Path: utzoo!dciem!nrcaer!sce!cognos!keithc
From: keithc@cognos.UUCP (Keith Campbell)
Newsgroups: comp.lang.eiffel
Subject: Re: Eiffel cleanup #1: The creation mechanism
Keywords: Separate operation on references from operations on objects
Message-ID: <7862@cognos.UUCP>
Date: 18 Jan 90 14:15:29 GMT
References: <226@eiffel.UUCP>
Reply-To: keithc@cognos.UUCP (Keith Campbell)
Organization: Cognos Inc., Ottawa, Canada
Lines: 65

In article <226@eiffel.UUCP> bertrand@eiffel.UUCP (Bertrand Meyer) writes:
[about plans for version 3]

>There is only one creation instruction.  Its most general form is

>    x !D!f(...)

>where x is a read-write entity of some type C and D is a descendant of
>C; f must be a creation procedure of D.  This creates an instance of
>D, applies the default initializations, calls f with the actual arguments
>given, and attaches the resulting object to x.

>D may be omitted in the most frequent case, for which it is the same
>as C, giving the form

>    x !!f(...)

>For a class which has no creation procedure (meaning that the default
>initializations are sufficient to ensure the invariant), the syntax
>becomes simply

>    x !!

In the interest of further reducing useless local variables, I suggest
that you go one step further and allow creation of anonymous objects.
Thus

    x !D!f(...)
would be equivalent to
    x := !D!f(...)

The improvement is that now we can pass newly created objects as actual
parameters without the need for the declaration of local variables:

    foo( !D!f(...), !E!g(...), ... );

If the compiler is given the ability to infer the (most general) class
of object which needs to be created, then

    x := !!f(...)
would be equivalent to
    x !!f(...)

and

    x := !!
would be equivalent to
    x !!

All of us are comfortable with the use of the symbol "1" to mean

    !Integer!.incremented

Why not extend this flexibility to all classes?

Note that "incremented" cannot be confused with a creation feature
because it appears following the dot ".".  In this case, the compiler
must generate code to first create this object (using the default
initializations) before the "incremented" function may be applied.

-- 
Keith Campbell               Cognos Incorporated   S-mail: P.O. Box 9707
Voice: (613) 738-1338 x5222                                3755 Riverside Drive
  FAX: (613) 738-0002                                      Ottawa, Ontario
 UUCP: uunet!mitel!sce!cognos!keithc                       CANADA  K1G 3Z4