TIP:            53
Title:          Addition of 'assert' Command
Version:        $Revision: 1.4 $
Author:         Gerald W. Lester <Gerald.Lester@aspentech.com>
Author:         Kevin Kenny <kennykb@acm.org>
State:          Withdrawn
Type:           Project
Vote:           Pending
Created:        14-Aug-2001
Post-History:   
Keywords:       bytecode,compiler
Tcl-Version:    8.4

~ Abstract

This TIP proposes the addition of an ''assert'' command and supporting
infrastructure to the Tcl core.

~ Rationale

Many languages, including other scripting languages, have assertion
checking features that can be used to assist in validating program
correctness.  Typically, these assertion checking features can be
"compiled out" of production systems so as not to impact performance. 
To have a similar effect in Tcl, the assertion checking features must
be implemented at the byte code compiler level.

If, doing byte code compilation, an assert command is encountered the
byte code stream generated will be dependent on the value of the
''assert_enabled'' command line option.  If the option is true, a byte
code stream will be emitted to implement the assert command.  If the
option is not true, no byte code will be emitted.

Similarly, if the interpreter encounters an ''assert'' command (either
compiled or uncompiled), it will only execute it if the
''assert_enabled'' command line option is true.

It is acceptable for the compiler to throw an error if the
''booleanExpression'' is not brace quoted.

~ Tcl-Level Specification

The manual entry for the ''assert'' command is included here:

----

~NAME

 > assert - Assert a run time validation condition

~SYNOPSIS

|   assert booleanExpression messageText

~DESCRIPTION

 > This command has no effect if the assert_enabled command line
   option is not true at both compile and run time.  If the
   ''assert_enabled'' command line option is true at both compile and
   run time, the following behavior will occur:

 > 1. The ''booleanExpression'' will be evaluated

 > 2. If the ''booleanExpression'' evaluates to a true value,
      ''assert::failed'' will be called at the global level with
      ''messageText'' as its one and only parameter.

 > The default implementation of ''assert::failed'' will write
   ''messageText'' to standard out and ''exit'' with a status code of
   1.

----

~ Remarks

This TIP has been withdrawn because of other changes, both inside and
outside the Tcl core.

 1. The bytecode compiler (8.4a4) contains code that recognizes a
    no-op procedure of the form ''proc no-op args {}'' and generates
    no bytecode if such a procedure is called with arguments that have
    no side effects.

 2. The ''control'' package within tcllib implements a
    ''::control::assert'' procedure that provides all the requested
    functionality.

These two, taken together, provide an implementation of the requested
functionality that is acceptable to the original author of this TIP.

~ Copyright

This TIP is in the public domain.