AppleScript: The Definitive Guide, 2nd Edition

Recursion

A handler is visible from within itself. This means that recursion is possible; a handler may call itself.

Explaining the elegances and dangers of recursion is beyond the scope of this book. The best way to learn about recursion is through a language like Scheme or LISP , where recursion is the primary form of looping. In fact, in conjunction with lists, AppleScript's recursion allows some remarkably Scheme-like (or LISP-like) modes of expression (see "LISP-likeness " in Chapter 4).

Tip

The best way to learn Scheme is to read Harold Abelson et al., Structure and Interpretation of Computer Programs, 2nd Edition (MIT Press, 1996), the best computer book ever written.

For example, here's a recursive routine for filtering a list. We'll remove from the list everything that isn't a number:

on filter(L)
    if L = {} then return L
    if {class of item 1 of L} is in {real, integer, number} then
        return {item 1 of L} & filter(rest of L)
    else
        return filter(rest of L)
    end if
end filter
filter({"hey", 1, "ho", 2, 3}) -- {1, 2, 3}

AppleScript is not a truly recursive language, however; recursion is limited by the depth of AppleScript's internal stack. If you recurse too deep (which usually means recursing through too long a list), you'll get a "stack overflow " error message. Unfortunately there's no way to know in advance what the limit is, as it depends on what happens during each recursion and on what environment is running the script. Just to give a typical example, using Script Editor on my ...