head-strict

A head-strict function will not necessarily evaluate every cons cell of its (list) argument, but whenever it does evaluate a cons cell it will also evaluate the element in the head of that cell. An example of a head-strict function is

	beforeZero :: [Int] -> [Int]
	beforeZero []     = []
	beforeZero (0:xs) = []
	beforeZero (x:xs) = x : beforeZero xs

which returns a list up to the first zero.

This pattern of evaluation is important because it is common in functions which operate on a list of inputs.