class GraphQL::Client::Schema::EnumType

def cast(value, _errors = nil)

Returns String or nil.

errors - Errors instance
values - JSON value

with the convenience predicate methods.
Internal: Cast JSON value to the enumeration's corresponding constant string instance 
def cast(value, _errors = nil)
  case value
  when String
    if @values.key?(value)
      @values[value]
    elsif schema_module.raise_on_unknown_enum_value
      raise Error, "unexpected enum value #{value}"
    else
      UnexpectedEnumValue.new(value).freeze
    end
  when NilClass
    value
  else
    raise InvariantError, "expected value to be a String, but was #{value.class}"
  end
end