[Haskell-beginners] List.sort

[Patrick LeBoutillier]

Hi,

> Patrick,
>
>>   Ord a => [a] -> [a]
>>
>> I don't understand how this type can allow it to sort pairs:
>>
>> Prelude> :m +List
>> Prelude List> sort [(1, "1"), (3, "3"), (2, "2")]
>> [(1,"1"),(2,"2"),(3,"3")]
>>
>> How can a pair by of type "Ord a"? Is it a rule that the first element
>> is automatically used?
>
> "Ord a" isn't really the type of a pair... it's a type *constraint* on the
> argument to the sort function, which is otherwise any type "a".
>
> So it means List.sort can be applied to any type "a" which satisfies "Ord a".
> But what does that mean?  It means that there is an instance for the
> typeclass Ord for whatever type is given to Listsort.
>
> In the case of pairs, it's exactly as you guessed... the leftmost parts are
> compared first.  This is so because somewhere there's an "instance Ord (a,b)"
> that defines how to compare pairs, and it does it that way (left-to-right).
> (And this generalizes to tuples bigger than pairs, up to some
> compiler-specific limit.)
>
> Does this make sense?

Yes, perfectly. I guess you just have to know about it... :)


>
> John
>
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-- 
=====================
Patrick LeBoutillier
Rosemère, Québec, Canada