Nice public API

[lasseebert]

It should be simple and "nice" to define schemas, so Validation should have a nice public API.
It should still be possible to use the underlaying functions to define schemas, rules and predicates as they might be more convenient when doing custom stuff or when dynamically defining schemas.

Take inspiration in dry-validation.

[lasseebert]

Maybe something like this:

Validation.schema do
  optional(:name, filled?)
  required(:email, filled? and match?(~r/@/))
  required(:contact_info) do
    required(:phone)
    optional(:fax)
  end
end

One could perhaps use a custom predicate like so:

Validation.schema do
  required(:email, &MyCustomPredicates.email/1)
end

And a custom rule like so:

Validation.schema do
  optional(:beer)
  optional(:wine)
  &(MyCustomRules.mutually_exclusive(&1, [:beer, :wine]))
end

Define :strict or :whitelist schemas:

Validation.schema(:strict) do
  ...
end

Validation.schema(:whitelist) do
  ...
end

It should also be possible to add a custom preprocessor.

[lasseebert]

• :strict and :whitelist options should default to be inherited to nested schemas

[lasseebert]

After working with this, it seems like a better idea not to "precompile" the different parts of the schema, but instead make the schema function and all nested functions at runtime. This will make it more simple to create a good DSL.

[lasseebert]

Or we could skip compile-time expansion of the DSL. Then we could allow anonymous functions in the schema tree, which might be a needed feature after all.