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