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digitalmars.D.learn - Conpile-Time module constructor

reply Ruby The Roobster <rubytheroobster yandex.com> writes:
Is it possible to make a module constructor run at compile-time?  
If so, how?
Jun 19 2022
parent reply Paul Backus <snarwin gmail.com> writes:
On Sunday, 19 June 2022 at 14:51:26 UTC, Ruby The Roobster wrote:

 Is it possible to make a module constructor run at 
 compile-time?  If so, how?


No, it's not.

What are you trying to accomplish that lead you to ask this 
question? There is probably a different way to do it without 
involving module constructors.
Jun 19 2022
parent reply Ruby The Roobster <rubytheroobster yandex.com> writes:
On 6/19/2022 11:19 AM, Paul Backus wrote:

 On Sunday, 19 June 2022 at 14:51:26 UTC, Ruby The Roobster wrote:

 Is it possible to make a module constructor run at compile-time?  If 
 so, how?


 
 No, it's not.
 
 What are you trying to accomplish that lead you to ask this question? 
 There is probably a different way to do it without involving module 
 constructors.


What I was trying to accomplish was to change a compile time array 
whenever a certain module was included.  The value of the array needs to 
be known at compile time.
Jun 19 2022
parent reply Paul Backus <snarwin gmail.com> writes:
On Sunday, 19 June 2022 at 15:34:48 UTC, Ruby The Roobster wrote:

 On 6/19/2022 11:19 AM, Paul Backus wrote:

 On Sunday, 19 June 2022 at 14:51:26 UTC, Ruby The Roobster 
 wrote:

 Is it possible to make a module constructor run at 
 compile-time?  If so, how?


 
 No, it's not.
 
 What are you trying to accomplish that lead you to ask this 
 question? There is probably a different way to do it without 
 involving module constructors.


 What I was trying to accomplish was to change a compile time 
 array whenever a certain module was included.  The value of the 
 array needs to be known at compile time.


You can do this with a `static if` check:

     static if (__traits(compiles, { import mymodule; }))
     {
         // mymodule is included
         enum compileTimeArray = ...;
     }
     else
     {
         // mymodule is not included
         enum compileTimeArray = ...;
     }
Jun 19 2022
parent reply Ruby The Roobster <rubytheroobster yandex.com> writes:
On 6/19/2022 11:55 AM, Paul Backus wrote:

 On Sunday, 19 June 2022 at 15:34:48 UTC, Ruby The Roobster wrote:

 On 6/19/2022 11:19 AM, Paul Backus wrote:

 On Sunday, 19 June 2022 at 14:51:26 UTC, Ruby The Roobster wrote:

 Is it possible to make a module constructor run at compile-time?  If 
 so, how?


 No, it's not.

 What are you trying to accomplish that lead you to ask this question? 
 There is probably a different way to do it without involving module 
 constructors.


 What I was trying to accomplish was to change a compile time array 
 whenever a certain module was included.  The value of the array needs 
 to be known at compile time.


 
 You can do this with a `static if` check:
 
      static if (__traits(compiles, { import mymodule; }))
      {
          // mymodule is included
          enum compileTimeArray = ...;
      }


Curious - how exactly does this determine if the module is included?  I 
thought __traits(compiles, ...) only checks if an expression is 
SEMANTICALLY correct.
Jun 19 2022
parent Paul Backus <snarwin gmail.com> writes:
On Sunday, 19 June 2022 at 15:57:41 UTC, Ruby The Roobster wrote:

 On 6/19/2022 11:55 AM, Paul Backus wrote:

 
 You can do this with a `static if` check:
 
      static if (__traits(compiles, { import mymodule; }))
      {
          // mymodule is included
          enum compileTimeArray = ...;
      }


 Curious - how exactly does this determine if the module is 
 included?  I thought __traits(compiles, ...) only checks if an 
 expression is SEMANTICALLY correct.


You're right--technically, it doesn't check whether the module is 
*included* in the final build, only whether the module is 
*available* for inclusion at compile time.

Because D supports separate compilation of modules, there is no 
way to check at compile time what modules will be included in the 
final build. If that's what you want to test for, you will have 
to move the check out of the code itself and into your build 
system (for example, by passing a `-version=HasMyModule` flag for 
builds that include the module in question).
Jun 19 2022