This is a drop-in replacement for the standard for command. It supports an expanded syntax for looping over iterators. It can be downloaded as a part of ycl.
package require ycl::iter for {varname1 ...} in iterator_name1 [{varname {varnameX ... } in iterator_nameX] ... script
like foreach does with list, it can operate on multiple iterators and take an arbitrary number of items each time from each iterator. Also like foreach, empty strings are produced as necessary for any iterator which becomes depleted before the others.
note that return -code break within an iterator will cause the main for loop to break, so it should be used with care.
newer code may be available in the ycl repository
rename ::for for_core interp alias {} ::for {} [namespace current]::for proc for args { if {[lindex $args 1] eq "in"} { if {[llength $args] % 3 != 1} { return -code error "wrong number of arguments for [info level 0]" } set assigns {} set assign_template { if {[namespace which $iter] eq {}} { variable $varname {} } else { variable $varname [$iter] } } set conditions {} set condition_template {[namespace which $iter] ne {}} while {[llength $args] > 1} { set args [lassign $args[unset args] varnames {} iter] foreach varname $varnames { lappend assigns [string map [list \$iter [list $iter] \$varname [list $varname]] \ $assign_template] } lappend conditions [string map [list \$iter [list $iter]] $condition_template] } set assigns [join $assigns \n] set args [join $args \n] uplevel "while {[join $conditions { || }]} {$assigns\n$args}" } else { uplevel [list [namespace current]::for_core {*}$args] } }
example
set result [list] for {key val} in [coroutine students apply {{} { yield [info coroutine] set db { Jack 97 Sally 89 Bob 83 Jill 77 John 72 } set index -1 foreach {name score} $db { yield $name yield $score } return -code break }}] prize in [coroutine prizes apply {{} { yield [info coroutine] foreach item { first second third } { yield $item } }}] { lappend result $key $val $prize } puts $result
produces:
Jack 97 first Sally 89 second Bob 83 third Jill 77 {} John 72 {}