Reflect.getDefaultParameterValues

[Tab Atkins Jr.]

On Mon, Oct 5, 2015 at 9:00 AM, Benjamin Gruenbaum <benjamingr at gmail.com> wrote:
> Well, I've personally thought about building a small pattern matching
> library using the syntax, but that's hardly a general use case:
>
> ```js
> match(
> (x = 1) => doFoo(...)
> (x = {y : 3}) => doBar(...)
> ```

That's just syntax punning/abuse, not a real use-case. ^_^

> However, there are several use cases people mention. Here are the first
> twofrom questions asking for this on StackOverflow in other languages:
>
>  - Exposing functions in a module over a JSON API. If the caller omits
> certain function arguments, return a specific error that names the specific
> function argument that was omitted. If a client omits an argument, but
> there's a default provided in the function signature, I want to use that
> default.

I'm confused.  If the argument defaulted or not?  If it's defaulted,
omitting it isn't an error.  If omitting it is an error, then there's
no default, by definition.

>  - Building an ORM that uses the default values in the database and returns
> correct error messages when an insert of a default value failed.

What about that requires the knowledge of the defaulted value, rather
than just the argument value?  I *might* see a use-case here for
knowing *whether or not the argument was defaulted*, so you can output
a more helpful error message (saying that the default is wrong, rather
than saying the passed value is wrong), but knowing the default value
doesn't help there (unless it's a unique sentinel value, which defeats
the point of default values in the first place).

~TJ