Set optional function parameter

[Jonathan M Davis]

On Friday, August 17, 2018 2:52:53 AM MDT Andrey via Digitalmars-d-learn 
wrote:
> Hello,
>
> In D there is a nice function:
> > auto Tuple!(int,"status",string,"output") executeShell (
> >
> >  scope const(char)[] command,
> >  const(string[string]) env = cast(const(string[string]))null,
> >  Config config = cast(Config)0,
> >  ulong maxOutput = 18446744073709551615LU,
> >  scope const(char)[] workDir = null,
> >  string shellPath = nativeShell()
> >
> >) @trusted;
>
> It has got 5 optional parameters. For example I want set only the
> fifth and all other leave with default values.
>
> If it was C/C++ then I would write:
> > executeShell("my_command", nullptr, 0, 18446744073709551615,
> > nullptr, "my/path");
>
> Long and boring...
>
> What about D? Does it support something like this:
> > executeShell("my_command", shellPath = "my/path");
>
> or
>
> > executeShell("my_command", default, default, default, default,
> > "my/path");
>
> I mean - can I skip some arguments and set only one that I want?
> Hm, Python, it seems to me, support this feature.

The short answer is no.

D is more like C++ or Java in this respect. Python has named arguments,
which allow you to indicate which parameter an argument goes with by name
rather than by position, thus allowing you to skip giving values for some
parameters if they have default values, whereas most C-derived languages -
including D - do not have such a feature.

Usually, it's a non-issue, because having more than one - maybe two
- default arguments is pretty rare, and having long parameter lists is
usually considered bad design. Unfortunately, executeShell is one of the few
cases in Phobos where a function has quite a few parameters, and D does not
directly provide a way to skip any parameters when providing arguments so
that you can provide a value for an argument later in the parameter list
while not providing parameters earlier in the list.

There have been some templated solutions floating around from time to time
to allow folks to implement a form of named arguments themselves, but as
I've never done anything with them, I don't know if they're designed to help
you in a situation like this or if they're designed such that the person who
writes the function can give you a way to use named arguments with that
particular function. You may find something floating around on
code.dlang.org that would help you.

However, if you're calling executeShell more than once, you could simply
create a wrapper function that only had the parameters you wanted and then
passed the full list of arguments to executeShell.

- Jonathan M Davis