Robert Dockins writes:
On Thursday 06 April 2006 06:44 pm, John Meacham wrote:
On Thu, Apr 06, 2006 at 10:52:52PM +0100, Brian Hulley wrote:
[snip a question about Eq and Ord classes]
well, there are a few reasons you would want to use inheritance in haskell, some good, some bad.
1. one really does logically derive from the other, Eq and Ord are like this, the rules of Eq says it must be an equivalance relation and that Ord defines a total order over that equivalance relation. this is a good thing, as it lets you write code that depends on these properties.
<PUBLIC SERVICE ANNOUNCEMENT>
Many of you probably know this already, but for those who might not know:
Prelude> let x = read "NaN" :: Float Prelude> x == x False Prelude> x == 0 False Prelude> 0 < x False Prelude> x < 0 False
Ewwwwww! Be careful how far you depend on properties of typeclasses, and make sure you document it when you do.
It's worse than that.
Prelude> let x = read "NaN" :: Float
Prelude> compare x x
GT
Prelude> x > x
False
So far as I can tell, report does not actualy *require* that |x > y| iff
|compare x y == GT|, but this is an unfortunate inconsistency.
--
David Menendez