Re: Design by contract and control inversion

[Shark8 <onewingedshark@gmail.com>]

On Wednesday, October 31, 2012 1:28:33 PM UTC-6, Hibou57 (Yannick Duchêne) wrote:
> Hi all,
> 
> I wondered if there are known idioms to express predicates for callbacks,  
> which may be via access to subprogram or interface/tagged types.

Hm, it seems to me that you could have an intermediate/pass-through handler.
(It would be rather limited, unless you make pre-/post-condition parameters... and if you do that it could get cumbersome quick.)
[NOTE: Not compiled or checked, just brainstorming/psudocode.]

Type Obj_Handle is not null access Object'class;
Type Handler is Access Procedure ( A, B : Integer; C : Obj_Handle );

-- this version applies the same preconditions to all handlers.
Procedure Intermediate (A, B : Integer; C : Obj_Handle; D : Handler)
   with Inline, pre => A > 0 or B in positive'Range;

Procedure Intermediate (A, B : Integer; C : Obj_Handle; D : Handler) is
begin
   D( A, B, C );
end;

To extend these with pre-/post-conditions we could add another type and another parameter to the handler.

Type Condition is access function( P : Parameter ) return Boolean;
Type Conditions is record
  Pre, Post : Condition;
end record;

-- This version applies the psudo-pre/post-conditions, as supplied.
Procedure Intermediate (A, B : Integer; C : Obj_Handle; D : Handler; E : Conditions) with inline,
 pre  => (if condition'(E.Pre)  /= Null then E.Pre),
 post => (if condition'(E.Post) /= Null then E.Post);
Procedure Intermediate (A, B : Integer; C : Obj_Handle; D : Handler; E : Conditions) is
begin
   D( A, B, C );
end;


Or something like it, no?