From: Neel Krishnaswami <neelk@alum.mit.edu>
To: caml-list@inria.fr
Subject: Re: [Caml-list] How to find out if a function is tail recursive?
Date: Thu, 12 Jun 2003 10:49:57 -0400 [thread overview]
Message-ID: <16104.37653.394164.683089@h00045a4799d6.ne.client2.attbi.com> (raw)
In-Reply-To: <20030612141549.GA14762@redhat.com>
Richard Jones writes:
> I was writing the section on tail recursion in the OCaml tutorial, and
> was surprised to find out that the range function (below) isn't tail
> recursive. Or at least it causes a stack overflow on a
> large-but-not-unreasonable input value.
>
> let rec range a b =
> if a > b then []
> else a :: range (a+1) b
> ;;
>
> let list = range 1 1000000;;
>
> Printf.printf "length = %d\n" (List.length list);;
>
> Can you tell me why this function isn't tail recursive, and share any
> useful tips on how to tell whether a function is or is not tail
> recursive?
A function call is a tail call if it is the last thing that the
function does before returning. In this example:
let rec range a b =
if a > b then
[]
else
a :: range (a+1) b
The two expressions '[]' and 'a :: range (a+1) b' are in tail
position. The recursive call to range is *not* in tail position,
because you need to do the 'a :: <value>' before returning.
You can identify 'tail position' as a purely syntactic criterion, and
then a 'tail call' is any function call in tail position.
With the function definition
let f x = <expr>
<expr> is an expression in tail position.
If you have an expression <expr> in tail position, then
If <expr> = <f> <x>, then:
o neither subexpression <f> nor <x> is in tail position,
o the call '<f> <x>' is in tail position
If <expr> = if <test>
then <e_1>
else <e_2>, then:
o <test> is not in tail position
o <e_1> and <e_2> are in tail position
If <expr> = match <m> with
| pat -> <e_1>
| ...
| pat -> <e_n>, then:
o <m> is not in tail position
o <e_1> ... <e_n> are in tail position.
If <expr> = begin
<e_1>;
...
<e_n-1>;
<e_n>
end, then:
o <e1> ... <e_n-1> are not in tail position
o <e_n> is in tail position
If <expr> = try <body> with exn -> <handler>, then:
o <body> is not in tail position
o <handler> is in tail position
--
Neel Krishnaswami
neelk@alum.mit.edu
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next prev parent reply other threads:[~2003-06-12 14:38 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-06-12 14:15 Richard Jones
2003-06-12 14:31 ` Paul Steckler
2003-06-12 14:43 ` Luc Maranget
2003-06-12 16:15 ` John Max Skaller
2003-06-12 16:37 ` John Max Skaller
2003-06-13 7:23 ` [Caml-list] This is so nice John Max Skaller
2003-06-12 14:37 ` [Caml-list] How to find out if a function is tail recursive? Wolfgang Müller
2003-06-12 14:49 ` Neel Krishnaswami [this message]
2003-06-12 15:14 ` Chris Uzdavinis
2003-06-12 15:33 ` Brian Hurt
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