try & catch / repeating code - DRY

[Jonathan M Davis]

On Thursday, May 24, 2018 19:39:07 Jacob Carlborg via Digitalmars-d-learn 
wrote:
> On 2018-05-24 08:05, Robert M. Münch wrote:
> > Hi, great! Thanks for the examples... BTW: Is there a place where such
> > generic and fundamental examples are collected?
>
> Not as far as I know.
>
> >> void handleException1(alias dg)()
> >> {
> >>      try dg();
> >>      catch (Exception e) { /* handle exception */ }
> >> }
> >>
> >> void handleException2(lazy void dg)
> >> {
> >>      try dg();
> >>      catch (Exception e) { /* handle exception */ }
> >> }
> >>
> >> void handleException3(scope void delegate () dg)
> >> {
> >>      try dg();
> >>      catch (Exception e) { /* handle exception */ }
> >> }
> >>
> >> void main()
> >> {
> >>      handleException1!({
> >>          writeln("asd");
> >>      });
> >>
> >>      handleException1!(() => writeln("asd"));
> >>
> >>      handleException2(writeln("asd"));
> >>
> >>      handleException3({
> >>          writeln("asd");
> >>      });
> >> }
> >
> > What is exactly the difference between handleException1 and 3?
>
> With handleException1 the delegate needs to be passed as a template
> argument, in the other case as a regular argument. I thought that the
> lambda syntax, () => writeln("asd"), did not work as a regular argument,
> but I checked now and it does.
>
> Passing it as a template argument might allow the compiler to inline it.
> All range functions in Phobos are using template argument approach.

With a template alias, it will accept pretty much any symbol (which would
then normally be restricted by a template constraint so that it's a symbol
which is usable in the target context), whereas an explicit delegate will
only accept anything that implicitly converts to a delegate with that
signature. What matches, I don't know, since I pretty much enver declare
explicit delegates, though I don't find it surprising that a lambda works,
since that's basically what a lambda is. But I expect that if you did
something like pass a functor, it would work with the alias but wouldn't
work with the delegate parameter.

- Jonathan M Davis