Raise on non-existent key

[Fryguy]

Another useful feature might be an option that instead of returning nil on a non-existent key, would instead raise an exception. I feel this could be similar to Hash#fetch.

cc @cben

[aspirewit]

@Fryguy You can use the validation, more details

[jcoyne]

@aspirewit I'm not sure validations would work in every case. For example I have a configuration like this:

app:
  driver:  'foxtrot'
  foxtrot:
   beats: 4
  waltz:
    beats: 3

Now I might want to resolve:

Settings.app.fetch(Settings.driver)

[amyin]

+1 I would also love run-time errors on non-existent keys, something like a ENV.fetch

[cben]

https://github.com/obrok/closed_struct may be useful for implementing this.

EDIT: it's also immutable, which is not so desired...

[pkuczynski]

Interesting, but I have no time to work on this at the moment...