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Software Engineering		 Old Precursor Specification

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Precursor construct: the original proposal

The Precursor mechanism is a recently introduced Eiffel construct The mechanism was proposed in mid-1997 (see credits at the end) to the language committee of NICE, the Nonprofit International Consortium for Eiffel, and formally approved by the committee in a vote of October 1997.

The rest of this document contains the text of the formal proposal as adopted by the NICE language committee. You should first have read the companion document presenting an

January 1999 note: For an up-to-date version see a small revision of this proposal. The changes are a change of order of components in the syntax (the old order is still supported in ISE Eiffel 4.3) and a simplification/refinement of the validity rule. informal description of the mechanism.

 

New reserved word

(To be added to list in "Eiffel: The Language", page 553.)

 

    Precursor

 

Syntax change

Page 342, definition of Unqualified_call (previously Entity [Actuals]) becomes

 

    Unqualified_call == Feature_of_call [Actuals]

with the new construct definitions

 

    Feature_of_call == Entity | Precursor
    Precursor == [Parent_qualification] Precursor
    Parent_qualification == "{" Class_name "}"

 

Validity changes

Note (not subject to vote): everything could be done within VUEX, page 368. I have found it more clear to introduce two new constraints. In VUEX, page 368, replace condition 1 by

 

    1. The call is an Unqualified_call.

With the new validity rule VUNQ

 

    An Unqualified_call appearing in a routine r of a class C is valid if and only if it satisfies one of the following two conditions:
    • 1. Its Feature_of_call is the final name of a feature of C.
    • 2. Its Feature_of_call is of the Precursor kind.

and the new validity rule VUPR (including a definition):

 

    A Precursor is valid if and only if it satisfies the following three conditions:
    • 1. It appears in a Routine_body.
    • 2. That Routine_body is part of the declaration of exactly one routine r of C.
    • 3. That declaration is a redefinition.
    • 4. If it is is the redefinition of routines from more than one parent, the Parent_qualification part is present.
    • 5. If the Parent_qualification part is present, its Class_name is the name of one of the parents of C from which C redefines r.

    The "immediate precursor" of r in this case is the routine being redefined from a parent of C, determined in case of ambiguity (under condition 2 of the rule) by the Class_name of the given Parent_qualification.

Also, in VUAR, page 369, rewrite the parenthetical sentence on line 2 of the rule by

 

    (For an Unqualified_call take target to be Current; if the Feature_of_call is of the Precursor kind, take fname to be the immediate precursor of the enclosing routine.)

 

Semantics change

Insert a new section on page 345 between 21.8 and 21.9:

 

    Precursor calls

    A call of the Precursor form, that is

     

      Precursor (...)
    or

     

      {PARENT} Precursor (...)

    may only appear in the redefinition of a routine r. The original version of r in the parent is known as the "immediate precursor".

    In this case the feature of the call, fname, is the immediate precursor.

Also, the next section (21.9) should begin

 

    Now assume target_value is not void and the call is not of the Precursor form.

 

Note

(This note is not part of the proposal and the vote does not apply to it.) Since Object-Oriented Software Construction, second edition (Prentice Hall, 1997) uses double braces, as in {{PARENT}}, rather than single braces as in {PARENT}, it may be desirable for compilers to accept both forms.

Credits

 

(This section is not part of the proposal and the vote does not apply to it.)

The Precursor proposal was put together and presented to NICE by Bertrand Meyer of ISE but the language construct is the result of collective work. The contributions of the following people are acknowledged (please signal any omissions): Eric Bezault, Roger Browne, James McKim, Xavier Le Vourch, Bertrand Meyer, Frieder Monninger, Steve Tynor, Dino Valente, Kim Walden.