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Botet Escriba" Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: resumable functions and monads Date: Sat, 07 Dec 2013 19:43:56 +0100 Lines: 468 Approved: news@gmane.org Message-ID: <52A36C6C.30507@wanadoo.fr> References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="------------050000080900030105030007" X-Trace: ger.gmane.org 1386441833 27364 80.91.229.3 (7 Dec 2013 18:43:53 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sat, 7 Dec 2013 18:43:53 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDH67CONY4PBB3OYRWKQKGQEPQCXEFQ@isocpp.org Sat Dec 07 19:43:59 2013 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-lb0-f197.google.com ([209.85.217.197]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1VpMr8-0006Pl-OY for gclcip-std-proposals@m.gmane.org; Sat, 07 Dec 2013 19:43:58 +0100 Original-Received: by mail-lb0-f197.google.com with SMTP id u14sf2775547lbd.4 for ; Sat, 07 Dec 2013 10:43:58 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:message-id:date:from:user-agent:mime-version:to :subject:references:in-reply-to:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe:content-type; bh=Nl84/Ta2P3lkd7BBJvL7t0CxO3+baaQNa7Z2cpPYOZs=; b=mcv7myGD+wLCydsmAhCM/FNiQRruShJ4SdROI/umoVtiYeDm7vaf+KuXzm9JYDZzDn V+j7UAhc/2iYBtqfyrNk2mKa9xhJDtL1F0STprOHjzpGUQvW01lWU5AUMHCEjFFCl7rs Go9Bc5bqc/bzkqIPugPOfTc7kYyv4Q7KO/I/EBqjOXgFwmpu2RzXBYjj57Avl6bekd9E hPxp+E27bgpBrPCMXOiRLOW7FTEEs5Alupc4g4MaBAATdSuCuF01n5ycP3GewRl4iAw9 qxI9AwqF8Nyx3GHmjDOtEzPadMLV0ZU+EELjYxEMxL6Deo33AIHMCn12xOAqRCz/uvf6 q2YQ== X-Gm-Message-State: ALoCoQlggcwiZ0KOTYec3nTnQzgF8KqnJhQ0k3HtthAbrOLKliIBHEnSOC/MKCUDJIVpYbc0htib X-Received: by 10.152.1.37 with SMTP id 5mr4228774laj.3.1386441838280; Sat, 07 Dec 2013 10:43:58 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.180.90.148 with SMTP id bw20ls269425wib.20.gmail; Sat, 07 Dec 2013 10:43:57 -0800 (PST) X-Received: by 10.180.39.140 with SMTP id p12mr7820419wik.12.1386441837180; Sat, 07 Dec 2013 10:43:57 -0800 (PST) Original-Received: from smtp.smtpout.orange.fr (smtp09.smtpout.orange.fr. [80.12.242.131]) by mx.google.com with ESMTP id d7si1289929wiy.3.2013.12.07.10.43.57 for ; Sat, 07 Dec 2013 10:43:57 -0800 (PST) Received-SPF: neutral (google.com: 80.12.242.131 is neither permitted nor denied by best guess record for domain of vicente.botet@wanadoo.fr) client-ip=80.12.242.131; Original-Received: from iMac-de-Vicente-Botet-Escriba.local ([92.139.34.180]) by mwinf5d69 with ME id yijw1m00G3tBVfg03ijwJV; Sat, 07 Dec 2013 19:43:57 +0100 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:24.0) Gecko/20100101 Thunderbird/24.1.1 In-Reply-To: X-Original-Sender: vicente.botet@wanadoo.fr X-Original-Authentication-Results: mx.google.com; spf=neutral (google.com: 80.12.242.131 is neither permitted nor denied by best guess record for domain of vicente.botet@wanadoo.fr) smtp.mail=vicente.botet@wanadoo.fr 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:8061 Archived-At: This is a multi-part message in MIME format. --------------050000080900030105030007 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: quoted-printable Le 07/12/13 15:00, stepik-777@mail.ru a =E9crit : > I'm reading this proposal:=20 > http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3722.pdf > It looks like adding do-notation for monads from Haskell to C++. It=20 > focuses on std::future, but proposes generalization to user defined types= .. > It is not clear to me how it will be applied to user defined types.=20 > Say, I want to extend it for std::list, will something like this work? > > | > template > structListMonad:list > { > usinglist::list;// inherit constructors > > // bind (>>=3D), apply Func to all elements in *this and concatenate resu= lts > template > autothen(TFunc&&Func) > { > autoiter =3Dbegin(); > autoNewList=3DFunc(ListMonad({*iter })); > for(++iter;iter !=3Dend();++iter) > NewList.splice(NewList.end(),Func(ListMonad({*iter }))); > returnNewList; > } > > // get element from unit list > constA&get() > { > if(size()!=3D1) > throw""; > return*begin(); > } > }; > > // construct unit list > template > ListMonadmreturn(A a) > { > returnListMonad({a}); > } > > ListMonadcomputeProducts()resumable > { > inta =3Dawait ListMonad({1,2,3}); > intb =3Dawait ListMonad({1,10,100}); > returnmreturn(a*b); > } > /* returns list [1, 10, 100, 2, 20, 200, 3, 30, 300], like in Haskell: > computeProducts =3D do > a <- [1, 2, 3] > b <- [1, 10, 100] > return (a*b) > */ > | > Hi, I think you have misunderstood the n3722 proposal. await is only=20 applicable to expressions that return a std::future. The expression |inta =3Dawait ListMonad({1,2,3}); is incorrect as ||ListMonad({1,2,3})| is not a std::future. || You can define monad operations to your list without needing to use await. What is the continuation function that you want to pass to the bind=20 function to obtain the result you expect? I don't see any bind function call on the Haskell code. Sorry if this is=20 trivial for some one used to work on Hashkell. Vicente --=20 ---=20 You received this message because you are subscribed to the Google Groups "= ISO C++ Standard - Future Proposals" group. To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposa= ls/. --------------050000080900030105030007 Content-Type: text/html; charset=ISO-8859-1
Le 07/12/13 15:00, stepik-777@mail.ru a écrit :
I'm reading this proposal: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3722.pdf
It looks like adding do-notation for monads from Haskell to C++. It focuses on std::future, but proposes generalization to user defined types.
It is not clear to me how it will be applied to user defined types. Say, I want to extend it for std::list, will something like this work?

