Validates that a value is a member of a given collection using an Eq comparator.

In enforces a membership constraint of the form: "value must be one of these".

Membership is defined by an Eq instance, not by structural equality or Elixir's in operator.

Options

• :values (required) The list of allowed values to compare against.

• :eq (optional) An equality comparator. Defaults to Funx.Eq.Protocol.

• :message (optional) A custom error message callback (value -> String.t()).

Semantics

• Succeeds if the value equals any element in :values under Eq, or if the value is a struct whose module is listed in :values.
• Fails otherwise.
• Nothing passes through.
• Just is unwrapped before comparison.