2.6: Higher order functions

You may frequently avoid direct recursion by use of higher-order functions: pass in a function to another function, which then handles the details of iteration or aggregation using your passed in the function. Examples include:

• map f xs, which applies a function f to each element in xs, creating a new list
• List.filter p xs, which filters a list using a predicate p, creating a new list containing any items that returned true for p x
• foldr c s xs lets you combine the items of a list to a summary value, using some combining operation c and a starting value s