Source : Free On-Line Dictionary of Computing

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