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digitalmars.D.learn - Template pattern delegate?

reply frame <frame86 live.com> writes:
Is there a possibility to write templated code / custom trait 
pattern with usage like a delegate?

I have a try-catch block with different types and don't want to 
repeat myself in every method again. It's always the same, just 
what's tried changes, eg.:

pseudo code:

template myStuff(mixin code)
{
     try {
         code();
     }
     catch (X e) {
         ...
     }
     catch (Y e) {
         ...
     }
     ...
}

static myStuff!({
    writeln("...");
});
Oct 25 2020
parent reply =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
On 10/25/20 4:30 AM, frame wrote:

 Is there a possibility to write templated code / custom trait pattern=20
 with usage like a delegate?
=20
 I have a try-catch block with different types and don't want to repeat =



 myself in every method again. It's always the same, just what's tried=20
 changes, eg.:
=20
 pseudo code:
=20
 template myStuff(mixin code)
 {
  =C2=A0=C2=A0=C2=A0 try {
  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 code();
  =C2=A0=C2=A0=C2=A0 }
  =C2=A0=C2=A0=C2=A0 catch (X e) {
  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 ...
  =C2=A0=C2=A0=C2=A0 }
  =C2=A0=C2=A0=C2=A0 catch (Y e) {
  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 ...
  =C2=A0=C2=A0=C2=A0 }
  =C2=A0=C2=A0=C2=A0 ...
 }
=20
 static myStuff!({
  =C2=A0=C2=A0 writeln("...");
 });
=20


That's one of my DConf Onlide slides but for a limited use case! :)

int tried(Func, string functionName =3D __FUNCTION__, string file =3D=20
__FILE__, size_t line =3D __LINE__)(Func func) {
   static if (!is (ReturnType!Func =3D=3D int)) {
     pragma(msg, format!"\n%s(%s): Error: %s must return int error=20
code"(file, line, functionName));
     static assert(false);
   }

   void printError(T)(T err) {
     stderr.writefln!"\n%s(%s): Failed to execute %s: %s"(file, line,=20
functionName, err);
   }

   try {
     return func();

   } catch (Exception exc) {
     printError(exc.msg);
     return 1;

   } catch (Error err) {
     printError(err);
     import core.stdc.stdlib : abort;
     abort();
   }

   assert(false);
}

Then, all extern(C) functions would be written the same way:

extern(C) int foo() {
   return tried({
     // ...
     return 0;
   });
}

Ali
Oct 25 2020
parent reply frame <frame86 live.com> writes:
On Sunday, 25 October 2020 at 12:02:10 UTC, Ali Çehreli wrote:

 On 10/25/20 4:30 AM, frame wrote:

 Is there a possibility to write templated code / custom trait 
 pattern with usage like a delegate?
 
 I have a try-catch block with different types and don't want 
 to repeat myself in every method again. It's always the same, 
 just what's tried changes, eg.:
 
 pseudo code:
 
 template myStuff(mixin code)
 {
      try {
          code();
      }
      catch (X e) {
          ...
      }
      catch (Y e) {
          ...
      }
      ...
 }
 
 static myStuff!({
     writeln("...");
 });
 


 That's one of my DConf Onlide slides but for a limited use 
 case! :)

 int tried(Func, string functionName = __FUNCTION__, string file 
 = __FILE__, size_t line = __LINE__)(Func func) {
   static if (!is (ReturnType!Func == int)) {
     pragma(msg, format!"\n%s(%s): Error: %s must return int 
 error code"(file, line, functionName));
     static assert(false);
   }

   void printError(T)(T err) {
     stderr.writefln!"\n%s(%s): Failed to execute %s: %s"(file, 
 line, functionName, err);
   }

   try {
     return func();

   } catch (Exception exc) {
     printError(exc.msg);
     return 1;

   } catch (Error err) {
     printError(err);
     import core.stdc.stdlib : abort;
     abort();
   }

   assert(false);
 }

 Then, all extern(C) functions would be written the same way:

 extern(C) int foo() {
   return tried({
     // ...
     return 0;
   });
 }

 Ali


I see that your approach can handle functions and delegates but 
isn't that not equivalent like this template for a function?

auto myStuff(T)(T function() fn) {
     try {
         return fn();
     }
     catch (Exception e) {
         //
     }
}

I wonder if I could use such a template as static variant and the 
compiler just expands the code? Just thinking that using a 
function or delegate is an overhead. Maybe not a function but a 
delegate will allocate GC memory I think.
Oct 25 2020
parent reply Jacob Carlborg <doob me.com> writes:
On Monday, 26 October 2020 at 00:56:26 UTC, frame wrote:


 I see that your approach can handle functions and delegates but 
 isn't that not equivalent like this template for a function?

 auto myStuff(T)(T function() fn) {
     try {
         return fn();
     }
     catch (Exception e) {
         //
     }
 }


You cannot pass a delegate to something that expects a function 
pointer.


 I wonder if I could use such a template as static variant and 
 the compiler just expands the code? Just thinking that using a 
 function or delegate is an overhead. Maybe not a function but a 
 delegate will allocate GC memory I think.


If you pass the delegate as a template parameter/alias parameter, 
it's more likely to be inlined:

auto myStuff(alias fn)() {
     try return fn();
     catch (Exception e) { }
}

myStuff!( { /* some code */ } );

--
/Jacob Carlborg
Oct 26 2020
parent frame <frame86 live.com> writes:
On Monday, 26 October 2020 at 09:25:03 UTC, Jacob Carlborg wrote:

 On Monday, 26 October 2020 at 00:56:26 UTC, frame wrote:


 If you pass the delegate as a template parameter/alias 
 parameter, it's more likely to be inlined:

 auto myStuff(alias fn)() {
     try return fn();
     catch (Exception e) { }
 }

 myStuff!( { /* some code */ } );

 --
 /Jacob Carlborg


Does not work in my case. Seems to conflict with overloads, but 
thanks for the hint.
Oct 26 2020