>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:
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.
Lars H remarks that all direct copies of a string are shared automatically (it's the same Tcl_Obj). For example, in the result of
proc lineindex {filename} {
set res {}
set F [open $filename r]
while {![eof $F]} {
lappend res [list $filename [tell $F]]
gets $F ; # Skip ahead one line
}
close $F
return $res
}there is one list element for every line of the file, but all of them share the Tcl_Obj for the $filename.
NEM has interned strings for the first reason (fast comparisons). In Tcl, I'd do that like this:
set INTERN_COUNTER 0
proc intern str {
if {![info exists ::INTERN($str)]} {
set ::INTERN($str) [incr ::INTERN_COUNTER]
}
return $::INTERN($str)
}That returns a unique integer for each string and you can then do integer tests for equality. Interned strings are used often for programming language interpreters to represent symbols/ atoms/variables names to optimise for equality checks and hash-table lookups.
A quick look at tclExecute.c shows that the byte code interpreter in fact contains the trivial optimization of string compare for comparing an instance to itself - JBR
"tclExecute.c" line 2457 of 6089
if (valuePtr == value2Ptr) {
/*
In the pure equality case, set lengths too for
the checks below (or we could goto beyond it).
*/
iResult = s1len = s2len = 0;
} else if ((valuePtr->typePtr == &tclByteArrayType)
Interning a string using method 1 above optimized string compare and memory usage.
AK I have done Tcl-level interning of strings as a form of compression. Pathnames again, several thousand times the same ones. Thing versions of the same file in a version control system, for example.