I am trying to learn D and (partly) replace my C++ projects with D. Lately in C++ I have become a fan of the type of functional programming discussed here:
http://blog.knatten.org/2012/11/02/efficient-pure-functional-programming-in-c-using-move-semantics/
and was wondering if something similar is possible in D.

Basically the idea is to define functions as follows:

std::vector<double> add_to_vector( double x, std::vector<double> &&v ) {
 v.push_back(x);
 return v;
}

and call it as follows:

std::vector<double> v;
v = add_to_vector( 1.2, std::move( v ) );

I know I could do the same by passing a reference value, but this makes it explicit that I am changing v, while being as efficient as passing a reference (since it just reuses the memory allocated to v instead of making a copy).

Is this possible in D? I had a look through many tutorials and some of the docs, but could not find out how to do it.

Any suggestions would be appreciated.

On 3/6/2014 8:21 PM, Edwin van Leeuwen wrote:

>
> and call it as follows:
>
> std::vector<double> v;
> v = add_to_vector( 1.2, std::move( v ) );
>
> I know I could do the same by passing a reference value, but this makes
> it explicit that I am changing v, while being as efficient as passing a
> reference (since it just reuses the memory allocated to v instead of
> making a copy).
>
> Is this possible in D? I had a look through many tutorials and some of
> the docs, but could not find out how to do it.
>
> Any suggestions would be appreciated.

See std.algorithm.move

http://dlang.org/phobos/std_algorithm.html#move

On Thursday, 6 March 2014 at 11:28:21 UTC, Mike Parker wrote:
>
> See std.algorithm.move
>

Thank you, can't believe I missed that. How do I specify that the function expects a temporary/xvalue (&&) parameter though?

Edwin van Leeuwen:

> Thank you, can't believe I missed that. How do I specify that the function expects a temporary/xvalue (&&) parameter though?

Thankfully D lacks the && operator.

Bye,
bearophile

On Thursday, 6 March 2014 at 12:01:25 UTC, bearophile wrote:
> Edwin van Leeuwen:
>
>> Thank you, can't believe I missed that. How do I specify that the function expects a temporary/xvalue (&&) parameter though?
>
> Thankfully D lacks the && operator.
>
> Bye,
> bearophile

But is also locks r-value references, and this is a problem.

On Thursday, 6 March 2014 at 11:49:51 UTC, Edwin van Leeuwen
wrote:
> On Thursday, 6 March 2014 at 11:28:21 UTC, Mike Parker wrote:
>>
>> See std.algorithm.move
>>
>
> Thank you, can't believe I missed that. How do I specify that the function expects a temporary/xvalue (&&) parameter though?

What are you trying to do? D is not C++11. Pure functional
programming in D follows a different path.

On 3/6/2014 8:49 PM, Edwin van Leeuwen wrote:
> On Thursday, 6 March 2014 at 11:28:21 UTC, Mike Parker wrote:
>>
>> See std.algorithm.move
>>
>
> Thank you, can't believe I missed that. How do I specify that the
> function expects a temporary/xvalue (&&) parameter though?

D doesn't have anything like that. The compiler will generally move for you when possible. Passing an RValue to a function will be done as a move. When returning structs from a function, they're moved. For LValues, it's up to the caller to make the move with std.algorithm.move. There's been a good deal of discussion here in the NG about this. Hit google with 'dlang move semantics' and you'll get some hits. There's also Ali's presentation (video[1] and slides[2]) from last year's DConf.

[1] http://www.youtube.com/watch?v=mPr2UspS0fE
[2] http://dconf.org/2013/talks/cehreli.pdf

On Thursday, 6 March 2014 at 12:08:35 UTC, Nicolas Sicard wrote:
> On Thursday, 6 March 2014 at 11:49:51 UTC, Edwin van Leeuwen
> wrote:
>> On Thursday, 6 March 2014 at 11:28:21 UTC, Mike Parker wrote:
>>>
>>> See std.algorithm.move
>>>
>>
>> Thank you, can't believe I missed that. How do I specify that the function expects a temporary/xvalue (&&) parameter though?
>
> What are you trying to do? D is not C++11. Pure functional
> programming in D follows a different path.

I guess what I am trying to do is allow code that looks like this:

v = add_to_vector( x, v );

while enforcing that v is never copied. I know there are other ways to do this, i.e.
void add_to_vector( x, ref v );

but for me the earlier pattern is more explicit about what is happening, while still being as efficient.

On Thursday, 6 March 2014 at 12:10:40 UTC, Mike Parker wrote:
> On 3/6/2014 8:49 PM, Edwin van Leeuwen wrote:
>> On Thursday, 6 March 2014 at 11:28:21 UTC, Mike Parker wrote:
>>>
>>> See std.algorithm.move
>>>
>>
>> Thank you, can't believe I missed that. How do I specify that the
>> function expects a temporary/xvalue (&&) parameter though?
>
> D doesn't have anything like that. The compiler will generally move for you when possible. Passing an RValue to a function will be done as a move. When returning structs from a function, they're moved. For LValues, it's up to the caller to make the move with std.algorithm.move. There's been a good deal of discussion here in the NG about this. Hit google with 'dlang move semantics' and you'll get some hits. There's also Ali's presentation (video[1] and slides[2]) from last year's DConf.
>
> [1] http://www.youtube.com/watch?v=mPr2UspS0fE
> [2] http://dconf.org/2013/talks/cehreli.pdf

Thank you for those links/pointers. I'll have a look at those.

On 3/6/2014 9:38 PM, Edwin van Leeuwen wrote:

> I guess what I am trying to do is allow code that looks like this:
>
> v = add_to_vector( x, v );
>
> while enforcing that v is never copied. I know there are other ways to
> do this, i.e.
> void add_to_vector( x, ref v );
>
> but for me the earlier pattern is more explicit about what is happening,
> while still being as efficient.
>

import std.stdio;

struct Foo {
    int x, y;
}

Foo add( Foo f, int val ) {
    writefln( "By val verision" );
    return Foo( f.x + val, f.y + val );
}

Foo add( ref Foo f, int val ) {
    writefln( "Ref version" );
    return Foo( f.x + val, f.y + val );
}

void main() {
    Foo f1 = Foo( 1, 2 );
    Foo f2 = add( f1, 1 ); // by ref version

    import std.algorithm : move;
    Foo f3 = add( f2.move, 4 ); // by val version, no copy

    // First is by ref, second is by val with no copy
    Foo f4 = f3.add( 2 ).add( 5 );
}