TIP:            282
Title:          Enhanced Expression Syntax
Version:        $Revision: 1.4 $
Author:         Will Duquette <William.H.Duquette@jpl.nasa.gov>
Author:         Don Porter <dgp@users.sourceforge.net>
State:          Draft
Type:           Project
Vote:           Pending
Created:        13-Oct-2006
Post-History:   
Keywords:       expr,operator,assignment
Tcl-Version:    8.7

~ Abstract

This TIP extends the syntax of the '''expr''' command to allow a sequence of
mathematical computations to be expressed clearly and concisely.

~ Rationale

The ugliness of mathematical code written in Tcl is a perennial source of
complaint. Consider the following computation:

|    set x [expr {5*$y + 6*$z}]
|    set w [expr {$x**2 + $y**2}]
|    set v [expr {$w**2 + $y**2}]

The [['''expr {...}''']] constructs make the code considerably harder to read.
But suppose '''expr''' syntax included an assignment operator:

|    expr {x = 5*$y  + 6*$z}
|    expr {w = $x**2 + $y**2}
|    expr {v = $w**2 + $y**2}

Next, suppose that an '''expr''' expression could include subexpressions,
delimited by a ";" character:

|    expr {
|        x = 5*$y  + 6*$z;
|        w = $x**2 + $y**2;
|        v = $w**2 + $y**2
|    }

The sequence of computations is now much clearer.

~ Specification

~~ Assignment Operator

The '''expr''' syntax should be extended with an assignment operator which has
C-like semantics, i.e., the result of an assignment is the assigned value.
This operator shall be written as "'''='''". The term on the left side of the
operator shall be a variable name, and the term on the right side of the
operator shall be any expression. The result of the overall assignment
expression shall be the result of the subexpression on the right hand side of
the operator.

~~ Expression Separator Operator

An '''expr''' expression can be separated into subexpressions by the semicolon
("''';'''") character, which will have the same semantics as the "," operator
in C. Thus, the return value of a call to '''expr''' is the value of the final
subexpression, and the both sides of the expression shall be expressions.

~ Reference Implementation

See [http://sourceforge.net/tracker/?func=detail&aid=1969722&group_id=10894&atid=310894]

~ Copyright

This document has been placed in the public domain.
