On Friday 21 May 2010 19:06:51, R J wrote:
Why does the following, trivial code snippet below hang GHCi when I type"Scalene > Failure", and what's the fix?
For an Ord instance, you need to define at least one of compare and (<=) or the other functions from the class won't work. All methods have default implementations in terms of compare, compare has a default implementation in terms of (<=). If you only implement (<) and then query (>), it's going in circles. Fixes: a) implement compare or (<=) b) choose the correct order for the constructors and add Ord to the deriving clause.
data Triangle = Failure | Equilateral | Isosceles | Scalene deriving (Eq, Show) instance Ord Triangle where Failure < Failure = False Failure < _ = True Equilateral < Failure = False Equilateral < Equilateral = False Equilateral < _ = True Isosceles < Scalene = True Isosceles < _ = False Scalene < _ = False
Your newlines never make it to my mail programme, is your mail programme configured to send only '\r' and not '\n' ?
(I tried submitting this to beginners@haskell.org, but even though I've signed up for that mailing list, I got a bounce-back saying that I needed admin approval to submit anything to that list, and I haven't heard from an admin, so I'm posting it here.)
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