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Re: [std-proposals] Named auto

From: Edward Catmur <ecatmur_at_[hidden]>
Date: Wed, 28 Sep 2022 18:48:35 +0100
On Wed, 28 Sept 2022 at 17:56, Keenan Horrigan <friedkeenan_at_[hidden]>

> > We already have syntax for function template parameter named deduction;
> we don't need another. What we don't have is a syntax for block scope named
> deduction.
> Well, we also currently have ways to get the types deduced through your
> syntax suggestions, they're just suboptimal (using decltype, remove_cvref
> etc.).

Pattern matching the type is very much suboptimal as well; it requires
creating helper metaclasses, alias templates etc., or abusing lambdas.

> Similarly, I find the current state of named function parameter deduction
> suboptimal. We have the normal 'template<...>' syntax, and then we have the
> terse syntax with auto-parameters. Right now I steer away from the terse
> syntax because when I deduce a type for a function parameter, I very often
> want to also name that deduced type, so I might as well use the
> 'template<...>' syntax. However, the terse syntax is more concise, and I
> would argue more readable were it not for the fact that it necessitates
> some use of decltype to name the type, and similarly may require
> remove_cvref etc. to be applied. I find this suboptimal, and would very
> much enjoy being able to use the terse syntax and some form of "named auto".

There's no reason that `auto<class T> T x` couldn't be accepted in function
template parameter position as well; it just feels redundant.

Also there's the issue that when supplying function template argument
explicitly (as f<A...>(a...)) to a terse function template it's not
entirely obvious which template argument corresponds to which argument. But
then again, named deduction wouldn't exactly worsen this issue.

Truthfully as well, I cannot recall a time where I've wanted to name a
> deduced type of a variable that wasn't a function parameter. Is this a
> common occurrence for others? It of course would be better to have syntax
> that allows for it, but I personally don't find a solution that doesn't
> allow named deduction with the abbreviated function template syntax
> appealing.

Yes, it's fairly common, and getting more so as previously callback-style
code (that could use lambda explicit template syntax) moves to coroutines.
It's frustrating when a nicely deduced, constrained and/or pattern matched
parameter becomes a coroutine local variable that now can only be matched
using the meagre `auto` syntax.

------- Original Message -------
> On Wednesday, September 28th, 2022 at 10:39 AM, Edward Catmur <
> ecatmur_at_[hidden]> wrote:
> On Wed, 28 Sept 2022 at 16:24, Keenan Horrigan <friedkeenan_at_[hidden]>
> wrote:
>> My issue with the 'using' syntax was that it seems redundant when used
>> with function parameter type deduction (because why wouldn't you just type
>> out the normal 'template<...>'), I don't see how your 'auto<class T> T var'
>> syntax addresses that beyond maybe putting the deduced entities closer to
>> their source? Not a hugely compelling reason to use it over normal template
>> parameters in my opinion.
> We already have syntax for function template parameter named deduction; we
> don't need another. What we don't have is a syntax for block scope named
> deduction.
>> ------- Original Message -------
>> On Wednesday, September 28th, 2022 at 10:08 AM, Edward Catmur via
>> Std-Proposals <std-proposals_at_[hidden]> wrote:
>> On Wed, 28 Sept 2022 at 15:14, Jason McKesson via Std-Proposals <
>> std-proposals_at_[hidden]> wrote:
>>> On Wed, Sep 28, 2022 at 10:06 AM S├ębastien Bini via Std-Proposals
>>> <std-proposals_at_[hidden]> wrote:
>>> >
>>> > How about a syntax similar to that of parameters then?
>>> >
>>> > template <class T> T var = foo();
>>> > template <class K, class V> std::pair<K const, V>& kv = *map.begin();
>>> // or [k, v]?
>>> > template <std::copyable T, auto I> std::array<T, I> arr = getArr();
>>> > template <std::size_t... I> [[maybe_unused]] std::index_sequence<I...>
>>> seq = std::index_sequence_for<Args...>();
>>> >
>>> > Then we don't need to reinvent the syntax for parameters.
>> That syntax is already in use for variable templates. Maybe named
>> deduction is at block scope and variable templates are at namespace scope,
>> but that kind of overloading of meaning is pretty horrible - `static` is
>> bad enough.
>> Let's re-examine the problem that people seem to want to solve.
>>> My understanding was that the principal problem was that you want to
>>> gain access to the actual typename that the placeholder deduction
>>> syntax yields, and sticking a `using name = decltype(var_name);` is
>>> both wordy and error-prone (I deliberately did that wrong, for
>>> example).
>>> The main reason people deduce types is because they don't want to have
>>> to type out the full name of the type
>> or because the type is unnameable, or because it's dependent, or because
>> the code you're calling could change its return type and so typing it out
>> is a hostage to fortune.
>>> (or because it's already
>>> mentioned in the initializing expression).
>> If you're using AAA with types on the RHS then you don't need to deduce
>> them again, so you wouldn't need named deduction.
>>> So it seems to me that
>>> injecting a bunch more words into such a declaration is...
>>> counter-productive.
>>> Yes, any of these declarations might be useful. But... are they really
>>> *that* useful? So useful that we want to type out (and read) all of
>>> that?
>> We are proposing that a single statement introduce not just potentially
>> multiple entity names, but also potentially multiple type aliases (also,
>> maybe, packs, templates, concepts). This is a novel challenge for compilers
>> and other tools, and it needs *some* introducing syntax to shunt the parser
>> down the new code paths; lexing is difficult enough as it is.
>> If `using` is too much typing, how about the introducer being `auto <`?
>> It seems that the lexer should be able to detect that pretty much
>> immediately.
>> auto<class T> T var = foo();
>> // etc.

Received on 2022-09-28 17:48:48