I'm trying to use mixins and enums to "expand" code in place but the results are not what I expected and I'm getting an weird error. I have created the smallest possible example to reproduce the error and it is the following:

enum base_digit(string ten, string sixteen, string two, string eight) = `
  if (base == 10)      i = ` ~ ten ~ `;
  else if (base == 16) i = ` ~ sixteen ~ `;
  else if (base == 2)  i = ` ~ two ~ `;
  else if (base == 8)  i = ` ~ eight ~ `;
`;

enum overflow_check(string type) = `
  if (num == ` ~ type ~ `) {
    min_num = true;
    ++num;
  }
`;

enum type_check(string which_if, string type, string signed,
    string ten, string sixteen, string two, string eight) =
  which_if ~ `(is_same!(num, ` ~ type ~ `)) {
    mixin(base_digit!("` ~ ten ~ `", "` ~ sixteen
          ~ `", "` ~ two ~ `", "` ~ eight ~ `"));
    static if (` ~ signed ~ `) {
      mixin(overflow_check!"` ~ type ~ `.min"); }
  }
`;

void test(bool signed)(int num, int base) {
  static if (signed) {
    mixin(type_check!("static if", "i8", "true", "5", "4", "10", "5"));
    mixin(type_check!("else static if", "i16", "true", "7", "6", "18", "8"));
    mixin(type_check!("else static if", "i32", "true", "12", "10", "34", "13"));
    mixin(type_check!("else", "i64", "true", "21", "18", "66", "24"));
  }

  else {
    mixin(type_check!("static if", "u8", "false", "4", "3", "9", "4"));
    mixin(type_check!("else static if", "u16", "false", "6", "5", "17", "7"));
    mixin(type_check!("else static if", "u32", "false", "11", "9", "33", "12"));
    mixin(type_check!("else", "u64", "false", "21", "17", "65", "23"));
  }
}

extern (C) void main() {
  test!true(10, 16);
}

In my normal code, the "test" function is actually part of an emum that is expanded from another function (told you, the whole code will just be to complicated for anyone to read). In the example, trying to compile will give the following error:

test.d-mixin-27(27): Error: template instance `is_same!(num, i8)` template `is_same` is not defined
test.d-mixin-28(28): Error: found `else` without a corresponding `if`, `version` or `debug` statement
test.d-mixin-29(29): Error: found `else` without a corresponding `if`, `version` or `debug` statement
test.d-mixin-30(30): Error: found `else` without a corresponding `if`, `version` or `debug` statement
test.d(42): Error: template instance `test.test!true` error instantiating

Now as you can see, there are three times this error occurs in the expansion of the mixin. The error will go away if we comment the three mixins in the first "static if" branch (as in our example, "signed" is set to true so the first branch will get compiled). So from me understanding, when a mixin is expanded, the compiler checks the code from the mixin SEPARATELY and sees if it makes sense. So for that reason, it sees as there is only an "else static if" without a "static if" before and thus it gives us the error. Tho this doesn't seem right because how can it see the other variables like "num" or "base" that were not defined INSIDE the enum as its parameters? So it may be another sneaky bug that I can't understand. Or like it has happened to the past, it may be something obvious that I'm so blind to see and someone will tell me and I'll get embarrassed again (this is why I don't like asking only for help fixing bugs)...

On Wednesday, 5 January 2022 at 08:40:15 UTC, rempas wrote:

Try this:

pragma(msg, type_check!("static if", "i8", "true", "5", "4", "10", "5"));

Result:

static if(is_same!(num, i8)) {
    mixin(base_digit!("5", "4", "10", "5"));
    static if (true) {
      mixin(overflow_check!"i8.min"); }
  }

is_same - is undefined
i8 - is undefined
overflow_check!"i8.min" - ?

This code with mixins is horrible :)

On Wednesday, 5 January 2022 at 08:40:15 UTC, rempas wrote:

And you cannot have else statement in different mixin:

extern (C) void main() {
    ///OK:
    mixin("static if(true){}else{}");

    ///ERROR:
    mixin("static if(true){}");
    mixin("else{}");
}

On Wednesday, 5 January 2022 at 09:02:53 UTC, vit wrote:

pragma(msg, type_check!("static if", "i8", "true", "5", "4", "10", "5"));

static if(is_same!(num, i8)) {
    mixin(base_digit!("5", "4", "10", "5"));
    static if (true) {
      mixin(overflow_check!"i8.min"); }
  }

Hi and thanks for your time. "pragma(msg, ...)" will indeed work but it will print the code in the standard output. Is there something that I'm missing?

On Wednesday, 5 January 2022 at 09:17:54 UTC, rempas wrote:

pragma(msg, type_check!("static if", "i8", "true", "5", "4", "10", "5"));

static if(is_same!(num, i8)) {
    mixin(base_digit!("5", "4", "10", "5"));
    static if (true) {
      mixin(overflow_check!"i8.min"); }
  }

1. That printed code on standard output is wrong, missing declaration for is_same and i8.

2. if-else must be in same mixin:

    mixin(""
        + type_check!("static if", "i8", "true", "5", "4", "10", "5")
        + type_check!("else static if", "i16", "true", "7", "6", "18", "8")
    	+ type_check!("else static if", "i32", "true", "12", "10", "34", "13")
    	+ type_check!("else", "i64", "true", "21", "18", "66", "24")
    );

3. last type_checkin signed and unsigned part of function test has else with condition.

else(is_same!(num, i64)) {
    mixin(base_digit!("21", "18", "66", "24"));
    static if (true) {
      mixin(overflow_check!"i64.min"); }
  }

On Wednesday, 5 January 2022 at 09:33:07 UTC, vit wrote:

    mixin(""
        + type_check!("static if", "i8", "true", "5", "4", "10", "5")
        + type_check!("else static if", "i16", "true", "7", "6", "18", "8")
    	+ type_check!("else static if", "i32", "true", "12", "10", "34", "13")
    	+ type_check!("else", "i64", "true", "21", "18", "66", "24")
    );

Yeah, sorry, you added the last comment (before this one) after I posted mine. Also, the fact that they must be in the same mixin is what I said that the problem is but thankfully there is a way around it. Tho this code will still not work. First, we must change "+" to "~" to concatenate strings but still, I'm getting the following error:

test.d-mixin-27(47): Error: found `End of File` when expecting `;` following statement
test.d(45): Error: template instance `test.test!true` error instantiating

There may be a problem in the "type_check" enum but I wanted to post this reply before I search any any case there is something else happening and you happen to know. I will update if I found it

else(is_same!(num, i64)) {
    mixin(base_digit!("21", "18", "66", "24"));
    static if (true) {
      mixin(overflow_check!"i64.min"); }
  }

What does this mean? Yes they do, the else is used to mach the 64-bit type.

On Wednesday, 5 January 2022 at 10:47:44 UTC, rempas wrote:

Well, it is specifically the "static if" part that cases the problem. I can't seem to be able to find anything tho

On Wednesday, 5 January 2022 at 08:40:15 UTC, rempas wrote:

Back when i was working on bitmanip with the bitfield mixin, i had to re-write it with newlines and tabs, and then have it expand it as text and output it and see what the actual output was before i could debug it.

The output was... informative.

That said, rolling your own mixins should really be the last resort. You're dumping a lot into a single line of code you can't trace, follow, debug, or look at.