template<typename A>
struct ListMonad : list<A>
{
    using list<A>::list; // inherit constructors

    // bind (>>=), apply Func to all elements in *this and concatenate results
    template<typename TFunc>
    auto then(TFunc&& Func)
    {
        auto iter = begin();
        auto NewList = Func(ListMonad<A>({ *iter }));
        for (++iter; iter != end(); ++iter)
            NewList.splice(NewList.end(), Func(ListMonad<A>({ *iter })));
        return NewList;
    }

    // get element from unit list
    const A& get()
    {
        if (size() != 1)
            throw "";
        return *begin();
    }
};

// construct unit list
template<typename A>
ListMonad<A> mreturn(A a)
{
    return ListMonad<A>({a});
}

ListMonad<int> computeProducts() resumable
{
    int a = await ListMonad({1, 2, 3});
    int b = await ListMonad({1, 10, 100});
    return mreturn(a*b);
}
/* returns list [1, 10, 100, 2, 20, 200, 3, 30, 300], like in Haskell:
computeProducts = do
    a <- [1, 2, 3]
    b <- [1, 10, 100]
    return (a*b)
*/

Hi,

I think you have misunderstood the n3722 proposal. await is only applicable to expressions that return a std::future. The expression

    int a = await ListMonad({1, 2, 3});


is incorrect as ListMonad({1, 2, 3}) is not a std::future<int>.


You can define monad operations to your list without needing to use await.

What is the continuation function that you want to pass to the bind function to obtain the result you expect?
I don't see any bind function call on the Haskell code. Sorry if this is trivial for some one used to work on Hashkell.

Vicente

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