Generic creation

This technical note is the text of proposal submitted by ISE to the Nonprofit International Consortium for Eiffel (NICE). It describes a set of extensions to Eiffel.

This is a working document, precise for the most part but informal in places, and is currently under discussion.

Because the note followed earlier discussions within the language committee of NICE, it lacks the customary "introduction" and "rationale" parts explaining the purpose of the proposed extensions. The rationale and applications should, however, be immediately clear to any reader familiar with Eiffel: how to allow creation instructions (of the form !! x, or create x according to the proposed new syntax) when the type of x is a formal generic parameter.

All page numbers refer to the current definition of Eiffel: the book Eiffel: The Language.

1. Syntax

The production for Constraint, on page 201. which reads

Constraint == "->" Class_type

becomes

Constraint == "->" Class_type [Creation_constraint]

with (see page 285) Creation_constraint == create Feature_list end

Note: in the syntax for Creators, page 285,
the keyword creation will be replaced by create.

2. Validity

New validity rule VTCC, page 201:



A Constraint is valid if and only if every identifier listed
in the Feature_list of its Generic_creators, if any, is the name
of a procedure of the base class of its Creator_type.




Adaptation of VTCG, page 203 (with clause 3 already adapted for
the "Generics in Constraints" change) now reads:



Let C be a constrained generic class. A Class_type CT having
C as base class is valid if and only if CT satisfies the
Unconstrained Genericity rule (VTUG, page 201) and in addition,
for any Formal_generic parameter G in the declaration of C
having a constraint, then if D is the Class_type listed in
the constraint and T, with base class BT, is the Type
corresponding to G in the Actual_generics list of CT:



3. T conforms to the type obtained from D by substituting
any occurrence of a formal generic parameter of C by the
corresponding actual parameter.


4. If the constraint for G has a Generic_creators part,
then the C version of every procedure of BT listed (as
per rule VTCC, page 201) in that part is declared in BT
as a creation procedure.





In VGCC, page 286, replace clause 1 by



If T is a Formal_generic_name (that is to say, a formal
parameter of the class where the instruction appears), then
the corresponding formal generic parameter has a Constraint
part with a Generic_creators part, and the creation instruction
has a Creators part whose feature is one of the procedures
listed (as per rule VTCC, page 201) listed in the Generic_creators
part. Cases 2 to 6 assume that T is not a Formal_generic_name.



3. Semantics

The semantics of a creation call of the form


create x.proc (arguments)



where the type of x is (as per VGCC (1)) a formal generic parameter
with a constraint listing proc in its Generic_creators part,
is to create an object of the type T used as the corresponding actual
generic þarameter in the generic derivation considered, using
as creation procedure the version of proc in the base class
BT of T. As per VGCC (3), BT must declare that version as a creation
procedure.