Let's assign with let

Let's assign with let presents techniques to assign data to multiple variables. For example, one might transpose the value of two variables.

Let

proc let args {
    if {[llength $args] == 2} {
        if {[lindex $args 1] eq {++}} {
            set result [uplevel 1 [list ::incr [lindex $args 0 ]]]
        } elseif {[lindex $args 1] eq {--}} {
            set result [uplevel 1 [list ::incr [lindex $args 0] -1]]
        } else {
            set result [uplevel 1 [list ::set {*}$args]]
        }
    } else {
        regexp {([^=:+\-*/&|@]*)([:+\-*/&|@]?)([@]*)=(.*)} $args -> vars op optional rest
        if {![info exists op]} {
            return -code error -errorcode 1 "no valid assignment operator in $args"
        }
        switch $op {
            : {
                if {[llength [info commands [lindex $rest 0]]]} {
                    set result [uplevel 1 $rest]
                } else {
                    # this should always work...
                    set result $rest
                }
                if { $optional eq {@}} {
                    set max [llength $result]
                    foreach var $vars res $result {
                        uplevel 1 [list ::set $var $res]
                    }
                } else {
                    foreach var $vars {
                        set result [uplevel 1 [list ::set $var $result]]
                    }
                }
            }
            @ {
                if {$optional eq {:}} {
                    set rest [uplevel 1 [list {*}$rest]]
                }
                set max [llength $rest]
                if {$max == 1} {
                    eval set rest $rest
                    set max [llength $rest]
                }
                foreach var $vars res $rest {
                    set result [uplevel 1 [list ::set $var $res]]
                }
            }
            + - - - * - / - & - | {
                set script {}
                foreach var $vars {
                    append script "
                        ::set [list $var] \[
                            ::expr {\[::set [list $var]] $op ($rest)}]
                    "
                }
                set result [uplevel 1 $script]
            }
            = -
            default {
                if {[catch {set result [uplevel 1 ::expr \{$rest\}]}]} {
                    # this should always work...
                    set result $rest
                }
                foreach var $vars {
                    set result [uplevel 1 [list ::set $var $result]]
                }
            }
        }
    }
    return $result
}

Example: Transposing two variables:

let a b @= $b $a

Which, by the way, also allows:

let a b c = 1               ;#this sets a b and c to 1
let a b c = 1 + 4           ;#"=" uses expr to process the value to assign
let a b c += 1              ;#computed assignments are allowed, +-*/&| supported
let a b c := info commands  ;#uses eval to process right side
let a b c @= 1 2 3          ;#instead of assigning the list {1 2 3} to a b and c,
                            ;#it instead assigns the elements in order, resulting
                            ;#in a getting the value 1, b getting 2 and so on.
let a b c @:= info commands ;#uses eval to get result and uses @= for assignment
let a ++                    ;#incr and
let a --                    ;#decr are supported.

RS The code fragment (appears twice above)

set i 0
foreach var $vars {
    uplevel set $var [ lindex $result $i ]
    incr i
}

seems like it can be replaced by

foreach var $vars res $result {
   uplevel 1 [list set $var $res]
}

- if foreach can do a job once, it might as well do it twice ;-) A variable saved, and possibly safer with the list wrapper...

Larry Smith: I'd clean forgotten that feature. Yes, it's much more elegant that way. The above code has been corrected.

foreach

My suggestion will swap a-b and vice versa:

foreach {a b} [list $b $a] {}

Although let has many other features of course.

Avoiding Brackets

aspect 2015: For interactive use I like to have the following two definitions active (from .tclshrc):

# let <varName> <cmd> <arg>....
proc let {name args} {
    tailcall ::set $name [uplevel 1 $args]
}
interp alias {} = {} expr

The mostly saves me having to contort my fingers to write [expr { and }]. The specific combination using = as a pun for expr I find more Tclish than = as an always-present keyword argument.

let pi = 4*atan(1)

Of course the resulting code is suboptimal and dangerous (Brace your expressions!), but it is a lot more fun to write. The only problem is that before publication I need to clean up, but that's a few editor macros away.

You will find some of the toys I paste to the wiki use this combination, where I feel readability trumps robustness.

Page Authors

pyk

Category Uncategorized