sorting lists using sortBy

Question

I am trying to sort my list of tuples based on the 4th element in each of the tuples. The fourth element contains a string that is a person\'s name. I want to put tuples that c

Answer 1

Start with the type of sortBy:

> :t sortBy
sortBy :: (a -> a -> Ordering) -> [a] -> [a]

This means byName needs to have type a -> a -> Ordering. In this case, a is a tuple whose fifth element as type String; byName will ignore the other fields. So you'll need to define a function like

type MyType = (String, Int, String, Int, String, Double)
byName :: MyType -> MyType -> Ordering
byName (_, _, _, _, a, _) (_, _, _, _, b, _) = ...

I leave replacing the ... with the correct expression as an exercise.

(Recall that Ordering is a type with three values, LT, EQ, and GT, where byName a b == LT if a < b, byName a b == EQ if a == b, and byName a b == GT if a > b. In your case, two tuples will compare as equal as long as they have the same name. It sounds like you don't actually care whether byName returns LT or GT otherwise.)