Passing $ as a function argument

[Simen Kjærås]

On Wednesday, 10 October 2018 at 08:46:42 UTC, James Japherson 
wrote:
> Would be nice to be able to pass $ as a function argument to be 
> used in automatic path length traversing.
>
>
> void foo(int loc)
> {
>    return bar[loc];
> }
>
> then foo($) would essentilly become
>
> foo(&)
>
>   becomes ==>
>
>    return bar[$];
>
>
> instead of having do to thinks like foo(bar.length).
>
> The usefulness comes from the case when bar is local:
>
> void foo(int loc)
> {
>    auto bar = double[RandomPInt+1];
>
>    return bar[loc];
> }
>
>
> then foo($) always returns a value and the outside world does 
> not need to know about foo. Since $ is a compile thing 
> expression and not used anywhere else this can always be 
> done(it is a symbolic substitution and has a direct translation 
> in to standard D code except $ cannot be used as arguments like 
> this the current D language grammar).

$ requires context (the array) for its value to be known - it's 
not a compile-time expression any more than rand() + 
currentWeather(getGpsCoordinates()) is. If $ were a valid 
identifier, you could do something like this:

struct Sentinel {}
Sentinel $;

void foo(T)(T loc) {
     auto bar = double[RandomPInt+1];
     static if (is(T == Sentinel)) {
         return bar[$];
     } else {
         return bar[loc];
     }
}

unittest {
     foo($);
}

Note that this would turn foo into a template, so that foo($) 
creates a separate function from foo(3).

Since $ isn't a valid identifier, this is currently impossible, 
but bachmeier's suggestion of foo!"$" works:

void foo(string s = "")(int loc = 0)
if (s == "" || s == "$") {
     auto bar = double[RandomPInt+1];
     static if (s == "$") {
         return bar[$];
     } else {
         return bar[loc];
     }
}

--
   Simen