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Subject: Re: Making the new expression smart
From: Richard Hodges (hodges.r_at_[hidden])
Date: 2020-09-18 01:34:40

On Thu, 17 Sep 2020 at 23:52, Ville Voutilainen <ville.voutilainen_at_[hidden]>

> On Thu, 17 Sep 2020 at 23:51, Richard Hodges via Std-Discussion
> <std-discussion_at_[hidden]> wrote:
> Personally I tend to prefer keywords that express intent rather than
> library functions,
> Why?

Thanks for asking. My position is pretty simple. It’s about elegance,
readability and ultimately, performance.

*Elegance & readability:*
Library functions are wordy, namespaced and by definition, a compromise
when seeking to express intent to both programmer and compiler. This
inevitably results in wordiness, preconditions that are left unchecked, the
possibility of UB and complicated syntax that Is expressed in and limited
by the syntax of templates and function calls. A keyword with a specific
job allows expression in terms of precise intent.

E.g Compare (ignoring current syntax transgressions):

auto x = std::make_tuple(foo, bar);


auto x = { foo, bar }; // shorter and now a language type


auto x = tuple [ foo, &bar]; // bar is a tie


auto x = tuple & [ foo, bar ]; // equivalent to std::tie

In the above trivial example, the presence of the name std::make_tuple adds
almost no information. It also creates a type with a long and unwieldy name
(that actually causes some compilers to run out of string space!) and an
eye-wateringly complex implementation. The presence of std:: adds
absolutely none as a tuple is a globally understood concept. The last two
examples suggest a similar capture model to that of lambdas, which provides

Expression through library templates means two levels of translation of
intent. One from user intent to expression in terms of existing library
code and (sparse) core language features. There is plenty of opportunity
for missing concepts in the language to limit the quality of that
On the other hand, a keyword with code generation behind it means that a
users’ specific intent can be translated into optimal code every time.
Furthermore, as compiler technology improves, that translation can be
improved, improving every program upon recompilation.
Of course this is partially true of libraries, but the library still
suffers from not having access to direct code generation with knowledge of
the absolute intent of the programmer.

Many modern library features rely on compiler intrinsics - from the
programmer’s point of view these are required but undocumented language
extensions. This is untenable at scale. Better, in my view, to have no
intrinsics and simply extend the language to include these obviously
essential features. Once that is done, why bother with the complication of
a library? Make a tuple a compiler-supported type. Provide class reflection
as a language feature. Then it can be easily documented, maintained and
shipped with no secret interdependency between compiler and standard

> ..in my never-ending quest for rationale, as opposed to just blanket
> statements. ;)

Richard Hodges
office: +442032898513
home: +376841522
mobile: +376380212

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