Compile time executable calling?

[Timothee Cour]

On Fri, Jul 12, 2013 at 2:54 PM, Walter Bright
<newshound2 at digitalmars.com>wrote:

> On 7/12/2013 1:42 PM, Tofu Ninja wrote:
>
>> So I had an idea recently, wouldn't it be cool to have the ability to
>> call an
>> executable at compile time and capture its output. Something like the
>> string
>> imports but instead of opening and reading a text file, it run an
>> executable,
>> waits for it to finish, and grabs its output.
>>
>> It would get really cool if you could pass this executable some args and
>> then
>> mix in its out put into your own code. It could be used similarly to how
>> CTFE
>> are used but with out the overhead of trying to compile that function and
>> what
>> not and with out the limitations on what it can do.
>>
>> I could imagine all sorts of things that would be possible with this that
>> is
>> currently not.
>>
>> Not sure if this is something that could be implemented easily, but seems
>> like
>> something that could be done and something that would be really cool.
>>
>
> This is actually done in the makefile that builds DMD. A program
> (optabgen) is compiled and then run. Optabgen's output is several .c files
> which are then compiled into DMD's binary. The programs impcnvgen and idgen
> do similar things.
>
> It is a very powerful technique.
>

I think the OP was refering to something different:
ability to call an arbitrary executable / shell command during compile time
of a D function, whereas optabgen is during compiling dmd itself:

what we want to have is this:

----
import("some_source_file.txt"); //currently this is our only interaction
with outside world during compile time
import std.process; //or std.ctfeprocess ?

string getStringAtCompileTime(string command){
  if(!__ctfe) assert(0);
  //below is currently impossible, but would be great to have
  Tuple!(int,string,string) result=systemCaptureStdinStdout(command);
//likewise without redirecting stdin/stdout
  assert(!result[0]);
  return result[1];
}

void main(){
  enum host = getStringAtCompileTime("env | grep HOST");
}
----
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