How to insert code in place with templates/mixins?

[Tejas]

On Monday, 20 December 2021 at 11:30:09 UTC, rumbu wrote:
> On Monday, 20 December 2021 at 10:49:20 UTC, rempas wrote:
>> On Monday, 20 December 2021 at 09:30:30 UTC, rumbu wrote:
>>
>> Thanks a lot for the info. When I try to use this code, I'm 
>> getting the following error:
>>
>> ```
>> Error: expression expected, not `%`
>> Error: expression expected, not `%`
>> ```
>
> My fault, I forgot to put some char delimiters. You can find 
> tested code here:
>
> https://run.dlang.io/is/KfdED0
>
>> So I suppose there is a problem with string concatenation. I 
>> couldn't use it anyway because it is inefficient and because 
>> I'm using betterC. Do you know any other way that I can 
>> concatenate strings that does not depend an the Garbage 
>> Collector or the standard library?
>
> Enums (that's why the string is declarated as enum) are 
> evaluated at compile time, the concatenation op will not end in 
> your code as instruction, so you can do anything outside 
> betterC rules as long you do it at compile time. You are just 
> building some code to use later, the compiler does not generate 
> any instruction for it.
>
> In the example above you can press the AST button to see 
> exactly how your code is generated.
>
> Wnen you have doubts about a generated string you can always 
> test it with ```pragma msg```. In this case, if you write:
>
> ```
> pragma(msg, add_char!'%');
> ```
>
> you will have in the output exactly what the compiler will 
> generate for your mixin.

Ehh, it still fails; should've explicitly put the length of the 
array and the `extern (C)` in `main`

```d
module demo;

//i am just declaring these to have them.
size_t stdout_index;
enum STDOUT_BUF_LEN = 42;
char[STDOUT_BUF_LEN] stdout_buffer; /+indexing an uninitialized 
dynamic array resulted in out of bounds error even for index == 
0+/
alias i32 = int;
void sys_write(int i, void* p, int index) {}
//


enum add_char(char c) =

   `if (stdout_index < STDOUT_BUF_LEN) {
     stdout_buffer[stdout_index++] ='` ~ c ~ `';
     continue;
   } else {
     sys_write(1, stdout_buffer.ptr, cast(i32)stdout_index);
     stdout_index = 0;
     stdout_buffer[stdout_index++] ='` ~ c ~ `';
     continue;
   }`;

extern(C) /+added this because you used -betterC+/ void main()
{

     while (true) {
		mixin(add_char!'%');
		mixin(add_char!'$');
     }



}
```