Re: Let this() figure out T implicitly?

[bearophile]

kraybourne:

> Then it compiles. Is it possible to have this() figure out the type some 
> way?

Usually people write a small global function helper.


> (Side note: What _does_ that error message mean? I don't get it.)

I think the meaning is: when a class template is not yet instantiated, it's not 
a type yet.

Bye,
bearophile

Re: Let this() figure out T implicitly?

[Kevin Cox]

The error message is saying that you are trying to use Foo as a type but
Foo is not a type, it is a template for a type.
On Feb 17, 2012 7:20 AM, "kraybourne"  wrote:

> Hi!
>
> This doesn't work:
>
>import std.stdio;
>class Foo(T)
>{
>T t;
>this(T val)
>{
>t = val;
>}
>}
>
>void main()
>{
>auto o = new Foo(5);
>}
>_
>
>$ dmd foo
>foo.d(13): Error: class foo.Foo(T) is used as a type
>$ _
>
> So I must
>
>auto o = new Foo!(int)(5);
>
> Then it compiles. Is it possible to have this() figure out the type some
> way? (In this particular example it's perhaps not such a big deal. But
> imagine a lot more args.)
>
> (Side note: What _does_ that error message mean? I don't get it.)
>
> thanks
> /krbrn
>

Re: Let this() figure out T implicitly?

[kraybourne]

On 2/17/12 1:32 PM, bearophile wrote:

kraybourne:


Then it compiles. Is it possible to have this() figure out the type some
way?


Usually people write a small global function helper.



Hm, so, something like this:

Foo!(T) newFoo(T)(T val)
{
return new Foo!(T)(val);
}
...
auto o = newFoo(5);

Works! Ha! How silly.
Thanks!






(Side note: What _does_ that error message mean? I don't get it.)


I think the meaning is: when a class template is not yet instantiated, it's not 
a type yet.

Bye,
bearophile

Re: Let this() figure out T implicitly?

[kraybourne]

On 2/17/12 1:51 PM, Kevin Cox wrote:

The error message is saying that you are trying to use Foo as a type but
Foo is not a type, it is a template for a type.



Ah, so module.Foo is really not a class, but a template? I think I get 
it! Thanks!


(Is then module.Foo(int).Foo the actual class type? I think I've seen 
errors like that pop up...)




On Feb 17, 2012 7:20 AM, "kraybourne" mailto:st...@kraybourne.com>> wrote:

Hi!

This doesn't work:

import std.stdio;
class Foo(T)
{
T t;
this(T val)
{
t = val;
}
}

void main()
{
auto o = new Foo(5);
}
_

$ dmd foo
foo.d(13): Error: class foo.Foo(T) is used as a type
$ _

So I must

auto o = new Foo!(int)(5);

Then it compiles. Is it possible to have this() figure out the type
some way? (In this particular example it's perhaps not such a big
deal. But imagine a lot more args.)

(Side note: What _does_ that error message mean? I don't get it.)

thanks
/krbrn

Re: Let this() figure out T implicitly?

[Kevin Cox]

Yes.  At least as the compiler would say.  It's a little odd but I believe
that is how the D Nam mangling works.  I personally just think of
Foo!(Class) as the type.
On Feb 17, 2012 8:05 AM, "kraybourne"  wrote:

> On 2/17/12 1:51 PM, Kevin Cox wrote:
>
>> The error message is saying that you are trying to use Foo as a type but
>> Foo is not a type, it is a template for a type.
>>
>>
> Ah, so module.Foo is really not a class, but a template? I think I get it!
> Thanks!
>
> (Is then module.Foo(int).Foo the actual class type? I think I've seen
> errors like that pop up...)
>
>
>  On Feb 17, 2012 7:20 AM, "kraybourne" > > wrote:
>>
>>Hi!
>>
>>This doesn't work:
>>
>>import std.stdio;
>>class Foo(T)
>>{
>>T t;
>>this(T val)
>>{
>>t = val;
>>}
>>}
>>
>>void main()
>>{
>>auto o = new Foo(5);
>>}
>>_
>>
>>$ dmd foo
>>foo.d(13): Error: class foo.Foo(T) is used as a type
>>$ _
>>
>>So I must
>>
>>auto o = new Foo!(int)(5);
>>
>>Then it compiles. Is it possible to have this() figure out the type
>>some way? (In this particular example it's perhaps not such a big
>>deal. But imagine a lot more args.)
>>
>>(Side note: What _does_ that error message mean? I don't get it.)
>>
>>thanks
>>/krbrn
>>
>>
>

Re: Let this() figure out T implicitly?

