► |  | The Racket Guide

1 | Welcome to Racket
2 | Racket Essentials
3 | Built-In Datatypes
4 | Expressions and Definitions
5 | Programmer-Defined Datatypes
6 | Modules
7 | Contracts
8 | Input and Output
9 | Regular Expressions
10 | Exceptions and Control
11 | Iterations and Comprehensions
12 | Pattern Matching
13 | Classes and Objects
14 | Units (Components)
15 | Reflection and Dynamic Evaluation
16 | Macros
17 | Creating Languages
18 | Performance
19 | Running and Creating Executables
20 | More Libraries
21 | Dialects of Racket and Scheme
22 | Command-Line Tools and Your Editor of Choice
| Bibliography
| Index

▼ | 4 | Expressions and Definitions

4.1 | Notation
4.2 | Identifiers and Binding
4.3 | Function Calls (Procedure Applications)
4.4 | Functions (Procedures): lambda
4.5 | Definitions: define
4.6 | Local Binding
4.7 | Conditionals
4.8 | Sequencing
4.9 | Assignment: set!
4.10 | Quoting: quote and '
4.11 | Quasiquoting: quasiquote and ‘
4.12 | Simple Dispatch: case
4.13 | Dynamic Binding: parameterize

► | 4.3 | Function Calls (Procedure Applications)

4.3.1 | Evaluation Order and Arity
4.3.2 | Keyword Arguments
4.3.3 | The apply Function

On this page:

4.3.1`` Evaluation Order and Arity
4.3.2`` Keyword Arguments
4.3.3`` The apply Function

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#### 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.

Examples:

> > ( cons 1 null )
> '(1)
> > ( + 1 2 3)
> 6
> > ( cons 1 2 3)
> cons: arity mismatch;
> the expected number of arguments does not match the given
> number
> expected: 2
> given: 3
> arguments...:
> 1
> 2
> 3
> > (1 2 3)
> application: not a procedure;
> expected a procedure that can be applied to arguments
> given: 1
> arguments...:
> 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.

##### 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 too large for list
> index: 2
> in: '(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"))

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