Topic: Adding library support for recursive lambdas
Author: yegor.derevenets@gmail.com
Date: Wed, 30 Sep 2015 11:05:26 -0700 (PDT)
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Hi,
it looks like C++14 generic lambdas and Y combinator play really well
together
when you want to write a recursive lambda. So, I wrote a draft proposal to
include Y combinator into the standard:
http://derevenets.com/public/yc-proposal.html
Could you have a look and evaluate the idea?
--
Yegor Derevenets
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<div dir=3D"ltr">Hi,<br><br>it looks like C++14 generic lambdas and Y combi=
nator play really well together<br>when you want to write a recursive lambd=
a. So, I wrote a draft proposal to<br>include Y combinator into the standar=
d:<br><br>=C2=A0=C2=A0=C2=A0 http://derevenets.com/public/yc-proposal.html<=
br><br>Could you have a look and evaluate the idea?<br><br>--<br>Yegor Dere=
venets<br><br></div>
<p></p>
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Author: Richard Smith <richard@metafoo.co.uk>
Date: Wed, 30 Sep 2015 11:26:37 -0700
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On Wed, Sep 30, 2015 at 11:05 AM, <yegor.derevenets@gmail.com> wrote:
> Hi,
>
> it looks like C++14 generic lambdas and Y combinator play really well
> together
> when you want to write a recursive lambda. So, I wrote a draft proposal to
> include Y combinator into the standard:
>
> http://derevenets.com/public/yc-proposal.html
>
> Could you have a look and evaluate the idea?
>
I think this would be better as a first-class language feature. I ran out
of time for the pre-Kona meeting, but I was intending on writing a paper to
allow giving a lambda a name (scoped to its own body):
auto x = []fib(int a) { return a > 1 ? fib(a - 1) + fib(a - 2) : a; };
Here, 'fib' is the equivalent of the lambda's *this (with some annoying
special rules to allow this to work despite the lambda's closure type being
incomplete).
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<div dir=3D"ltr"><div class=3D"gmail_extra"><div class=3D"gmail_quote">On W=
ed, Sep 30, 2015 at 11:05 AM, <span dir=3D"ltr"><<a href=3D"mailto:yego=
r.derevenets@gmail.com" target=3D"_blank">yegor.derevenets@gmail.com</a>>=
;</span> wrote:<br><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 =
..8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir=3D"ltr">Hi,<br><=
br>it looks like C++14 generic lambdas and Y combinator play really well to=
gether<br>when you want to write a recursive lambda. So, I wrote a draft pr=
oposal to<br>include Y combinator into the standard:<br><br>=C2=A0=C2=A0=C2=
=A0 <a href=3D"http://derevenets.com/public/yc-proposal.html" target=3D"_bl=
ank">http://derevenets.com/public/yc-proposal.html</a><br><br>Could you hav=
e a look and evaluate the idea?</div></blockquote><div><br></div><div>I thi=
nk this would be better as a first-class language feature. I ran out of tim=
e for the pre-Kona meeting, but I was intending on writing a paper to allow=
giving a lambda a name (scoped to its own body):</div><div><br></div><div>=
=C2=A0 auto x =3D []fib(int a) { return a > 1 ? fib(a - 1) + fib(a - 2) =
: a; };</div><div><br></div><div>Here, 'fib' is the equivalent of t=
he lambda's *this (with some annoying special rules to allow this to wo=
rk despite the lambda's closure type being incomplete).</div></div></di=
v></div>
<p></p>
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Author: Nicol Bolas <jmckesson@gmail.com>
Date: Wed, 30 Sep 2015 11:35:37 -0700 (PDT)
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In the Design Decisions section, it claims that `y_wrapper` would have the
ability to store a value or reference, depending on a template parameter.
By default it stores a value, but if you construct one yourself, it can
store a reference.
That doesn't seem feasible, not in the intended use case. Remember that
lambda functions have a name, but they are untypeable. Calling `std::y` (a
terrible function name, BTW) uses template argument deduction to figure out
what the lambda's type is. But there's no such thing as template argument
deduction when instantiating a template class like `y_wrapper`. So... how
do you do this:
auto fib = y_wrapper<???, use_reference>([]...);
What goes in the ??? part?
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<div dir=3D"ltr">In the Design Decisions section, it claims that `y_wrapper=
` would have the ability to store a value or reference, depending on a temp=
late parameter. By default it stores a value, but if you construct one your=
self, it can store a reference.<br><br>That doesn't seem feasible, not =
in the intended use case. Remember that lambda functions have a name, but t=
hey are untypeable. Calling `std::y` (a terrible function name, BTW) uses t=
emplate argument deduction to figure out what the lambda's type is. But=
there's no such thing as template argument deduction when instantiatin=
g a template class like `y_wrapper`. So... how do you do this:<br><br><div =
class=3D"prettyprint" style=3D"background-color: rgb(250, 250, 250); border=
-color: rgb(187, 187, 187); border-style: solid; border-width: 1px; word-wr=
ap: break-word;"><code class=3D"prettyprint"><div class=3D"subprettyprint">=
<span style=3D"color: #008;" class=3D"styled-by-prettify">auto</span><span =
style=3D"color: #000;" class=3D"styled-by-prettify"> fib </span><span style=
=3D"color: #660;" class=3D"styled-by-prettify">=3D</span><span style=3D"col=
or: #000;" class=3D"styled-by-prettify"> y_wrapper</span><span style=3D"col=
or: #660;" class=3D"styled-by-prettify"><???,</span><span style=3D"color=
: #000;" class=3D"styled-by-prettify"> use_reference</span><span style=3D"c=
olor: #660;" class=3D"styled-by-prettify">>([]...);</span></div></code><=
/div><br>What goes in the ??? part?<br></div>
<p></p>
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.
Author: Yegor Derevenets <yegor.derevenets@gmail.com>
Date: Wed, 30 Sep 2015 20:56:28 +0200
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On Wed, Sep 30, 2015 at 11:35:37AM -0700, Nicol Bolas wrote:
> By default it stores a value, but if you construct one yourself, it can
> store a reference.
>
> That doesn't seem feasible, not in the intended use case.
I meant something like this:
auto almost_fib = [](auto fib, int x) -> int {
return x <= 1 ? x : fib(x - 1) + fib(x - 2);
};
auto fib = std::y_wrapper<decltype(almost_fib) &>(almost_fib);
std::cout << fib(42) << std::endl;
Perhaps, I should add this example to the paper.
Naming is not an important issue for now. std::y is good enough for the
start, we can always pick a better name later, once we agree we need
this function in the library.
--
Yegor Derevenets
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.