Re: 'Convertible' class?

You make some good arguments. Thanks. Let me ask about a few of them. On Thu, Feb 08, 2001 at 04:06:24AM +0000, Marcin 'Qrczak' Kowalczyk wrote:

Wed, 7 Feb 2001 15:43:59 -0500, Dylan Thurston pisze:

...

class Convertible a b where convert :: a -> b

So, e.g., fromInteger and fromRational could be replaced with convert. (But if you did the same thing with toInteger and toRational as well, you run into problems of overlapping instances.

... And convert cannot be a substitute for fromIntegral/realToFrac, because it needs a definition for every pair of types.

Right. Those could still be defined as appropriately typed versions of 'convert . convert'.

You can put Num a in some instance's context, but you can't put Convertible Integer a. It's because instance contexts must constrain only type variables, which ensures that context reduction terminates (but is sometimes overly restrictive). There is ghc's flag -fallow-undecidable-instances which relaxes this restriction, at the cost of undecidability.

Ah! Thanks for reminding me; I've been using Hugs, which allows these instances. Is there no way to relax this restriction while maintaining undecidability? Best, Dylan Thurston