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[Caml-list] Reading a large text file
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Date: 2004-05-01 (19:25)
From: skaller <skaller@u...>
Subject: Re: [Caml-list] Reading a large text file
On Sun, 2004-05-02 at 04:05, Richard Jones wrote:
> On Sat, May 01, 2004 at 11:43:51AM -0400, Rahul Siddharthan wrote:

> The short answer is no, because in OCaml (unlike in LISP) lists are
> immutable. 

> However, OCaml is a practical language and so allows you to create a
> mutable cons structure if you so desire.  eg:

It can now do slightly better than that. It is possible to use
the new 'private' keyword to *guard* your mutable list.

module MList = struct 
type 'a mylist = private { head : 'a; mutable tail : 'a mylist option; }
let splice a b = ...(* makes a new mylist of a @ b *)

Here, lists are immutable *publically*. However the splice
function provides a concatenation of two ordinary lists
into a mylist in 1 pass, with O(1) stack usage <grin>
and the result is in forward order, not reversed.

The list is still immutable! You can't modify it.
The mutable field is only used during construction
to append to the end, preserving the forward ordering.

Its possible for C language implementations to do
that kind of thing right now for actual Ocaml lists.
But now you can play the game in Ocaml.

John Skaller, mailto:skaller@users.sf.net
voice: 061-2-9660-0850, 
snail: PO BOX 401 Glebe NSW 2037 Australia
Checkout the Felix programming language http://felix.sf.net

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