Editing POP-2 (section)

===Functions===
Variables can hold values of any type, including functions, which are first-class objects. Thus, the following constructs

 function max x y; if x > y then x else y close end;

and

 vars max;
    lambda x y; if x > y then x else y close end -> max;

are equivalent.

An interesting operation on functions is [http://www.cs.bham.ac.uk/research/projects/poplog/doc/pophelp/partapply ''partial application''], (sometimes termed ''[[currying]]''). In partial application, some number of the rightmost arguments of the function (which are the last ones placed on the stack before the function is involved) are ''frozen'' to given values, to produce a new function of fewer arguments, which is a [[Closure (computer programming)|closure]] of the original function. For instance, consider a function for computing general second-degree polynomials:

 function poly2 x a b c; a * x * x + b * x + c end;

This can be bound, for instance as

 vars less1squared;
    poly2(% 1, -2, 1%) -> less1squared;

such that the expression

 less1squared(3)

applies the closure of poly2 with three arguments frozen, to the argument 3, returning the square of (3 - 1), which is 4. The application of the partially applied function causes the frozen values (in this case 1, -2, 1) to be added to whatever is already on the stack (in this case 3), after which the original function poly2 is invoked. It then uses the top four items on the stack, producing the same result as

 poly2(3, 1, -2, 1)

i.e.

 1*3*3 + (-2)*3 + 1