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From: Zhihao Yuan <lichray@gmail.com>
Newsgroups: gmane.comp.lang.c++.isocpp.proposals
Subject: Re: optional<T>, one more value or two states
Date: Thu, 24 Jan 2013 12:13:08 -0600
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On Thu, Jan 24, 2013 at 12:01 PM, Jeffrey Yasskin <jyasskin@googlers.com> wrote:
> optional<T> has good reasons to make its
> conversion implicit, and if convert() is adopted, those reasons

My reason to support explicit conversion is a logical reason.
Think of type T with possible values { a, b, c }, and U with
possible values { a, b, c, x }, we can say T is-a U, since T can
be used everywhere U can be used.  And in such a case,
an implicit conversion from T to U is logically.

But optional<T> should be different.  In type theory, it's
T' build upon T, with possible values { a', b', c', None }.  An
implicit conversion should not be allowed here.  And no
existing functional programming language has an logic
such that to use T instead of `Some T`.

--
Zhihao Yuan, ID lichray
The best way to predict the future is to invent it.
___________________________________________________
4BSD -- http://4bsd.biz/

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