I want to create a "CREATE TABLE data (...)" where the columns are derived from struct member names. Something like:

string s = "CREATE TABLE data(";

static foreach(f; FieldNameTuple!myStruct) {
 s ~= f ~ ",";
}

s ~= ");";

Which of course doesn't work... I didn't find any reference how to build-up strings in a statif foreach loop.

Is this possible at all?

-- 
Robert M. Münch
http://www.saphirion.com
smarter | better | faster

On Saturday, 7 March 2020 at 16:30:59 UTC, Robert M. Münch wrote:
> Which of course doesn't work... I didn't find any reference how to build-up strings in a statif foreach loop.
>
> Is this possible at all?

Use regular foreach with a regular string. Put that inside a function.

Then simply use that function to initialize your other thing and enjoy the magic of CTFE!

On Saturday, 7 March 2020 at 16:30:59 UTC, Robert M. Münch wrote:
> Is this possible at all?

You can use an anonymous lambda to build the string in CTFE:

------------------------------------------

struct S {
    int a;
    bool b;
}

import std;

enum string sql = {
    string s = "CREATE TABLE data(";

    static foreach(f; FieldNameTuple!S) {
    	s ~= f ~ ",";
    }

	s ~= ");";
    return s;
} ();

pragma(msg, sql);

------------------------------------------

This prints "CREATE TABLE data(a, b);"

On 2020-03-07 16:40:15 +0000, Adam D. Ruppe said:

> Use regular foreach with a regular string. Put that inside a function.
> 
> Then simply use that function to initialize your other thing and enjoy the magic of CTFE!

Perfect! This implicit CTFE is a tricky thing to see/remember/... Feeling a bit dumb but, hey it works :-)

-- 
Robert M. Münch
http://www.saphirion.com
smarter | better | faster

On 2020-03-07 16:41:47 +0000, MoonlightSentinel said:

> On Saturday, 7 March 2020 at 16:30:59 UTC, Robert M. Münch wrote:
>> Is this possible at all?
> 
> You can use an anonymous lambda to build the string in CTFE:
> 
> ------------------------------------------
> 
> struct S {
>      int a;
>      bool b;
> }
> 
> import std;
> 
> enum string sql = {
>      string s = "CREATE TABLE data(";
> 
>      static foreach(f; FieldNameTuple!S) {
>      	s ~= f ~ ",";
>      }
> 
> 	s ~= ");";
>      return s;
> } ();
> 
> pragma(msg, sql);
> 
> ------------------------------------------
> 
> This prints "CREATE TABLE data(a, b);"

Nice... is the enum a so called "manifest constant" for which the initializer is evaluated at compile time?

OT: The pragma seems to print the string twice... at least here on my side.

OT2: Looks like I have to read through the language spec again... and most likely over and over again, to have all these tricks at my finger-tips.

-- 
Robert M. Münch
http://www.saphirion.com
smarter | better | faster

On 2020-03-07 16:41:47 +0000, MoonlightSentinel said:

> You can use an anonymous lambda to build the string in CTFE:
> 
> ------------------------------------------
> 
> struct S {
>      int a;
>      bool b;
> }
> 
> import std;
> 
> enum string sql = {
>      string s = "CREATE TABLE data(";
> 
>      static foreach(f; FieldNameTuple!S) {
>      	s ~= f ~ ",";
>      }
> 
> 	s ~= ");";
>      return s;
> } ();
> 
> pragma(msg, sql);
> 
> ------------------------------------------
> 
> This prints "CREATE TABLE data(a, b);"

You can get rid of the enum und the static and it will work too.

-- 
Robert M. Münch
http://www.saphirion.com
smarter | better | faster

On 07.03.20 17:41, MoonlightSentinel wrote:
> On Saturday, 7 March 2020 at 16:30:59 UTC, Robert M. Münch wrote:
>> Is this possible at all?
> 
> You can use an anonymous lambda to build the string in CTFE:

It turns out that if you do use this standard idiom, you might end up getting blamed for an unrelated DMD bug though:
https://github.com/dlang/dmd/pull/9922

:o)

On Sunday, 8 March 2020 at 20:28:01 UTC, Robert M. Münch wrote:
> You can get rid of the enum [...]

That depends on your use case. You will need enum if you want to use the value at compile time (e.g. when using it as a template parameter). Otherwise a normal string will suffice.

> und the static and it will work too.

Yesh, I forgot about the old implicit static foreach loop.