220 2160 article Path: news.gmane.org!not-for-mail From: Zhihao Yuan Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: optional, one more value or two states Date: Thu, 24 Jan 2013 13:17:55 -0600 Lines: 55 Approved: news@gmane.org Message-ID: References:

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Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 X-Trace: ger.gmane.org 1359055076 18643 80.91.229.3 (24 Jan 2013 19:17:56 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 24 Jan 2013 19:17:56 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCUILMGRQMARBZERQ2EAKGQE3AHMY6I@isocpp.org Thu Jan 24 20:18:15 2013 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ea0-f197.google.com ([209.85.215.197]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1TySJS-0007Os-7P for gclcip-std-proposals@m.gmane.org; Thu, 24 Jan 2013 20:18:14 +0100 Original-Received: by mail-ea0-f197.google.com with SMTP id j13sf10644322eaa.8 for ; Thu, 24 Jan 2013 11:17:56 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=x-received:x-beenthere:x-received:x-received:received-spf :mime-version:x-received:in-reply-to:references:date:message-id :subject:from:to:x-original-sender:x-original-authentication-results :reply-to:precedence:mailing-list:list-id:x-google-group-id :list-post:list-help:list-archive:list-subscribe:list-unsubscribe :content-type; bh=B9eIEXm89du358R2ceIc7BAtAB8xRvydJUuyXG4lU2I=; b=PeBmz8+aNzggKzNc2cgrZ2DA9OEBrljImcZ8yq+fNsBRFlzyg9tmbN3XlsJkJVg7yc 2RAxrgbBS5i4GEZovVa9l350ZJkoatTJNFGxXN/0ffyJyy/L/LtQEeryztuMf00QgSi8 mnSP5t4eFzjY1bn7LYbVVASqNNDFshIk5sCwfclleUhD6TdxlWsVm1YizYwSXO/aDXIT 9CR4/nJA/t3sM6rJ8+W4wJRuwq0AzNOmIIxNPAwH0iOQgvuBb3nyGKoAtV7/d+1qWPhu ITeMXi17+wJAcnltsNICOOQrVggqjL4lHyo8MVVMc+J2fHLrC X-Received: by 10.152.46.212 with SMTP id x20mr947913lam.10.1359055076744; Thu, 24 Jan 2013 11:17:56 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.152.129.195 with SMTP id ny3ls49662lab.34.gmail; Thu, 24 Jan 2013 11:17:56 -0800 (PST) X-Received: by 10.112.9.37 with SMTP id w5mr1233951lba.62.1359055076170; Thu, 24 Jan 2013 11:17:56 -0800 (PST) X-Received: by 10.112.9.37 with SMTP id w5mr1233949lba.62.1359055076144; Thu, 24 Jan 2013 11:17:56 -0800 (PST) Original-Received: from mail-lb0-f182.google.com (mail-lb0-f182.google.com [209.85.217.182]) by mx.google.com with ESMTPS id k8si11014552lbl.286.2013.01.24.11.17.56 (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Thu, 24 Jan 2013 11:17:56 -0800 (PST) Received-SPF: pass (google.com: domain of lichray@gmail.com designates 209.85.217.182 as permitted sender) client-ip=209.85.217.182; Original-Received: by mail-lb0-f182.google.com with SMTP id gg6so7229130lbb.27 for ; Thu, 24 Jan 2013 11:17:56 -0800 (PST) X-Received: by 10.152.46.17 with SMTP id r17mr2857737lam.47.1359055075939; Thu, 24 Jan 2013 11:17:55 -0800 (PST) Original-Received: by 10.112.75.131 with HTTP; Thu, 24 Jan 2013 11:17:55 -0800 (PST) In-Reply-To: X-Original-Sender: lichray@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of lichray@gmail.com designates 209.85.217.182 as permitted sender) smtp.mail=lichray@gmail.com; dkim=pass header.i=@gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:2160 Archived-At: On Thu, Jan 24, 2013 at 12:35 PM, Zhihao Yuan wrote: >> Which functional languages have user-defined implicit conversions at all? > > Scala. Besides, let me say a few words about implicit conversion. An implicit conversion from T to U should ensure that T is-a U, the possible values of T is a subset of the possible values of U, T -> U holds a partial-order relation. And you will see any implicit conversion violating the above statements is a flaw. Narrowing conversion. Int type T can be converted to int type U even if T has a larger range. One of the criticism of C++'s type safety. char const* to std::string. Pointer is not a collection of chars, while std::string is. So `string(strdup("aha"))` result in a memory leak. And such a conversion implies the `array to pointer` conversion, which makes "nu\0\l" lost everything after '\0'. "Those are C's probem/C has NUL-terminated strings from the stone age. " OK. I'm not trying to argue these. But keep this in mind: an implicit conversion from T to U without T <= U is a flaw. Let's go back to `std::optional`. So that's why I ask, what is optional. A T with a larger range, or a T with two states. If (1), then an implicit conversion is OK, but I suggest that to ensure "undefined == undefined" (JavaScript), or "None == None" (Python). Although I don't like it, and I don't think it's safe, but this is *the* extended value. If optional is a T with two states, things go easy. We have numerous successful use cases in the functional programming area. -- Zhihao Yuan, ID lichray The best way to predict the future is to invent it. ___________________________________________________ 4BSD -- http://4bsd.biz/ -- --- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. To post to this group, send email to std-proposals@isocpp.org. To unsubscribe from this group, send email to std-proposals+unsubscribe@isocpp.org. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposals/?hl=en. .