Existing use of "external" transform_attribute

The transform_attribute class is used in 2 places

within rule::parse function

The purpose here is to transform the passed-in, or external, attribute type, Attribute_, to the rule's attribute, attribute_type.

within call_rule_definition function

The purpose here is the very same as the purpose above. The only difference is the name of the types. Here, instead of the passed-in attribute type being named Attribute_, the name is ActualAttribute, and instead of the rule's attribute being named attribute_type, the name is simply Attribute.

Proposed use of "RHS" transform_attribute

Currently, the x3 docs require that the rule attribute is "compatible" with the RHS attribute (page 96).

The propsed RHS transform would relieve the end-user of this compatibility constraint by transforming a possibly incompatible rule attribute to the RHS's attribute.

If such a RHS transform_attribute's post returns a bool value, then it could be used to replace some semantic actions and changes to the context, as shown here.

This RHS transform_attribute use could be placed somewhere after the external transform attribute use, possibly in the call_rule_defintion body just before parse_rhs call here.


  using rhs_attr=
    typename traits::attribute_of<RHS,Context>::type
    ;
  using transform=
    traits::transform_attribute_tagged
    < ActualAttribute
    , rhs_attr
    , rule_internal_id<ID>
    >;
  using transform_type=typename transform::type;
  transform_type rhs_actual=
    transform::pre
    ( attr_
    );
  ok_parse = parse_rhs
    ( rhs
    , first
    , last
    , context
    , attr_//RContext&
      //Rule Context, to allow actions to access rule attribute.
      //see val_context in core/call.hpp.
    , rhs_actual
    , has_action_and_not_explicit_propagation_t()
    );
  if(ok_parse)
  { 
      ok_parse=transform::post(attr_,rhs_actual);
  }

boostorg / spirit

need for "RHS" transform_attribute #780

Existing use of "external" transform_attribute

Proposed use of "RHS" transform_attribute