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digitalmars.D.learn - mixed in struct constructor is ignored when another (non mixed in)

reply ParticlePeter <ParticlePeter gmx.de> writes:
mixin template Common() {
   private int m_member;
   this( int m ) { m_member = m; }
}

struct Foo {
   mixin Common;
}

struct Bar {
   mixin Common;
   this( int m, float n ) { m_member = m * n; }
}


auto foo = Foo(1);       // ok
auto b_1 = Bar( 1, 2 );  // ok
auto b_2 = Bar( 3 );     // Error: constructor main.Bar.this (int 
m, int n) is not callable using argument types (int)

Is this expected behavior?
Feb 26 2018
next sibling parent reply Jonathan M Davis <newsgroup.d jmdavisprog.com> writes:
On Monday, February 26, 2018 12:30:24 ParticlePeter via Digitalmars-d-learn 
wrote:

 mixin template Common() {
    private int m_member;
    this( int m ) { m_member = m; }
 }

 struct Foo {
    mixin Common;
 }

 struct Bar {
    mixin Common;
    this( int m, float n ) { m_member = m * n; }
 }


 auto foo = Foo(1);       // ok
 auto b_1 = Bar( 1, 2 );  // ok
 auto b_2 = Bar( 3 );     // Error: constructor main.Bar.this (int
 m, int n) is not callable using argument types (int)

 Is this expected behavior?


Yes. Stuff that's mixed in is treated as having a different scope than other
mixins or stuff that wasn't mixed in. To quote towards the bottom of here:

https://dlang.org/spec/template-mixin.html

"Alias declarations can be used to overload together functions declared in
different mixins"

It gives an example of

mixin Foo!() F;
mixin Bar!() B;

alias func = F.func;
alias func = B.func;

I don't know if you can do the same thing with constructors or not, though
alias this statements don't allow the = syntax. So, maybe that won't
conflict, and it will work to do something like

alias this = Common.this;

If that doesn't work, then it seems like a good enhancement request.

- Jonathan M Davis
Feb 26 2018
parent reply ParticlePeter <ParticlePeter gmx.de> writes:
On Monday, 26 February 2018 at 12:47:48 UTC, Jonathan M Davis 
wrote:

 On Monday, February 26, 2018 12:30:24 ParticlePeter via 
 Digitalmars-d-learn wrote:

 mixin template Common() {
    private int m_member;
    this( int m ) { m_member = m; }
 }

 struct Foo {
    mixin Common;
 }

 struct Bar {
    mixin Common;
    this( int m, float n ) { m_member = m * n; }
 }


 auto foo = Foo(1);       // ok
 auto b_1 = Bar( 1, 2 );  // ok
 auto b_2 = Bar( 3 );     // Error: constructor main.Bar.this 
 (int
 m, int n) is not callable using argument types (int)

 Is this expected behavior?


 Yes. Stuff that's mixed in is treated as having a different 
 scope than other mixins or stuff that wasn't mixed in. To quote 
 towards the bottom of here:

 https://dlang.org/spec/template-mixin.html

 "Alias declarations can be used to overload together functions 
 declared in different mixins"

 It gives an example of

 mixin Foo!() F;
 mixin Bar!() B;

 alias func = F.func;
 alias func = B.func;

 I don't know if you can do the same thing with constructors or 
 not, though alias this statements don't allow the = syntax. So, 
 maybe that won't conflict, and it will work to do something like

 alias this = Common.this;

 If that doesn't work, then it seems like a good enhancement 
 request.

