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. See also tail-strict, hyperstrict. (1995-05-11)
