Re: [Haskell-cafe] ANN: convertible (first release)

On Wed, 2009-01-28 at 18:41 -0500, Michael D. Adams wrote:

On Wed, Jan 28, 2009 at 5:28 PM, wren ng thornton wrote:

...

John Goerzen wrote:

...

wren ng thornton wrote: I wonder if you would consider submitting a patch to base? It seems that this is a sore problem there, and ideally should be dealt with properly in base.

As Bertram Felgenhauer says, it's not as easy as fixing base; the problem is an error in the Haskell98 specification. The Float and Double types contain exceptional values which cannot be represented in Rational (by definition). The only Haskell98 solution is to raise an error when attempting to convert those values into Rational, which isn't much of an improvement.

Is there a good reason why Rational is defined in a way that it can not represent Nan, Inf and -Inf? (Any other exceptional values I forgot?) Would fixing the definition so that it can represent those values be sufficient to fix this entire problem?

It may well help conversions of floating point numbers through Rational but the contrary argument is that Rational is a well defined concept and that values like those do not belong in it. The problem seems to be that the various real/float classes in the Prelude cannot provide a class that allows conversions directly between Double and Float without going via a universal type because the class would either have to be multi-parameter or mention one of the types in the class definition. For integral types it's no problem to go via Integer but floating point types are a good deal more tricky. Duncan