 - Jonathan M Davis


Thanks for clarification, unfortunately your suggestion doesn't 
work.
Since when is alias this = something; supposed to work?
alias Common.this this; doesn't work as well and following also 
not:

struct Baz {
   Foo foo;
   alias foo this;
   this( int m, int n ) { m_member = m * n; }
}

auto baz = Baz(1); // Error, same as above

Not sure if I do require an enhancement, I just stumbled on that 
and was wondering.
Feb 26 2018
parent Simen =?UTF-8?B?S2rDpnLDpXM=?= <simen.kjaras gmail.com> writes:
On Monday, 26 February 2018 at 13:09:40 UTC, ParticlePeter wrote:

 Thanks for clarification, unfortunately your suggestion doesn't 
 work.
 Since when is alias this = something; supposed to work?
 alias Common.this this; doesn't work as well and following also 
 not:

 struct Baz {
   Foo foo;
   alias foo this;
   this( int m, int n ) { m_member = m * n; }
 }

 auto baz = Baz(1); // Error, same as above

 Not sure if I do require an enhancement, I just stumbled on 
 that and was wondering.


The workaround for now is to define your other constructor in 
another mixin template:

struct Bar {
     mixin Common;

     mixin template Common2() {this( int m, float n ) { m_member = 
cast(int)(m * n); } };
     mixin Common2;
}

It's not exactly pretty, but it works.

An issue has already been filed for this: 
https://issues.dlang.org/show_bug.cgi?id=17055.

--
   Simen
Feb 26 2018
prev sibling parent reply Adam D. Ruppe <destructionator gmail.com> writes:
On Monday, 26 February 2018 at 12:30:24 UTC, ParticlePeter wrote:

 Is this expected behavior?


yes sort of, but there are bugs associated with it too...

I wrote about this in the "Tip of the Week" section here before 
http://arsdnet.net/this-week-in-d/2016-feb-07.html

there's a workaround there and a bit more explanation of 
weirdness.
Feb 26 2018
parent reply ParticlePeter <ParticlePeter gmx.de> writes:
On Monday, 26 February 2018 at 14:02:56 UTC, Adam D. Ruppe wrote:

 On Monday, 26 February 2018 at 12:30:24 UTC, ParticlePeter 
 wrote:

 Is this expected behavior?


 yes sort of, but there are bugs associated with it too...

 I wrote about this in the "Tip of the Week" section here before 
 http://arsdnet.net/this-week-in-d/2016-feb-07.html

 there's a workaround there and a bit more explanation of 
 weirdness.


This cool, I didn't know that we can name mixins when 
instantiating but also never taught that there could be any 
purpose for naming. Works, thanks.
Feb 26 2018
parent reply Adam D. Ruppe <destructionator gmail.com> writes:
On Monday, 26 February 2018 at 14:38:22 UTC, ParticlePeter wrote:

 This cool, I didn't know that we can name mixins when 
 instantiating but also never taught that there could be any 
 purpose for naming. Works, thanks.


oh yes, there's a lot of cool things with mixin. You might want 
to skim my tip of the week index: 
http://arsdnet.net/this-week-in-d/totw-index.html and see if more 
jump out at you.

template mixins have behavior that look stupid until you 
understand why - then it gets pretty nice. Like the naming and 
overloading rules allow selective overriding and integration once 
you know how.
Feb 26 2018
parent ParticlePeter <ParticlePeter gmx.de> writes:
On Monday, 26 February 2018 at 14:42:58 UTC, Adam D. Ruppe wrote:

 On Monday, 26 February 2018 at 14:38:22 UTC, ParticlePeter 
 wrote:

 This cool, I didn't know that we can name mixins when 
 instantiating but also never taught that there could be any 
 purpose for naming. Works, thanks.


 oh yes, there's a lot of cool things with mixin. You might want 
 to skim my tip of the week index: 
 http://arsdnet.net/this-week-in-d/totw-index.html and see if 
 more jump out at you.

 template mixins have behavior that look stupid until you 
 understand why - then it gets pretty nice. Like the naming and 
 overloading rules allow selective overriding and integration 
 once you know how.


Ok and that is an awesome link as well, I wished having this on 
several occasions. By the way, I also remembered reading your tip 
already once ... yeah, while reading it again. Anyway, thanks.
Feb 26 2018