[TCLCORE] TIP #408: Allow Any Command for expr Functions

[Brian G. <bri...@me...>]

 TIP #408: ALLOW ANY COMMAND FOR EXPR FUNCTIONS 
================================================
 Version:      $Revision: 1.1 $
 Author:       Brian Griffin <brian_griffin_at_mentor.com>
 State:        Draft
 Type:         Project
 Tcl-Version:  8.7
 Vote:         Pending
 Created:      Friday, 17 August 2012
 URL:          http://purl.org/tcl/tip/408.html
 WebEdit:      http://purl.org/tcl/tip/edit/408
 Post-History: 

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 ABSTRACT 
==========

 Proposed expansion of what constitutes a function in an *expr* 
 expression. 

 RATIONALE: NESTED EXPR CALLS 
==============================

 The *expr* command in Tcl is the only way to perform math and 
 comparison operations. Tcl's variable and command substitution rules 
 allow for great flexibility, although the syntax does not lead to great 
 readability, especially due to the ordering imposed on substitutions. 
 For example, in the expression: 

    expr {[lindex $data $start] > 4}

 the command substitution ([...]) is performed before the expression can 
 be parsed for evaluation, and, in order for the command substitution to 
 be parsed, the variable substitution ($) must be performed. The 
 consequences of this ordering means that arguments to commands cannot 
 themselves be expressions unless the argument is itself a command 
 substitution involving *expr*. This means, for example, that to compute 
 an index value, *expr* must be recursively called: 

    expr {[lindex $data [expr {$start + 2}]] > 4}

 However, when using a math function, the command substitution ordering 
 does not apply since the function is part of the expression syntax: 

    expr {pow($x + 2, 2)}

 It seems reasonable and useful if any Tcl command can be called using 
 function syntax: 

    expr {lindex($data, $start + 2) > 4}

 The common command substitution pattern has lead many developers to end 
 up writing things like: 

    if {[expr {$a+$b}] > 7} ...

 In hindsight, such statements look ridiculous, but it works and it 
 follows the same pattern as the *lindex* case above. If subexpressions 
 could be evaluated directly by the enclosing *expr* operations, this 
 would greatly simplify the overall expression and make it more readable 
 and thereby more maintainable, and hopefully, less error prone. 

 The goal here is to reduce the instances of recursive *expr* calls in 
 an expression. It seems overly complex and confusing to read, and to 
 have to use command substitution syntax inside what is already an 
 *expr* operation. 

 PROPOSED CHANGES 
==================

 The proposal is to simply apply normal Tcl command resolution rules to 
 *expr* functions. The one caveat is that *tcl::mathfunc* namespace is 
 always searched first. 

 I see this as a continuation to [TIP #232]. 

 REJECTED ALTERNATIVES 
=======================

 This can already be accomplished today by creating alias functions in 
 the *tcl::mathfunc* namespace, but this mechanism is awkward in that: 

    1. It must be defined beforehand for the set of commands used or 
       intended to be used in expressions, 

    2. For OO and namespace code, some specialized duplication is also 
       required, and 

    3. It can lead to confusion when it's not clear why some commands 
       work as functions and others don't. 

 PROOF OF CONCEPT 
==================

 (Thanks and credit to Frédéric Bonnet) 

 A quick & dirty proof of concept with unknown handlers: 

   proc mathproc {cmd args} {
       if {[namespace qualifiers $cmd] eq "tcl::mathfunc"} {
           return [uplevel [list [namespace tail $cmd]] $args]
       }
       uplevel [list ::unknown $cmd] $args
   }
   namespace unknown mathproc
   
   set l [expr {list(1,2,3)}]
   => 1 2 3
   
   expr {lindex($l,2)}
   => 3
   
   expr {expr(join($l,"+"))}
   => 6

 REFERENCE IMPLEMENTATION 
==========================

 None currently 

 COPYRIGHT 
===========

 Copyright (c) 2012 by GriffinTk 

 This document is licensed under a Creative Commons 
 Attribution-ShareAlike 3.0 Unported License 
 [<URL:http://creativecommons.org/licenses/by-sa/3.0/deed.en_US>]. 

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