feat: use a schema as when condition

[vonagam]

Separate PR from #443.

[jquense]

Thanks for sketching this out. I'm not convinced this API is worth the added code, I've gone back and forth a few times on adding this and overall I don't think the caveats and gotchas are probably worth the convenience. The biggest problem is that you need a sync capable schema which is not obvious from the API. And if you do want to use a sync schema it's easy to use one explicitly with the function form of is.

Overall I think trying to keep the core library small is probably more valuable here than the API affordance but let me know what you think

[vonagam]

So, this change requires just 5 lines of code.

Change from:

let isFn =
  typeof is === 'function'	
    ? is
    : (...values) => values.every(value => value === is);

to:

let isFn;

if (typeof is === 'function') {
  isFn = is;
} else if (isSchema(is)) {
  isFn = (...values) => values.every(value => is.isValidSync(value));
} else {
  isFn = (...values) => values.every(value => value === is);
}

Plus i added here makeIsFn function so that we do not create and destroy arrays when we deal with only one value (i assume it is the most popular case). Optional 4 lines.

And the only api change here is that you can now use a sync schema as a condition.

So, i do not see here big costs nor in code increase, nor in api complication.

As for benefits, i think that it is expected that you can use validation schema as a condition (like, they are made for this) without writing custom helper. Plus joi provides such usage, and i see gain in providing same functionality for same named methods (less code change for a user when switching and all joi schemas are sync).

But yes, it does not provide functionality which you currently cannot do. So, while i prefer not to write custom helper for this in my codebase, i am ok with this PR not being merged. Feel free to close without any comments.

[vonagam]

Possible bonus point for this: if is, then and otherwise are schemas then maybe we can use this fact later when implementing describe for schema conditions. (If there will be any way to get describe() without finalisation)

[vonagam]

This pull should wait for #447.

So that is.isValidSync(value) could be easily replaced with is.isValidSync(value, options).