On Thu, May 31, 2007 at 10:03:05AM +0100, PR Stanley wrote:
What is the basic philosophy for Bool being a member of Ord?
I hear two questions, why is Bool a member of Ord at all, and why was it ordered with False before True. If I'm reading the reports correctly, the Ord instance was actually added in the Haskell 1.1 standard, by changing the definition of Bool from data Bool = True | False to data Bool = True | False deriving (Eq, Ord, Ix, Enum, Text, Binary) (Text was later broken into Read and Show) I imagine it was added because you generally can't derive an instance of a class for a new type unless all the types you mention in the definition already have instances. Also, there are Ord instances like (Ord a, Ord b) => Ord (a,b), and it's sometimes handy to be able to use types like (String, Bool) as keys in a Map, or keep them in a Set. Probably there are other useful things you can do.
What justifies False < True? in most interpretations this equals:
False == 0 True == 1
Indeed, it's the same in C but what justifies the decision in Haskell?
It seems like we actually get that order because deriving Ord makes constructors earlier in the definition compare greater than later constructors, and nobody was bothered by that ordering. I can't see how one of the orders could be more expressive or otherwise technically better than the other, so I suppose you might as well agree with other conventions. I think it's mathematical convention more than the C convention Haskell is agreeing with. But this is all just speculation - the members of the Haskell comittee could tell us why this all actually happened, and at least a few read this list. Brandon