[rust-dev] std::num::pow() is inadequate / language concepts

Patrick Walton pcwalton at mozilla.com
Fri Jul 25 10:02:38 PDT 2014

On 7/25/14 6:26 AM, Gregor Cramer wrote:
> And so the function call is as expected, like with other numeric types:
> pow(a) // a is BigInt
> But there is now a problem in this function definition, BigInt is given as
> a copy, and this is a software design issue (superfluous memory allocation).
> And this currently happens if the user is calling std::num::pow() with a
> numeric type like BigInt (apart from other performance penalties in pow()).

That solution doesn't work for generic code, because Rust doesn't do 
ad-hoc templates like C++. A function that is generic over the bigint 
and int "pow" functions has to have one signature for "pow". Otherwise 
you could get errors during template instantiation time, which is 
something Rust strictly avoids.

> That's what I've mentioned that the compiler should decide whether an
> argument is given by reference or by value.

That doesn't work. It would have numerous problems with the borrow 
check, etc.

> I try to show the problems if function specialization (overloading) is not
> supported.

Sorry, but it's not convincing to me.


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