TIP #144 Version 1.4: Argument Expansion Syntax

This is not necessarily the current version of this TIP.

Abstract

This TIP proposes to add syntax in Tcl to perform argument expansion in a safe and efficient manner.

Introduction

Many commands take a variable number of arguments and often you find yourself with those arguments in a list. This list must then be expanded into individual arguments to the command. This is currently done with eval:

eval destroy [winfo children .]

This is a bit obscure and also very error prone when the command becomes more complex. It is also inefficient and not object safe, why something specialised in doing this would be better.

Rationale

See also TIP #103. Please also see a summary of a poll of TCLCORE readers taken after TIP #103 was rejected. http://wiki.tcl.tk/9462

For examples three statements are used. This is the eval version:

eval destroy [winfo children .]
eval button .b $stdargs -text \$mytext -bd $border
eval exec \$prog $opts1 [getMoreopts] \$file1 \$file2

The eval version would be even more complex if the lists that are to be expanded are not known to be pure. To be really safe the last would be:

eval exec \$prog [lrange $opts1 0 end] [lrange [getMoreopts] 0 end] \$file1 \$file2

With the proposed syntax they become:

destroy {}[winfo children .]
button .b {}$stdargs -text $mytext -bd $border
exec $prog {}$opts1 {}[getMoreopts] $file1 $file2

The advantage of using syntax for this is that the command do not get obscured. In the examples destroy/button/exec is the most important information on each line and it gets to be first on the line.

Specification

If a word starts with a pair of braces, "{}", and is followed by a non whitespace character it signifies argument expansion. The braces are removed and the rest of the word is parsed as any other word. The character after the removed "{}" counts as a first character in the rules about open braces and double quotes. The word is then parsed as a list (as if with Tcl_SplitList() or Tcl_GetListFromObj) and each element of the list is added to the command being built as a separate word with no further parsing.

Before executing the command any word to be expanded is treated as a list where each element becomes one separate argument to the command.

Note 1: A word should really start with {} to trigger expansion which means that words like these are not expanded:

cmd "{}$temp" \{}[something]

Note 2: Expansion is typically most useful with words like:

cmd {}$var {}[somecmd $arg] {}$arr([cmd $arg])

But things like this are also legal:

cmd {}word {}$x,$y {}[foo]xy[apa] {}{apa bepa}

Reference Implementation

Not done yet.

Copyright

This document has been placed in the public domain.

This is not necessarily the current version of this TIP.