Richard Suchenwirth - After a weekend without fun project, here's a little midnight study on local variables - inspired by reading on Scheme's let construct. I preferred the name with which sounded even more commonsense to me.
Tcl variables are either global, or local to a procedure. You can shorten a variable's lifetime with unset. In LISP/Scheme as well as in C (as unequal they otherwise are), you can have variables local to a block of code - in LISP enclosed by let, in C by just bracing a block. And you can have that in Tcl too, as shall presently be demonstrated. The with command takes a non-empty list of variable names (with optional initial values, like a proc arglist) and a body which must refer to those variables, executes the body in caller's scope, and unsets the "local" variables afterwards.
proc with {locals body} {
set cleanup [list unset]
foreach i $locals {
set name [lindex $i 0]
if [uplevel info exists $name] {
return -code error "local variable $name shadows outer var."
}
lappend cleanup $name
if [llength $i]==2 {
uplevel 1 set $i ;# uplevel works like concat, so no eval
}
}
set res [uplevel 1 $body]
uplevel 1 $cleanup
set res
}# usage example and test code:
set x 17
with {xa xb {xc 5}} {
set xa [expr $xc*$xc]
set xb [incr xa $x]
puts xb=$xb
}
puts "now we have [info vars x*]" ;# should not show xa, xb, xcDiscussion: The handling of local variables with same name as one in outer scope is more strict than in C (where it may raise a warning) or Lisp (where it's no problem at all - the local temporarily supersedes the outer, which is later available unchanged). Since body is just upleveled, any other treatment seemed dangerous or overly difficult - but feel free to improve on that... Likewise, specifying a local variable which is not set to a value in body, or not giving any locals at all, raises an error on unsetting. As usual, another error is raised when retrieving a variable's value that has not been set before.
How useful this is, is another question. Good efficient code should be written inside procedures, and variables inside procedures are local anyway unless explicitly globalized.
Here's my take on let, where local variables in the block do supersede the ones at higher levels.
proc let {vars body} {
uplevel [subst -nocommands {
namespace eval __tmp__ {
variable __vv {} __var {} __value {}
if {[namespace parent] != "::"} {
foreach __vv [info vars [namespace parent]::*] {
variable [namespace tail \$__vv] [set \$__vv]
}
}
foreach __vv [list $vars] {
foreach {__var __value} \$__vv break
variable \$__var \$__value
}
unset __vv __var __value
$body
namespace delete [namespace current]
}
}]
}As a bonus, since it uses namespace you get block-local procedures also. I imagine this still isn't too efficient, creating and destroying the namespace each time through.
The quoting isn't too ugly here, especially considering what its doing.. but is there a clean way to get out of Quoting hell in this case?