TIP:            133
Title:          Extending [expr] Operators
Version:        $Revision: 1.6 $
Author:         Richard Suchenwirth <Richard.Suchenwirth-Bauersachs@siemens.com>
State:          Draft
Type:           Project
Vote:           Pending
Created:        08-Apr-2003
Post-History:   
Tcl-Version:    8.7

~ Abstract

This TIP proposes a way to define new operators for conditions and the
'''expr''' command.  It also includes demonstrations of how it might work
in the examples: '''in''' tests inclusion in a list, and '''and''',
'''or''', and '''not''' are aliases for "'''&&'''", "'''||'''", "'''!'''".

~ Rationale

Inclusion of a value in a list is frequently tested with the construct

|   if {[lsearch -exact $list $value] >= 0} {...}

The proposal, first brought by Reinhard Max in the Tcl Chatroom, is to
allow an ''in'' operator in the language understood by '''expr''', and
the condition parts of '''for''', '''if''' and '''while''', so that
the above can be written as

|   if {$value in $list} {...}

This is shorter to type and much better to read.

In the same vein, I propose to allow operators "'''and'''",
"'''or'''", "'''not'''" to be resolved exactly like the current
"'''&&'''", "'''||'''" resp "'''!'''"  The new "operator aliases" are
not shorter than the original operators, but definitely better
readable - and easier typed too, as no Shift (or Alt-Gr on German
keyboards) key is needed.

When Tcl was young, almost all users knew C, so the C style operators
were a good choice.  In recent years, there is tendency that Tcl is
used by persons who have no or less experience with C, but come from
other languages (etc. BASIC variants have the AND, OR, NOT operators)
or have Tcl as a first language. For all these, the option of
natural-language operators will make the learning just a little bit
easier.

~ Implementation Proposals

Donal K. Fellows remarked (on an earlier proposal relating to just an
'''in''' operator) in the Tcl Chatroom: "On the plus side, it
shouldn't be hard to implement (might need an extra opcode for
'''lsearch''', but that's pretty straightforward.)"

Pascal Scheffers proposed an extension mechanism for '''expr''' and
conditions, so the proposed extensions to the expression language can
be done in Tcl, with the commands:

| expr_register_operator in  {val list} {expr {[lsearch -exact $list $val]>=0}}
| expr_register_operator and {p q}      {expr {$p && $q}}
| expr_register_operator or  {p q}      {expr {$p || $q}}
| expr_register_operator not {p}        {expr {!$p}}

Such operator registrations can have one or two arguments (for unary
and binary operators, respective) in the second argument. The third
argument is a body that is evaluated, with local variables from the
argument list substituted, and returns the resulting value, to be
substituted for the operator and its operands.

Alternatively, one could stipulate that '''expr''' interprets its
arguments in the above sense when called like this:

| expr operator in  {val list} {expr {[lsearch -exact $list $val]>=0}}
| expr operator and {p q}      {expr {$p && $q}}

This would currently raise an error, hence cannot break existing code.

For a simple start, it shall be an error to define an operator both as
unary and binary.

Rules for operator precedence, and a way of specifying the precedence
level, will still be needed.

A feature sometimes discussed in news:comp.lang.tcl, an assignment
operator, could in the same way easily be added by users who so want:

| expr operator = {varName value} {upvar 1 $varName var; set var $value}

Reinhard Max has also proposed a unary '''empty''' operator:

|  expr operator empty {list} {expr {[llength $list]==0}}

~ Copyright

This document has been placed in the public domain.
