String Metaprogramming

[E.S. Quinn via Digitalmars-d-learn]

On Saturday, 18 July 2015 at 13:48:20 UTC, Clayton wrote:
> Am new to D programming, am considering it since it supports 
> compile-time function execution . My challenge is how can I 
> re-implement the function below so that it is fully executed in 
> compile-time. The function should result to tabel1 being 
> computed at compile-time. There seems to be a lot of mutation 
> happening here yet I have heard no mutation should take place 
> in meta-programming as it subscribes to functional programming 
> paradigm.
>
>
>
> void computeAtCompileTime( ref string pattern ,ref int[char] 
> tabel1){
> 	int size = to!int(pattern.length) ;
> 	
> 	foreach( c; ALPHABET){
> 		tabel1[c] = size;
> 	}
> 	
> 	for( int i=0;i<size -1 ; ++i){   //Initialise array
> 		tabel1[pattern[i]] = size -i-1;
>
>         pragma(msg, format("reached pattern  
> table1[pattern[i]]=(%s) here",
>         table1[pattern[i]].stringof  ~"     v="~ (size 
> -i-1).stringof));
> 	}
>
> 	
> 	
> 	
> }

Actually, the main things you can't do in CTFE are FPU math 
operations (much of std.math has issues unfortunately), compiler 
intrinsics, pointer/union operations, and I/O. I don't 
immediately see anything that will cause issues with CTFE in that 
function. However, sometimes the compiler isn't smart enough to 
figure out that it should be doing that, but you can force the 
compiler to try CTFE using this pattern

int ctfeFunc() {
}

void main() {
     enum val = ctfeFunc();

}

enums are manifest constants, and thus must be computable at 
compile time, so this will issue an error if something in your 
function can't CTFE.