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20. Funcall, Apply, and Mapcar

Earlier I promised to give some functions which take functions as arguments. Here they are:

> (funcall #'+ 3 4)
> (apply #'+ 3 4 '(3 4))
> (mapcar #'not '(t nil t nil t nil))

Funcall calls its first argument on its remaining arguments.

Apply is just like funcall, except that its final argument should be a list; the elements of that list are treated as if they were additional arguments to a funcall.

The first argument to mapcar must be a function of one argument; mapcar applies this function to each element of a list and collects the results in another list.

Funcall and apply are chiefly useful when their first argument is a variable. For instance, a search engine could take a heuristic function as a parameter and use funcall or apply to call that function on a state description. The sorting functions described later use funcall to call their comparison functions.

Mapcar, along with nameless functions (see below), can replace many loops.

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