[Timon Gehr]

On 02/17/2012 02:07 PM, Kevin Cox wrote:

Yes.  At least as the compiler would say.  It's a little odd but I
believe that is how the D Nam mangling works.  I personally just think
of Foo!(Class) as the type.



class Foo(T){ ... }

Is syntactic sugar for

template Foo(T){
class Foo{ ... }
}

Therefore the type is Foo!(Class).Foo. I'd prefer the compiler to output 
Foo!(Class) in error messages and for .stringof though, vote here:


http://d.puremagic.com/issues/show_bug.cgi?id=7064

Re: Let this() figure out T implicitly?

[kraybourne]

On 2/17/12 2:38 PM, Timon Gehr wrote:

On 02/17/2012 02:07 PM, Kevin Cox wrote:

Yes. At least as the compiler would say. It's a little odd but I
believe that is how the D Nam mangling works. I personally just think
of Foo!(Class) as the type.



class Foo(T){ ... }

Is syntactic sugar for

template Foo(T){
class Foo{ ... }
}

Therefore the type is Foo!(Class).Foo.


Aaah, I see, clever, thx


I'd prefer the compiler to output
Foo!(Class) in error messages and for .stringof though, vote here:

http://d.puremagic.com/issues/show_bug.cgi?id=7064

Re: Let this() figure out T implicitly?

[Steven Schveighoffer]

On Fri, 17 Feb 2012 07:19:28 -0500, kraybourne   
wrote:



Hi!

This doesn't work:

import std.stdio;
class Foo(T)
{
T t;
this(T val)
{
t = val;
}
}

void main()
{
auto o = new Foo(5);
}
_

$ dmd foo
foo.d(13): Error: class foo.Foo(T) is used as a type
$ _

So I must

auto o = new Foo!(int)(5);

Then it compiles. Is it possible to have this() figure out the type some  
way? (In this particular example it's perhaps not such a big deal. But  
imagine a lot more args.)


What you are asking for is IFTI (Implicit Function Template Instantiation)  
on constructors, and is perfectly possible, but not implemented:


http://d.puremagic.com/issues/show_bug.cgi?id=6082

Vote up if you want to see it happen!

-Steve

Re: Let this() figure out T implicitly?

[Ali Çehreli]

On 02/17/2012 09:08 AM, Steven Schveighoffer wrote:

> What you are asking for is IFTI (Implicit Function Template
> Instantiation) on constructors, and is perfectly possible, but not
> implemented:
>
> http://d.puremagic.com/issues/show_bug.cgi?id=6082

What was the resolution for the case when the constructor is a template? 
I think that's why IFTI doesn't work for structs and classes in C++. 
Although, I can't get the following to compile in D anyway (that old and 
annoying error message again! :p):


  Error: no property 'opCall' for type 'deneme.B'

import std.conv;

class B
{
string s;

this(T)(T t)   // <-- Is the struct a template
   // or the constructor a template?
{
s = to!string(t);
}
}

void main()
{
auto b0 = B(42);
}

I wasn't around when the static opCall() was designed but it is probably 
the very first thing that bugged me about D. :) I want B(42) to be 
object construction, not opCall(), which is not even defined.


Ali

Re: Let this() figure out T implicitly?

[Timon Gehr]

On 02/18/2012 12:04 AM, Ali Çehreli wrote:

On 02/17/2012 09:08 AM, Steven Schveighoffer wrote:

 > What you are asking for is IFTI (Implicit Function Template
 > Instantiation) on constructors, and is perfectly possible, but not
 > implemented:
 >
 > http://d.puremagic.com/issues/show_bug.cgi?id=6082

What was the resolution for the case when the constructor is a template?
I think that's why IFTI doesn't work for structs and classes in C++.
Although, I can't get the following to compile in D anyway (that old and
annoying error message again! :p):

   Error: no property 'opCall' for type 'deneme.B'

import std.conv;

class B
{
 string s;

 this(T)(T t)   // <-- Is the struct a template
// or the constructor a template?


The constructor it is, and B is a class, not a struct.



 {
 s = to!string(t);
 }
}

void main()
{
 auto b0 = B(42);
}

I wasn't around when the static opCall() was designed but it is probably
the very first thing that bugged me about D. :) I want B(42) to be
object construction, not opCall(), which is not even defined.

Ali



Why? What useful semantics would that have for classes?

Re: Let this() figure out T implicitly?

[Ali Çehreli]

On 02/17/2012 05:59 PM, Timon Gehr wrote:
> On 02/18/2012 12:04 AM, Ali Çehreli wrote:
>> On 02/17/2012 09:08 AM, Steven Schveighoffer wrote:
>>
>> > What you are asking for is IFTI (Implicit Function Template
>> > Instantiation) on constructors, and is perfectly possible, but not
>> > implemented:
>> >
>> > http://d.puremagic.com/issues/show_bug.cgi?id=6082
>>
>> What was the resolution for the case when the constructor is a template?
>> I think that's why IFTI doesn't work for structs and classes in C++.
>> Although, I can't get the following to compile in D anyway (that old and
>> annoying error message again! :p):
>>
>> Error: no property 'opCall' for type 'deneme.B'
>>
>> import std.conv;
>>
>> class B
>> {
>> string s;
>>
>> this(T)(T t) // <-- Is the struct a template
>> // or the constructor a template?
>
> The constructor it is, and B is a class, not a struct.

