The identity function accepts one value and simply returns that value .
The identity function is trivial to implement in Tcl :
proc identity val {return $val}return -level 0 $x is the canonical built-in identity function . It simply sets the interpreter result to $x , and returns to the current level , allowing Tcl to continue processing the sript at the current level . It's an unusual , but often quite useful way to employ return .
Various built-in commands simply return their argument and can also be used as an identity function .
set x {some\{value}
#the following all act as identify functions
return -level 0 $x
lindex $x
apply {x {set x}} $x
#brace-quoted for safety
expr {$x}
subst {$x}
dict remove $x
dict replace $x
K $x {}
K* $x
Let's say you have to pass a script to eval, and he value eval returns is used somehow. What script do you pass if you want eval to simply return a constant, the result of a substitution, or a concatenated combination thereof? All of the above methods work, and return -level 0 avoids the need for extra quoting or proc wrappers.
An example use on the if page:
set y [if {$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.