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From: Eric Niebler <eric.niebler@gmail.com>
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Subject: template template aliases?
Date: Mon, 30 Jan 2017 09:24:00 -0800
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Crazy thought.

Why not permit template template aliases?

  template <class A>
  template <class B>
  using Foo = pair<A, B>;

Foo<A> is a unary template such that Foo<A><B> is pair<A,B>.

Why?

Higher order template metaprogramming sometimes requires passing a template
to a template. For example:

  // with C++17 fold expressions
  template <template <class Ts> class Pred, class...Ts>
  constexpr bool all_of = (... && Pred<Ts>::value);

  static_assert(all_of<std::is_scalar, int, short, float>);

Q: How would you use all_of to check that all types in a pack are
convertible to bool?
A: With a "bind"-like template template alias:

  template <template<class...> class Fn, class...As>
  template <class...Bs>
  using bind_back = Fn<Bs..., As...>;

  template <template<class...> class Fn>
  template <class T>
  using unary = Fn<T>;

  static_assert(all_of<unary<bind_back<std::is_convertible, bool>>, int,
short, float>);

What would this language feature break?

Thanks,
Eric

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<div dir=3D"ltr">Crazy thought.<div><br></div><div>Why not permit template =
template aliases?</div><div><br></div><div>=C2=A0 template &lt;class A&gt;<=
/div><div>=C2=A0 template &lt;class B&gt;</div><div>=C2=A0 using Foo =3D pa=
ir&lt;A, B&gt;;</div><div><br></div><div>Foo&lt;A&gt; is a unary template s=
uch that Foo&lt;A&gt;&lt;B&gt; is pair&lt;A,B&gt;.</div><div><br></div><div=
>Why?</div><div><br></div><div>Higher order template metaprogramming someti=
mes requires passing a template to a template. For example:<div><br></div><=
div>=C2=A0 // with C++17 fold expressions</div><div>=C2=A0 template &lt;tem=
plate &lt;class Ts&gt; class Pred, class...Ts&gt;</div><div>=C2=A0 constexp=
r bool all_of =3D (... &amp;&amp; Pred&lt;Ts&gt;::value);</div><div><br></d=
iv><div>=C2=A0 static_assert(all_of&lt;std::is_scalar, int, short, float&gt=
;);</div><div><br></div><div>Q: How would you use all_of to check that all =
types in a pack are convertible to bool?</div><div>A: With a &quot;bind&quo=
t;-like template template alias:</div><div><br></div><div>=C2=A0 template &=
lt;template&lt;class...&gt; class Fn, class...As&gt;</div><div>=C2=A0 templ=
ate &lt;class...Bs&gt;</div><div>=C2=A0 using bind_back =3D Fn&lt;Bs..., As=
....&gt;;</div><div><br></div><div>=C2=A0 template &lt;template&lt;class...&=
gt; class Fn&gt;</div><div>=C2=A0 template &lt;class T&gt;</div><div>=C2=A0=
 using unary =3D Fn&lt;T&gt;;</div><div><br></div><div>=C2=A0 static_assert=
(all_of&lt;unary&lt;bind_back&lt;std::is_convertible, bool&gt;&gt;, int, sh=
ort, float&gt;);</div><div><br></div><div>What would this language feature =
break?</div></div><div><br></div><div>Thanks,</div><div>Eric</div><div><br>=
</div></div>

<p></p>

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