4.3 Function Calls (Procedure Applications)

An expression of the form

(proc-expr arg-expr ...)

is a function call—also known as a procedure application—when proc-expr is not an identifier that is bound as a syntax transformer (such as if or define).

4.3.1 Evaluation Order and Arity

A function call is evaluated by first evaluating the proc-expr and all arg-exprs in order (left to right). Then, if proc-expr produces a function that accepts as many arguments as supplied arg-exprs, the function is called. Otherwise, an exception is raised.

Some functions, such as cons, accept a fixed number of arguments. Some functions, such as + or list, accept any number of arguments. Some functions accept a range of argument counts; for example substring accepts either two or three arguments. A function’s arity is the number of arguments that it accepts.

4.3.2 Keyword Arguments

Some functions accept keyword arguments in addition to by-position arguments. For that case, an arg can be an arg-keyword arg-expr sequence instead of just a arg-expr:

Keywords introduces keywords.

(proc-expr arg ...)

arg   =   arg-expr     |   arg-keyword arg-expr

For example,

(go "super.rkt" #:mode 'fast)

calls the function bound to go with "super.rkt" as a by-position argument, and with 'fast as an argument associated with the #:mode keyword. A keyword is implicitly paired with the expression that follows it.

Since a keyword by itself is not an expression, then

(go "super.rkt" #:mode #:fast)

is a syntax error. The #:mode keyword must be followed by an expression to produce an argument value, and #:fast is not an expression.

The order of keyword args determines the order in which arg-exprs are evaluated, but a function accepts keyword arguments independent of their position in the argument list. The above call to go can be equivalently written

(go #:mode 'fast "super.rkt")

Procedure Applications and #%app in The Racket Reference provides more on procedure applications.

4.3.3 The apply Function

The syntax for function calls supports any number of arguments, but a specific call always specifies a fixed number of arguments. As a result, a function that takes a list of arguments cannot directly apply a function like + to all of the items in a list:

(define (avg lst) ; doesn’t work...   (/ (+ lst) (length lst)))

> (avg '(1 2 3)) +: contract violation   expected: number?   given: '(1 2 3)

(define (avg lst) ; doesn’t always work...   (/ (+ (list-ref lst 0) (list-ref lst 1) (list-ref lst 2))      (length lst)))

> (avg '(1 2 3)) 2 > (avg '(1 2)) list-ref: index 2 is out of range for list '(1 2)

The apply function offers a way around this restriction. It takes a function and a list argument, and it applies the function to the values in the list:

(define (avg lst)   (/ (apply + lst) (length lst)))

> (avg '(1 2 3)) 2 > (avg '(1 2)) 3/2 > (avg '(1 2 3 4)) 5/2

As a convenience, the apply function accepts additional arguments between the function and the list. The additional arguments are effectively consed onto the argument list:

(define (anti-sum lst)   (apply - 0 lst))

> (anti-sum '(1 2 3)) -6

The apply function accepts keyword arguments, too, and it passes them along to the called function:

(apply go #:mode 'fast '("super.rkt"))

(apply go '("super.rkt") #:mode 'fast)

Keywords that are included in apply’s list argument do not count as keyword arguments for the called function; instead, all arguments in this list are treated as by-position arguments. To pass a list of keyword arguments to a function, use the keyword-apply function, which accepts a function to apply and three lists. The first two lists are in parallel, where the first list contains keywords (sorted by keyword<?), and the second list contains a corresponding argument for each keyword. The third list contains by-position function arguments, as for apply.

(keyword-apply go

'(#:mode)

'(fast)

'("super.rkt"))