From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <oleg@okmij.org>
Received: from mail2-relais-roc.national.inria.fr (mail2-relais-roc.national.inria.fr [192.134.164.83])
	by sympa.inria.fr (Postfix) with ESMTPS id E688C7FACD
	for <caml-list@sympa.inria.fr>; Tue, 30 Sep 2014 10:47:11 +0200 (CEST)
Received-SPF: None (mail2-smtp-roc.national.inria.fr: no sender
  authenticity information available from domain of
  oleg@okmij.org) identity=pra; client-ip=66.39.3.115;
  receiver=mail2-smtp-roc.national.inria.fr;
  envelope-from="oleg@okmij.org"; x-sender="oleg@okmij.org";
  x-conformance=sidf_compatible
Received-SPF: Pass (mail2-smtp-roc.national.inria.fr: domain of
  oleg@okmij.org designates 66.39.3.115 as permitted sender)
  identity=mailfrom; client-ip=66.39.3.115;
  receiver=mail2-smtp-roc.national.inria.fr;
  envelope-from="oleg@okmij.org"; x-sender="oleg@okmij.org";
  x-conformance=sidf_compatible; x-record-type="v=spf1"
Received-SPF: None (mail2-smtp-roc.national.inria.fr: no sender
  authenticity information available from domain of
  postmaster@www1.g3.pair.com) identity=helo;
  client-ip=66.39.3.115;
  receiver=mail2-smtp-roc.national.inria.fr;
  envelope-from="oleg@okmij.org";
  x-sender="postmaster@www1.g3.pair.com";
  x-conformance=sidf_compatible
X-IronPort-Anti-Spam-Filtered: true
X-IronPort-Anti-Spam-Result: AtQCANttKlRCJwNzaWdsb2JhbABghmrPPgKBERYBEQ0HDQdChAQCBHkbIRMBIB0rFIhDvxwYjzZoBxaDGIEdBZ0nAZlcIYJ5AQEB
X-IPAS-Result: AtQCANttKlRCJwNzaWdsb2JhbABghmrPPgKBERYBEQ0HDQdChAQCBHkbIRMBIB0rFIhDvxwYjzZoBxaDGIEdBZ0nAZlcIYJ5AQEB
X-IronPort-AV: E=Sophos;i="5.04,625,1406584800"; 
   d="scan'208";a="98569994"
Received: from www1.g3.pair.com ([66.39.3.115])
  by mail2-smtp-roc.national.inria.fr with ESMTP/TLS/ADH-AES256-SHA; 30 Sep 2014 10:46:43 +0200
Received: by www1.g3.pair.com (Postfix, from userid 9370)
	id 1FFBAC3825; Tue, 30 Sep 2014 04:46:41 -0400 (EDT)
From: oleg@okmij.org
To: gabriel.scherer@gmail.com
CC: goswin-v-b@web.de, caml-list@inria.fr
In-reply-to:
	<CAPFanBHFO_t3N+f2hTZ+hytEj=8fhBAik-PYFcrcn2VayJ+dvw@mail.gmail.com>
Message-Id: <20140930084641.1FFBAC3825@www1.g3.pair.com>
Date: Tue, 30 Sep 2014 04:46:41 -0400 (EDT)
Subject: Re: [Caml-list] Why List.map does not be implemented


Gabriel Scherer wrote
> it is safe to Obj.magic a mutable data-structure into an immutable
> one. The Obj.magic-using code for List.map, implemented in Extlib and
> inherited by Batteries, is careful to use an unsafe cast in exactly
> the second situation. This is a feature that other languages
> (eg. Mezzo) safely provide.

Let me relay a short anecdote about the List.map function in
Batteries. Once I received a message from a person who was using the
Hansei library of probabilistic programming. He reported a problem:
his program produced clearly wrong results. He even traced the problem
to the function 'List.map' -- saying that if he wrote List.map himself,
the problem disappeared. I was initially puzzled: how one can possibly
mis-write List.map. I asked for more details and he said that he was
using Batteries. That gave me a hunch: I went to the Battery
repository to look at the source code and found what I expected --
mutation. In probabilistic programs, mutation has to be done
carefully. Mutation to the global heap forces correlation on what
should be independent ``possible worlds''. Mutation has to be done
with world-local heaps, which Hansei provides for that purpose. There
are times when hidden optimizations come to bite us. 

I guess in Mezzo I would have seen from the type that a mutation is
performed, and would have avoided the optimized Map (or the type
checker would make me avoid it).