const(n)

💖 const(n) — Tcl Built-In Commands 💖

okay bestie, buckle up, because this page is about the const command from Tcl's built-in command set, and it's literally the "make it permanent" button for your variables 💅✨

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🎀 NAME 🎀

const — create and initialize a constant 🫶

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🌸 SYNOPSIS 🌸

const varName value

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🦋 DESCRIPTION 🦋

So here's the vibe 🌈: this command is normally used within a procedure body (or a method body, or a lambda term) to create a constant inside that procedure — or inside a namespace eval body to create a constant inside that namespace. Basically it makes a variable that nobody, and I mean nobody, is allowed to touch afterwards 😤

The constant is an unmodifiable variable named varName, and it gets initialized with value. And the result of const? Always the empty string on success. Iconic, quiet, no drama 👑

If a variable varName does not exist yet, it gets created with its value set to value and gets marked as a constant 🎀 The way this slaps: that means no other command (like set, append, incr, unset — yeah, those ones 😭) may modify or remove the variable. And plot twist, variables are checked for whether they're constants before any traces are called 👀

If a variable varName already exists, that's an error — unless that variable is already marked as a constant, and in that case const is a total no-op, like "been there, done that" 💅

Now the rules, because there are always rules 🙈 varName may not be a qualified name and may not reference an element of an array by any means. And if the variable exists and it's an array? That's an error, bestie, no cap 💀

Constants are normally only removed by their containing procedure exiting or their namespace being deleted. That's it. That's the exit strategy 🦋

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✨ EXAMPLES ✨

Create a constant in a procedure 🍓

proc foo {a b} {
    const BAR 12345
    return [expr {$a + $b + $BAR}]
}

Create a constant in a namespace to factor out a regular expression 💜

namespace eval someNS {
    const FOO_MATCHER {(?i)}\mfoo\M}

    proc findFoos str {
        variable FOO_MATCHER
        regexp -all $FOO_MATCHER $str
    }

    proc findFooIndices str {
        variable FOO_MATCHER
        regexp -all -indices $FOO_MATCHER $str
    }
}

Making a constant in a loop doesn't error 🔥

proc foo {n} {
    set result {}
    for {set i 0} {$i < $n} {incr i} {
        const X 123
  lappend result [expr {$X + $i**2}]
    }
}

yes, that lappend line is indented like that in the original — copy it exactly, indentation and all, no notes 🙈

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🌟 SEE ALSO 🌟

proc(n), namespace(n), set(n), unset(n) — go read these next, bestie, they're the whole friend group 🫶

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💫 KEYWORDS 💫

namespace, procedure, variable, constant

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and that's the page, periodt 🎀 this was const(n) from the Tcl Built-In Commands manual, version Tcl 9.0, dated... well, the footer just says Tcl 9.0 and honestly same, we don't do dates either 💅✨ love u, go make some unmodifiable variables 🦋💖