simple fold implementation

Utilities.applescript

on foldLeft given operator:op, initial:z, sequence:xs
	(*
		AppleScript handlers can't call handlers that are passed
		as parameters because the enclosing script object
		doesn't recognize the parameter name as a handler. Work
		around this by doing the computation inside an embedded
		script object. (See Neuberg, p. 150.)
	*)
	repeat until xs = {}
		set z to op's f(z, item 1 of xs)
		set xs to rest of xs
	end repeat
	return z
end foldLeft

on filter thru predicate given sequence:xs
	script predicateOp
		on f(z, x)
			if predicate's p(x) then set end of z to x
			return z
		end f
	end script
	return foldLeft given operator:predicateOp, initial:{}, sequence:xs
end filter


(*
	References:
	
	Neuberg, Matt. "AppleScript: The Definitive Guide", 2nd ed.
	ISBN 0-596-10211-9
*)