• Steve Teale (4/4) Mar 06 2014 Pretty much what the subject says. Why can't template mixins
• Frustrated (38/42) Mar 06 2014 template mixins mix in directly into the code as if you typed
• Gary Willoughby (15/59) Mar 07 2014 Interesting. I regularly use template mixins referring to 'this'
• Dejan Lekic (6/6) Mar 06 2014 Template mixins can't contain statements, only declarations, because the...
• Steve Teale (20/26) Mar 07 2014 If I side-step slightly, the compiler does not appear to have

"Steve Teale" <steve.teale britseyeview.com> writes:

Pretty much what the subject says. Why can't template mixins 
include statements ans so on?

Is it just too hard, or is it just too much like C macros?

Steve

"Frustrated" <Frustrated nowhere.com> writes:

On Thursday, 6 March 2014 at 17:27:35 UTC, Steve Teale wrote:
 Pretty much what the subject says. Why can't template mixins 
 include statements ans so on?

 Is it just too hard, or is it just too much like C macros?

 Steve

template mixins mix in directly into the code as if you typed 
them. If they contained statements then you could mixin 
statements into classes, say, and it would then be illegal.

I guess there is no reason per se, but I guess that wasn't the 
desired behavior for template mixins. I imagine there could be a 
definite downside to having template mixins containing 
statements. Also, they can't be self contained.

e.g.,

mixin template C()
{
    i = i + 1;  // invalid
}

...

int i = 0;
mixin C();


The template itself can't be semantically checked in place 
because i is unknown inside the template. (it is not self 
contained so to speak)

In any case, just seems wrong for templates to do that. They are 
not grouping expressions but grouping definitions and 
declarations of things so you don't have to do them multiple 
times.

string mixins, OTOH, could do the above.

template C()
{
     string C()
     {
         return "i = i + 1;";
     }
}

...

int i = 0;
mixin(C);

and this will work. This is because the statement is contained 
within a string and the compiler simply inserts the string 
directly. The template can still be validated in place(since "i = 
i + 1" is a string and has no other meaning in the template).

"Gary Willoughby" <dev nomad.so> writes:

On Thursday, 6 March 2014 at 18:31:02 UTC, Frustrated wrote:
 On Thursday, 6 March 2014 at 17:27:35 UTC, Steve Teale wrote:
 Pretty much what the subject says. Why can't template mixins 
 include statements ans so on?

 Is it just too hard, or is it just too much like C macros?

 Steve

 template mixins mix in directly into the code as if you typed 
 them. If they contained statements then you could mixin 
 statements into classes, say, and it would then be illegal.

 I guess there is no reason per se, but I guess that wasn't the 
 desired behavior for template mixins. I imagine there could be 
 a definite downside to having template mixins containing 
 statements. Also, they can't be self contained.

 e.g.,

 mixin template C()
 {
    i = i + 1;  // invalid
 }

 ...

 int i = 0;
 mixin C();


 The template itself can't be semantically checked in place 
 because i is unknown inside the template. (it is not self 
 contained so to speak)

 In any case, just seems wrong for templates to do that. They 
 are not grouping expressions but grouping definitions and 
 declarations of things so you don't have to do them multiple 
 times.

 string mixins, OTOH, could do the above.

 template C()
 {
     string C()
     {
         return "i = i + 1;";
     }
 }

 ...

 int i = 0;
 mixin(C);

 and this will work. This is because the statement is contained 
 within a string and the compiler simply inserts the string 
 directly. The template can still be validated in place(since "i 
 = i + 1" is a string and has no other meaning in the template).

Interesting. I regularly use template mixins referring to 'this' 
and they work fine. e.g.:

mixin template Bar()
{
     public int getFoo()
     {
         return this.foo;
     }
}

class Foo
{
     private int foo;
     mixin Bar;
}

Dejan Lekic <dejan.lekic gmail.com> writes:

Template mixins can't contain statements, only declarations, because they 
(template mixins) are a way to inject code into the context.

Therefore it makes sense to forbid statements, as they can't appear in ANY 
context.

-- 
http://dejan.lekic.org

"Steve Teale" <steve.teale britseyeview.com> writes:

On Thursday, 6 March 2014 at 18:36:12 UTC, Dejan Lekic wrote:
 Template mixins can't contain statements, only declarations, 
 because they
 (template mixins) are a way to inject code into the context.

 Therefore it makes sense to forbid statements, as they can't 
 appear in ANY
 context.

If I side-step slightly, the compiler does not appear to have 
difficulty coping:

import std.stdio;
import std.conv;

string codeString(string A, string B, int I)()
{
    enum n = I*3;
    return "writeln(\""~A~"\"); writeln(\""~B~"\"); 
writefln(\"%d\","~to!string(n)~");";
}

void foo()
{
    mixin(codeString!("One", "Two", 1)());
    writeln("Hello world");
}

void main()
{
    foo();
}