Scheme Programmer's Manual

NAME

assert - raise an assertion violation if an expression is false

LIBRARY

(import (rnrs))                     ;R6RS
(import (rnrs base))                ;R6RS

SYNOPSIS

(assert expression)

DESCRIPTION

An assert form is evaluated by evaluating expression. If expression returns #f, an exception with condition types &assertion and &message is raised. The message provided in the condition object is implementation-dependent.

IMPLEMENTATION NOTES

Ikarus Scheme
See "BUGS".

RETURN VALUES

If expression returns a true value, that value is returned from the assert expression.

EXAMPLES

(define (integer-add x y)
  (assert (and (integer? x) (integer? y)))
  (+ x y))

(integer-add 1 2)
   => 3 (integer-add 1 2.1)
   => raises &assertion and &message

APPLICATION USAGE

Applications can use this form to guard against invalid uses of procedures. It is often more convenient than manual use of assertion-violation.

RATIONALE

Implementations can exploit the fact that assert is syntax to provide as much information as possible about the location of the assertion failure.

ERRORS

This procedure can raise exceptions with the following condition types:
&assertion (R6RS)
The wrong number of arguments was passed or an argument was outside its domain.
&syntax (R6RS)
More or fewer than one expression was passed.

SEE ALSO

assertion-violation(3)

STANDARDS

R6RS

HISTORY

The assert macro is new in R6RS, but has a long history in programming.

AUTHORS

This page is part of the scheme-manpages project. It includes materials from the RnRS documents. More information can be found at

BUGS

The implementation in psyntax from the r6rs-libraries project is buggy and does not return the true value. This bug has spread into some Scheme compilers.

Index

NAME
LIBRARY
SYNOPSIS
DESCRIPTION
IMPLEMENTATION NOTES
RETURN VALUES
EXAMPLES
APPLICATION USAGE
RATIONALE
ERRORS
SEE ALSO
STANDARDS
HISTORY
AUTHORS
BUGS
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