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digitalmars.D.learn - static foreach / How to construct concatenated string?

reply =?iso-8859-1?Q?Robert_M._M=FCnch?= <robert.muench saphirion.com> writes:
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
Mar 07 2020
next sibling parent reply Adam D. Ruppe <destructionator gmail.com> writes:
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!
Mar 07 2020
parent =?iso-8859-1?Q?Robert_M._M=FCnch?= <robert.muench saphirion.com> writes:
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
Mar 07 2020
prev sibling parent reply MoonlightSentinel <moonlightsentinel disroot.org> writes:
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);"
Mar 07 2020
next sibling parent =?iso-8859-1?Q?Robert_M._M=FCnch?= <robert.muench saphirion.com> writes:
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
Mar 08 2020
prev sibling next sibling parent reply =?iso-8859-1?Q?Robert_M._M=FCnch?= <robert.muench saphirion.com> writes:
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
Mar 08 2020
parent MoonlightSentinel <moonlightsentinel disroot.org> writes:
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.
Mar 09 2020
prev sibling parent Timon Gehr <timon.gehr gmx.ch> writes:
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)
Mar 09 2020