[Richard Suchenwirth] 2005-04-09 - A trampoline is a software device to repeatedly call some code until a terminal condition is met. It can give the effect of [recursion] without consuming much stack. Slightly rewritten from [Playing TRAC], here is a simple trampoline:

 proc trampoline form {
    while 1 {
      set new [subst $form]
       if {$new eq $form} break
       set form $new
    }
    set form
 }
#-- Testing with the classic [factorial] - either constant 1 is returned, or a bracketed expression for the next trampoline jump:
 proc fac n {
    expr {$n<=1? 1: "\[expr $n * \[fac [incr n -1]]]"}
 }
 % fac 5
 [expr 5 * [fac 4]]
 % trampoline {[fac 5]}
 120

In the dialect developed in [Playing TRAC], factorial looks like this:
 DS fac {[GR $1 1 "\[ML $1 \[CL fac [SU $1 1]]]" 1]}
The structural similarity should be evident.
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See also [Tail call optimization] | [Tail call optimisation] (both have their merits :)

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[Category Concept] | [Arts and crafts of Tcl-Tk programming]