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Subject: Re: Reducing the number of numeric algorithms in namespace ranges
From: Yehezkel Bernat (yehezkelshb_at_[hidden])
Date: 2019-06-25 10:47:07

On Tue, Jun 25, 2019 at 5:49 PM Christopher Di Bella via SG20 <
sg20_at_[hidden]> wrote:

> Are there any practical reasons for reduce(begin(v), end(v), 0, plus{})
>> to permit out-of-order computation? For example, is this allowed as an
>> implementation?
>>
>> ```
>> template<class InputIterator, class T, class BOp>
>> T reduce(InputIterator first, InputIterator last, T init, BOp bop)
>> {
>> #if NDEBUG
>> return reduce(execution::par_unseq, first, last, init, bop);
>> #else
>> return accumulate(first, last, init, bop);
>> #endif // NDEBUG
>> }
>> ```
>>
>
My understanding is that it isn't allowed.
What the standard mentions explicitly is:

[ Note <http://eel.is/c++draft/reduce#8.note-1>
:
The difference between reduce and accumulate is that reduce applies
binary_Â­op in an unspecified order, which yields a nondeterministic result
for non-associative or non-commutative binary_Â­op such as floating-point
addition. <http://eel.is/c++draft/reduce#8.sentence-1>
â€” *end note*
]
http://eel.is/c++draft/reduce#8

So this is equivalent to passing `std::execution::seq`, which allows
invocations that are "indeterminately sequenced". This is different than
`accumulate`, which is guaranteed to perform a left fold.

> If either question can be answered with "yes", then forgetting accumulate
>> is a non-starter,
>>
>
I'd think the question should go the opposite direction:
What are the reasons to *forbid* out-of-order computation (which is the
only reason to keep having accumulate around)?
The numerical algorithms are intended for numerical usages.
Maybe using a numerical type that isn't associative or isn't commutative is
a task that shouldn't be done by a generic algorithm anyway?
Using other types (e.g. `std::string`; its operator+ isn't commutative of
course) is not what the algorithm was designed for (this is why it is in
`numeric` instead of `algorithm`) so maybe we are fine with disallowing
such a usage going forward.

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