tailcall(n)

💫 tailcall(n) 💫

ok so this is a Tcl Built-In Command bestie, and it's genuinely one of the slayest little tricks in the whole interpreter. sit down, we're doing recursion the efficient way 🫶

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✨ NAME ✨

tailcall — replace the current procedure with another command 💅

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🌸 SYNOPSIS 🌸

tailcall command ?arg ...?

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🦋 DESCRIPTION 🦋

okay so here's the vibe: tailcall swaps out the procedure, lambda application, or method that's currently running and puts another command in its place. main character energy, honestly.

That command — and the arg ... you hand it, if you hand it any — gets looked up in the current namespace context, NOT in the caller's. that's the one lil plot twist. Apart from that difference in resolution, it is equivalent to:

return [uplevel 1 [list command ?arg ...?]]

and that's the whole thing fr fr. no cap. 🎀

🚨 okay important caveat bestie: this command may not be invoked from within an uplevel into a procedure, or inside a catch inside a procedure or lambda. it's giving forbidden. don't do it. 💀

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🔥 EXAMPLE 🔥

Compute the factorial of a number 🍓

proc factorial {n {accum 1}} {
    if {$n < 2} {
        return $accum
    }
    tailcall factorial [expr {$n - 1}] [expr {$accum * $n}]
}

the way that tailcall just re-invokes itself and never grows the stack?? obsessed. periodt. ✨

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Print the elements of a list with alternating lines having different indentations 🌈

proc printList {theList} {
    if {[llength $theList]} {
        puts "> [lindex $theList 0]"
        tailcall printList2 [lrange $theList 1 end]
    }
}
proc printList2 {theList} {
    if {[llength $theList]} {
        puts "< [lindex $theList 0]"
        tailcall printList [lrange $theList 1 end]
    }
}

look at them passing the baton back and forth like it's a relay race 💖 > then < then > then <… iconic behavior honestly.

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👑 SEE ALSO 👑

• apply(n)
• proc(n)
• uplevel(n)

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💜 KEYWORDS 💜

• call
• recursion
• tail recursion

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ok bye bestie, go write some stack-safe recursion and think of me 💅✨

Tcl 8.6 — tailcall(n) 🎀 mwah, go slay 🫶