apply(n)

💖 apply(n) — the Tcl Built-In Commands era 💖

okay bestie, gather round, because today we're talking about apply — a built-in Tcl command and honestly? she's giving main character energy. no cap.

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✨ NAME ✨

apply — apply an anonymous function 🦋

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🌸 SYNOPSIS 🌸

apply func ?arg1 arg2 ...?

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💅 DESCRIPTION 💅

So. The apply command takes your function func, applies it to the arguments arg1 arg2 ..., and hands you back the result. that's it, that's the whole vibe. ✨

Now, the function func is literally just a list, and it comes in two flavors:

• a two element list: {args body}
• a three element list: {args body namespace}

(as if the list command had been used — because, lowkey, that's exactly the energy)

The first element, args, specifies the formal arguments to func. and here's the tea: the specification of the formal arguments args is shared with the proc command, and it's described in detail in the corresponding manual page. so if you already know proc, you already know this. iykyk. 👑

The contents of body are executed by the Tcl interpreter after the local variables corresponding to the formal arguments are given the values of the actual parameters arg1 arg2 .... While body is being executed, variable names normally refer to local variables — which get created automatically when referenced and deleted when apply returns. not me describing the full lifecycle of a variable 😭

One local variable is automatically created for each of the function's arguments. that's just how she operates.

🫶 Heads up bestie: global variables can only be accessed by invoking the global command or the upvar command. Namespace variables can only be accessed by invoking the variable command or the upvar command. no shortcuts, no sneaky backdoors, periodt.

The invocation of apply adds a call frame to Tcl's evaluation stack (you know, the stack of frames accessed via uplevel). The execution of body proceeds in this call frame, in the namespace given by namespace, or in the global namespace if none was specified. And if namespace is given, it's interpreted relative to the global namespace even if its name does not start with ::. iconic behavior tbh.

The semantics of apply can also be described by approximately this:

proc apply {fun args} {
    set len [llength $fun]
    if {($len < 2) || ($len > 3)} {
        error "can't interpret \"$fun\" as anonymous function"
    }
    lassign $fun argList body ns
    set name ::$ns::[getGloballyUniqueName]
    set body0 {
        rename [lindex [info level 0] 0] {}
    }
    proc $name $argList ${body0}$body
    set code [catch {uplevel 1 $name $args} res opt]
    return -options $opt $res
}

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🎀 EXAMPLES 🎀

This first one shows how to make a simple general command that applies a transformation to each element of a list. chef's kiss fr fr.

proc map {lambda list} {
    set result {}
    foreach item $list {
        lappend result [apply $lambda $item]
    }
    return $result
}
map {x {return [string length $x]:$x}} {a bb ccc dddd}
      → 1:a 2:bb 3:ccc 4:dddd
map {x {expr {$x**2 + 3*$x - 2}}} {-4 -3 -2 -1 0 1 2 3 4}
      → 2 -2 -4 -4 -2 2 8 16 26

The apply command is also useful for defining callbacks for use in the trace command. look at her being versatile 💜

set vbl "123abc"
trace add variable vbl write {apply {{v1 v2 op} {
    upvar 1 $v1 v
    puts "updated variable to \"$v\""
}}}
set vbl 123
set vbl abc

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🌈 SEE ALSO 🌈

proc(n), uplevel(n)

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⭐ KEYWORDS ⭐

anonymous function, argument, lambda, procedure

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and that's the whole page, bestie. this little icon came to you straight out of the Tcl manual, sitting pretty under the apply(n) name in the Tcl Built-In Commands section. go forth and lambda, you absolute legend 💖✨🎀🦋