identity function

The identity function accepts one value and simply returns that value.

Tcl 8.6 introduces a purpose built identity function in the core:

string cat $x

Prior to that, different forms were common:

return -level 0 $x -- a command intended for the purpose, but quite awkward to type
lindex $x -- shorter but arguably less obvious

And in proc form, close variants of K serve:

K $x {}
proc identity {x} {set x}
proc K* {x args} {set x} -- a multi-purpose K variant

Reference

Identity function , Wikipedia

Description

Various built-in commands simply return their argument and can also be used as an identity function. Some commands are very near misses: they'll work most of the time, until they don't. Those are called out with counterexamples in a comment in the list below.

set x {some\{value}

#canonical identity functions

string cat $x           ;# 8.6+
return -level 0 $x

# emergent identity functions and near misses:

concat $x               ;# miss: " x "
expr {$x}               ;# miss:  09, 1.00, 007, 0x10, " 100"
lindex $x
list $x                 ;# miss: \{, \$, \", [
lrepeat 1 $x            ;# miss: \{, \$, \", [
regexp -inline .* $x
regsub .* $x $x
return -level 0 $x
string cat $x
string map {} $x
string range $x 0 end
string repeat $x 1
string replace $x -1 -1
string trim $x {}
string trimleft $x {}
string trimright $x {}

K $x {}
K* $x

There are various more obvious ways to get it wrong:

# misses due to under-quoting:
expr $x                 ;# miss: 2+2
subst $x                ;# miss: $somevar

# only work on well-formed lists
dict merge $x
join $x
lassign $x

# only work on even-length lists
dict remove $x
dict replace $x

Discussion

AMG: Let's say you have to pass a script to eval, and the value eval returns is used somehow. What script do you pass if you want eval to simply return a constant, the result of a substitution, or a concatenated combination thereof? All of the above methods work, and return -level 0 avoids the need for extra quoting or proc wrappers. This all can be quite useful in functional contexts.

An example use from the if page:

set y [if {$x} {lindex a} else {lindex b}]

Another approach, on the switch page, thanks to RS (2005-05-30):

proc is x {set x}
set type [switch -- $num {
    1 - 9         {is odd}
    2 - 3 - 5 - 7 {is prime}
    0 - 4 - 6 - 8 {is even}
}]

It's possible to delete the proc line and replace is with "lindex" or "return -level 0".

PYK: In the wild, list $x is seen being put to use as the identify function, but it's not a safe bet, because list will perform quoting on the argument when necessary to make the value well-formed item in a single-item list.

AMG: Brush offers the [:] command (that's a single colon) which acts as the identity function when passed one argument. Given zero arguments, it returns empty string. Given two arguments, it behaves as [K], returning only the first argument. Likewise three or more arguments: it returns the first. [:] can be implemented in Tcl thus:

proc : {args} {lindex $args 0}