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From: John English <je@bton.ac.uk>
Subject: Re: Run Time Dispatch Question
Date: 2000/02/25
Message-ID: <38B6837E.7208F8C8@bton.ac.uk>#1/1
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Date: 2000-02-25T13:25:25+00:00
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Michael Garrett wrote:
> I am having a problem understanding run time dispatching. ( Basic question I
> know, but I'm having a brain cramp. )
> [...snip...]

You need to put your tagged type t and primitive p in a package, not
a procedure. Otherwise, the procedure p(...) defined for each type
derived from t is just an overload of the name p, not a primitive of
the type. Dispatching only happens for primitives. To quote RM95
3.2.3:

    The primitive subprograms of a specific type are defined as
    follows: 
    a) The predefined operators of the type (see 4.5); 
    b) For a derived type, the inherited (see 3.4) user-defined
       subprograms; 
    c) For an enumeration type, the enumeration literals (which are
       considered parameterless functions -- see 3.5.1); 
    d) For a specific type declared immediately within a
       package_specification, any subprograms (in addition to the
       enumeration literals) that are explicitly declared immediately
       within the same package_specification and that operate on
       the type; 
    e) Any subprograms not covered above that are explicitly declared
       immediately within the same declarative region as the
       type and that override (see 8.3) other implicitly declared
       primitive subprograms of the type. 

So, if t and p were declared in a package then p would be a primitive
(by (d)), and overloading p for t1 and t2 would make them primitives
too (by (e)).

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