Version 6 of Interning a String

>Strick> In some languages ( e.g. [L1 ] ) you can "intern" a string, which gives you one unique "interned instance" of the string with that value.

Here's a proc to do it in Tcl:

 proc intern s {
        if { ! [info exists ::INTERN($s)] } { set ::INTERN($s) $s }
        set ::INTERN($s)
 }

I used Tcl a dozen years before needing this, but today i wrote a program that used many of the same long strings (file pathnames) in some Tcl lists.

There's two different reasons you might want to intern a string:

To make string comparisons as fast as pointer comparisons
To save memory, when strings with the same value are used many times

Most often you intern strings for the first reason, but in Tcl that can't work: there are no separate "value" and "address" comparisons. Today I interned for the second reason.

MS remarks that Tcl has a shared pool of literal strings. Any code within proc bodies reuses literals from the shared pool. In a sense, all literal strings are already "interned", and this mechanism is mainly useful for strings that are generated at runtime.