On 2006-03-17, Ross Paterson wrote:
Speaking of confusing, try
[0, 0.3 .. 2]::[Rational]
Right. I had forgotten that -- Rational is exact, yet has the weird "closest endpoint" behavior of Float and Double.
Also, toEnum and fromEnum would make more sense mapping from and to Integer. It seems that succ and pred are unused.
So, I think I'll put together a proposal, well two. Okay, three. First, change toEnum and fromEnum to Integer. Then there is a choice between: (1): Remove Double, Float, and Rational from Enum. They're no longer usable in arithmetic sequences. Pro: Very easy to do. Sequences can still be constructed by starting with integers, and scaling to convert. Con: Loses some functionality (though it's questionable functionality given rounding). (2): Split Enum into the classes Enum and ArithmeticSequence and change what the various [..] desugars to. class Enum a where succ, pred :: a -> a toEnum :: Integer -> a fromEnum :: a -> Integer I believe succ and pred aren't used directly by anything else, but I do like having them. instance Integer, Int Rational _could_ be added here by the diagonal representation, but probably sohuldn't. class ArithmeticSequence a where stepFrom :: a -> [a] -- [n..] stepFromBy :: a -> a -> [a] -- [n,n'..] stepFromTo :: a -> a -> [a] -- [n..m] stepFromByTo :: a -> a -> a -> [a] -- [n,n'..m] instance Int, Integer, Float, Double, Rational. (a) Make all of them have the "closest endpoint" behavior. (b) Make all of them have strict "no more than" behavior. Pros: Clearly divides two seperate uses, while keeping functionality. Can re-introduce relationship between Ix and Enum? Cons: Yet another typeclass. Slightly misleading name, as non-arithmetic structures _should_ be supported. Also a bit long. But doing so automatically is tricky, as toEnum and fromEnum are no longer accessible Keeps questionable functionality of non-exact arithmetic sequences. Personally, I'm for 2(a), but I think even (1) is an improvement. It's a pity we can't make Enum a subclass of ArithmeticSequence that provides the methods of its superclass. Would it be possible to have "data ... (deriving ArithmeticSequence)" check if (Num a, Ord a) is defined and use (+), else if Enum a is defined, use fromEnum/toEnum to go through Integer, else fail. Where I suppose defined must mean "defined in this module". Hmm. That's kind of ugly. I can see why these were combined, but it's still really ugly. Steppable might be a better name. Comments anyone? -- Aaron Denney -><-