Ah! Thanks!

So today the following works; and not B, but the constructor is a template:

import std.conv;

struct B
{
string s;

this(T)(T t)   // <-- constructor is a template
{
s = to!string(t);
}
}

void main()
{
auto b0 = B(42);   // construct with int
assert(b0.s == "42");

auto b1 = B(1.5);  // construct with double
assert(b1.s == "1.5");
}

With the proposed feature, the struct would be a template.

>> {
>> s = to!string(t);
>> }
>> }
>>
>> void main()
>> {
>> auto b0 = B(42);
>> }
>>
>> I wasn't around when the static opCall() was designed but it is probably
>> the very first thing that bugged me about D. :) I want B(42) to be
>> object construction, not opCall(), which is not even defined.
>>
>> Ali
>>
>
> Why? What useful semantics would that have for classes?

I am not sure but the point is, if function templates provide IFTI, then 
because of being functions the constructors could provide IFTI as well, 
as it does for the struct B above.


As an aside, I don't know why it is not the same with classes. I hope I 
am not again making an error in the following code. I have replaced 
'struct' with 'class' and inserted two 'new' keywords:


import std.conv;

class B
{
string s;

this(T)(T t)
{
s = to!string(t);
}
}

void main()
{
auto b0 = new B(42);   // line 61459
assert(b0.s == "42");

auto b1 = new B(1.5);  // line 61462
assert(b1.s == "1.5");
}

The compiler says:

deneme.d(61459): Error: no constructor for B
deneme.d(61462): Error: no constructor for B
deneme.d(61462): Warning: statement is not reachable

Ali

Re: Let this() figure out T implicitly?

[Timon Gehr]

On 02/18/2012 03:22 AM, Ali Çehreli wrote:

On 02/17/2012 05:59 PM, Timon Gehr wrote:
 > On 02/18/2012 12:04 AM, Ali Çehreli wrote:
 >> On 02/17/2012 09:08 AM, Steven Schveighoffer wrote:
 >>
 >> > What you are asking for is IFTI (Implicit Function Template
 >> > Instantiation) on constructors, and is perfectly possible, but not
 >> > implemented:
 >> >
 >> > http://d.puremagic.com/issues/show_bug.cgi?id=6082
 >>
 >> What was the resolution for the case when the constructor is a
template?
 >> I think that's why IFTI doesn't work for structs and classes in C++.
 >> Although, I can't get the following to compile in D anyway (that old
and
 >> annoying error message again! :p):
 >>
 >> Error: no property 'opCall' for type 'deneme.B'
 >>
 >> import std.conv;
 >>
 >> class B
 >> {
 >> string s;
 >>
 >> this(T)(T t) // <-- Is the struct a template
 >> // or the constructor a template?
 >
 > The constructor it is, and B is a class, not a struct.

Ah! Thanks!

So today the following works; and not B, but the constructor is a template:

import std.conv;

struct B
{
 string s;

 this(T)(T t)   // <-- constructor is a template
 {
 s = to!string(t);
 }
}

void main()
{
 auto b0 = B(42);   // construct with int
 assert(b0.s == "42");

 auto b1 = B(1.5);  // construct with double
 assert(b1.s == "1.5");
}

With the proposed feature, the struct would be a template.



With the proposed feature still the constructor would be a template. The 
proposed feature is just that IFTI for constructors should be able to 
deduce the parameters for the enclosing struct/class template 
additionally to just its own parameters.




 >> {
 >> s = to!string(t);
 >> }
 >> }
 >>
 >> void main()
 >> {
 >> auto b0 = B(42);
 >> }
 >>
 >> I wasn't around when the static opCall() was designed but it is
probably
 >> the very first thing that bugged me about D. :) I want B(42) to be
 >> object construction, not opCall(), which is not even defined.
 >>
 >> Ali
 >>
 >
 > Why? What useful semantics would that have for classes?

I am not sure but the point is, if function templates provide IFTI, then
because of being functions the constructors could provide IFTI as well,
as it does for the struct B above.

As an aside, I don't know why it is not the same with classes. I hope I
am not again making an error in the following code. I have replaced
'struct' with 'class' and inserted two 'new' keywords:

import std.conv;

class B
{
 string s;

 this(T)(T t)
 {
 s = to!string(t);
 }
}

void main()
{
 auto b0 = new B(42);   // line 61459
 assert(b0.s == "42");

 auto b1 = new B(1.5);  // line 61462
 assert(b1.s == "1.5");
}

The compiler says:

deneme.d(61459): Error: no constructor for B
deneme.d(61462): Error: no constructor for B
deneme.d(61462): Warning: statement is not reachable

Ali



This seems to be a bug.