Re: [Caml-list] List.rev

[Brian Hurt <bhurt@spnz.org>]

On Sat, 1 May 2004, Nicolas Cannasse wrote:

> > Are there automatic ways to transform non-tail-recursive functions
> > into tail-recursive ones?
> >
> > Rich.
> 
> You can have a look at ExtLib sources. We provide tail-recursive
> implementations for each List operations (with same "little o" complexity).
> 

This is actually quite bad advice (sorry, Nicolas)- many of the "tricks" 
we do in Ext-Lib are not for newbies.  I'm thinking specifically of the 
Obj.magic stuff.  If you find yourself doing this anywhere else, you are 
almost certainly screwing up.

There is a fairly standard set of tricks I use to turn non-tail-recursive 
functions into tail-recursive functions.  The two most important ones are:

1) build lists backwards, then reverse them when they're done.  For 
example, List.append could be implemented:
	let append alist blist =
		let revlist = List.rev_append blist (List.rev alist) in
		List.rev revlist
	;;

2) Hoist recursive calls out of try/catch clauses, introducing variables
to detect when an exception was thrown.  For example, to read all the 
lines of a channel into a list of strings, do:
	let readfile chan = 
		let rec loop accum =
			let eof, line = 
				try
					false, (input_line chan)
				with
					| End_of_file ->
						true, ""
			in
			if (eof) then
				List.rev accum
			else
				loop (line :: accum)
		in
		loop []
	;;


-- 
"Usenet is like a herd of performing elephants with diarrhea -- massive,
difficult to redirect, awe-inspiring, entertaining, and a source of
mind-boggling amounts of excrement when you least expect it."
                                - Gene Spafford 
Brian

-------------------
To unsubscribe, mail caml-list-request@inria.fr Archives: http://caml.inria.fr
Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: http://caml.inria.fr/FAQ/
Beginner's list: http://groups.yahoo.com/group/ocaml_beginners