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Subject: Re: Things that OO programming lacks
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NNTP-Posting-Date: Mon, 17 Nov 2014 19:21:43 UTC
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Date: Mon, 17 Nov 2014 12:21:42 -0700
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Date: 2014-11-17T12:21:42-07:00
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On 17-Nov-14 10:28, Dmitry A. Kazakov wrote:
> FSM are proven to be extremely
> complex to use, error-prone and unmaintainable. That is because states are
> as uncomposable as events are. If you add new or remove old states you are
> in deep trouble.

Not really; in Ada we'd just say:

Type Event is ( 'a', 'b', 'c', 'd' );
Type State is ( Start, middle_1, middle_2, Stop );

Type Transition_Table is array (Event, State) of State;

Type FSM(<>) is private;
Function  Create( Transitions : Transition_Table ) return FSM;
Procedure Do_Event( State_Machine : in out FSM; Action : Event );

private
   Type FSM(Transitions : not null access Transition_Table) is record
     Current : State:= Start;
   end record;

-- in the Body ...

Function Create( Transitions : Transition_Table ) return FSM is
( Transitions => New Transition_Table'(Transitions), others => <> );

Procedure Do_Event( State_Machine : in out FSM; Action : Event ) is
begin
   case Action is
    when others => null; -- Attach particular/special actions here.
                         -- Also allow Ada to do case-coverage where
                         -- possible.
   end case;

   State_Machine.Current:=
      State_Machine.Transitions(Action, State_Machine.Current);
end Do_Event;