D import idiom compilation time

[H. S. Teoh]

On Sat, Jan 05, 2019 at 05:57:57AM +0000, Neia Neutuladh via Digitalmars-d wrote:
> On Fri, 04 Jan 2019 20:13:05 -0800, H. S. Teoh wrote:
[...]
> > 	struct from {
> > 	  template opDispatch(string moduleName)
> > 	  {
> > 	    mixin("import opDispatch = " ~ moduleName ~ ";");
> > 	  }
> > 	}
> > 
> > Then just write:
> > 
> > 	from.std.stdio.writeln("...");
> 
> Error: module `std` is in file 'std.d' which cannot be read
> 
> You need the opDispatch to take the module name rather than a
> qualified package name.

Hmph.  But in that case, doesn't that mean that the original definition
of _std doesn't work with Phobos subpackages, like std.algorithm,
because it would be trying to lookup the symbol in the package rather
than the module? So something like _std.algorithm.searching.find would
fail, because `searching.find` cannot be found in
std/algorithm/package.d.


[...]
> ---
> struct FromImpl(string prefix)
> {
>     template opDispatch(string ident)
>     {
>         enum qualified = prefix == "" ? ident : prefix ~ "." ~ ident;
>         static if (__traits(compiles,
>            {mixin("import ", qualified, ";");}))
>         {
>             mixin("import opDispatch = ", qualified, ";");
>         }
>         else
>         {
>             enum opDispatch = FromImpl!(qualified)();
>         }
>     }
> }
> enum from = FromImpl!""();
> ---

Interesting.


> This is greedy, so it doesn't work for everything. Let's say you have:
> 
>   foo/
>     package.d
>     bar/
>       baz.d
> 
> In this case, from.foo.bar.baz.someFunc() will find foo/package.d and
> look for a symbol it defines named bar. If it doesn't define that
> symbol, you're out of luck.
> 
> Reader exercise: is there a way to make this second case work? How
> does this interact with metaprogramming, and what sorts of precautions
> should you take?

Hmm. Couldn't you make it work by making it lazy? I.e., have opDispatch
return a struct that encapsulates a partial package path, and that
contains a further opDispatch to resolve the next component in the
qualified name, and that only resolves to the actual symbol when it
actually references an actual symbol?


T

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