> which implies the injectivity and surjectivity conditions you specify.
> 
> I think this doesn't hold for the standard Read/Show instances for
> Double/Float: IEEE-754 NaNs have a sign bit and a payload, and  both
> parts are probably lost by the "read . show"-combination. The 2
> infinities are probably OK, but what about -0 vs. +0?

ghci> show (-0 :: Double)
"-0.0"
ghci> read (show (-0 :: Double)) :: Double
-0.0

It's true that extra information in NaNs is lost, but they all behave
the same anyway...

Cheers,

Bertram