testing: add runIfMain

[ry]

Once we support imports.meta.url and have a way for testing if the current script is the main script, we want to have a two line snippit of code that will allow any test to be executed as a standalone test deno foo_test.ts rather than running the main entry point.
Essentially something like this:
https://github.com/tensorflow/tensorflow/blob/c4e61ac1fd0b3b48461e2876b20fe56c09faa2ff/tensorflow/python/lib/io/tf_record_test.py#L484

The most basic form would be something like this:

if (args[1] == imports.meta.url) {
  testMain();
}

But perhaps we can make this shorter by having a function like runIfMain(imports.meta.url)

This came up in: #129

[ry]

import.meta.url is now available in 0.2.10 so runTestsIfMain should be doable now.

[bartlomieju]

I guess this belongs to testing/mod.ts but that file contains this notice: // Do not add imports in this file in order to be compatible with Node.. So import { args } from "deno"; isn't desired there. Any thoughts?

EDIT:
On the other hand

if __name__ == "__main__":
  do_something()

is a very commonly used idiom in Python. Maybe we could add runIfMain to deno itself? Then Python example would be written as:

import { runIfMain } from "deno";

runIfMain(doSomething);
// or
runIfMain(() => {
   doSomething1();
   doSomething2();
});

Just thinking out loud.

[bartlomieju]

@ry any more thoughts on this?

[ry]

@bartlomieju Yes - I think if we implement window.location as described in denoland/deno#1750 then we can do

if (imports.meta.url == window.location.toString()) {
  runTests();
}

[ry]

FYI window.location has landed in Deno v0.3.0

One can test if the script is main by using this:

if (window.location.toString() == import.meta.url) {
  console.log("main");
}

We should be able to adapt for testing.runIfMain(import.meta)

See the manual entry.

[bartlomieju]

Closed in #215

[bartlomieju]

CC @ry