I want to use an expression and put it in place inside the if parentheses. The expression is: is(typeof(val) == type). I want to use a template called "is_same" that will take the value and a type to place them to the respective places. I have tried the following but it doesn't seem to work:

mixin template is_same(val, type) {
  is(typeof(val) == type)
}

void main() {
  int val = 10;
  static if (is_same!(val, int)) {}
}

When trying to compile, I'm taking the following error message:

Error: declaration expected, not `is`

Is this a limitation of templates in D or is there a way to bypass this?

It isn't really about limitation of templates. You're trying to use mixin template and it's main purpose is to inject declarations. If you want to replace is expression with template you could use something like this:

bool is_same(alias value, T)() {
    return is(typeof(value) == T);
    }

void main() {
    int value = 10;
    static if (is_same!(value, int)) {
        writeln("it is true!");
    } else {
        writeln("it is false!");
    }
}

Personally, I don't see any benefit with replacing that kind of is expressions with templates. Perhaps I'm missing something :)

On Friday, 17 December 2021 at 08:44:39 UTC, Mitacha wrote:

bool is_same(alias value, T)() {
    return is(typeof(value) == T);
    }

void main() {
    int value = 10;
    static if (is_same!(value, int)) {
        writeln("it is true!");
    } else {
        writeln("it is false!");
    }
}

Oh! That's nice! I didn't even knew it was possible to create template functions like this! Thanks!

The benefits are typing less code and make it more readable and easy on the eyes ;)

I want to use an expression and put it in place inside the if parentheses. The expression is: is(typeof(val) == type). I want to use a template called "is_same" that will take the value and a type to place them to the respective places. I have tried the following but it doesn't seem to work:

mixin template is_same(val, type) {
  is(typeof(val) == type)
}

void main() {
  int val = 10;
  static if (is_same!(val, int)) {}
}

When trying to compile, I'm taking the following error message:

Error: declaration expected, not `is`

Is this a limitation of templates in D or is there a way to bypass this?

There is also a compiler trait [1] which can do that for you:

void main()
{
    int val = 10;
    static if (__traits(isSame, typeof(val), int)) {}
}

[1] https://dlang.org/spec/traits.html#isSame