Add strict option to Keyword.validate

[Mateusz Front]

Hi,

WDYT about adding a `strict: boolean` option to `Keyword.validate/2` that would make it return an error if a key that doesn't have a default value provided is absent? The returned value in case of an error could be `{:error, unknown_keys, missing_keys}`, for example:

{:error, [], [:b]} = Keyword.validate([a: :foo], [:a, :b], strict: true)

{:error, [:c], [:b]} = Keyword.validate([a: :foo, c: :bar], [:a, :b], strict: true)

{:error, [:c], []} = Keyword.validate([a: :foo, b: :foobar, c: :bar], [:a, :b], strict: true)

in case of success, the result would remain as it is now:

{:ok, [a: :foo, b: :bar]} = Keyword.validate([a: :foo, b: :bar], [:a, :b], strict: true)

{:ok, [b: :bar, a: :foo]} = Keyword.validate([a: :foo], [:a, b: :bar], strict: true)

The behaviour would be unchanged when the strict is set to false (default). The bang version would be adjusted accordingly.

Thanks ;)

[José Valim]

We considered this but the error we would raise is generic. It is much better for you to validate afterwards and raise good error messages that say it is missing and which value you should pass instead and what it is used for!

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[Mateusz Front]

Right, I understand. I'd argue that many times there isn't much more to say beyond 'We require this option, please provide it'. Also, a simple way of ensuring the options are there would be very useful when prototyping, as it would prevent from `nil`s sneaking deep into the logic, before friendly error messages are added.