An expression of the form
(proc-expr arg-expr ...)
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.
|> (cons 1 null)|
|> (+ 1 2 3)|
|> (cons 1 2 3)|
cons: expects 2 arguments, given 3: 1 2 3
|> (1 2 3)|
procedure application: expected procedure, given: 1;
arguments were: 2 3
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.
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
(go "super.ss" #:mode 'fast)
calls the function bound to go with "super.ss" 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.ss" #: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.ss")
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 the list:
The apply function offers a way around this restriction. It takes a function and a list arguments, and it applies the function to the arguments:
The apply function supports only by-position arguments. To apply a function with keyword arguments, 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.