Numeric literals

[Patrik Jansson]

On Tue, 28 Aug 2001, Mark Carroll wrote:
> How do I tell if an instance of reading will work, or catch that it
> didn't?

Short: check the resulting list from  reads "some string"

In the Read class we have also

  reads :: Read a => ReadS a

type ReadS a = String -> [(a,String)]

[A ghci test session follows]

Prelude> reads "123.45a" :: [(Float,String)]
[(123.45,"a")]
Prelude> reads "123a.45" :: [(Float,String)]
[(123.0,"a.45")]
Prelude> reads "a123.45" :: [(Float,String)]
[]
Prelude> reads "Infinity" :: [(Float,String)]
*** Exception: Ratio.%: zero denominator

/Patrik