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.

Last updated: 1995-05-11

Nearby terms:

head normalisation theoremheads downhead-strictheapheartbeat

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