D import idiom compilation time

[Neia Neutuladh]

On Fri, 04 Jan 2019 20:13:05 -0800, H. S. Teoh wrote:
> It doesn't have to be tied to a specific root package. Just call it
> "autoimport", or something short like "from":
> 
> 	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. To fix that, you can write something like:

---
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!""();
---

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?