[Haskell-cafe] FW: Why does this Ord-class instance crash?

[Daniel Fischer]

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 at 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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