AD>Расскажите, плиз, только попроще и по русски

FUNCTIONS THAT USE POINTERS OR REFERENCES TO BASE CLASSES MUST BE ABLE TO USE OBJECTS OF DERIVED CLASSES WITHOUT KNOWING IT
Above is a paraphrase of the Liskov Substitution Principle (LSP). Barbara Liskov first
it as follows nearly 8 years ago. What is wanted here is something like the following substitution property: If
for each object o of type S there is an object o of type T such that for all programs P defined in terms of T, the behavior of P is unchanged when o is substituted for o then S is a subtype of T.

void DrawShape(const Shape& s)
{
    if (typeid(s) == typeid(Square))
        DrawSquare(static_cast<Square&>(s));
    else if (typeid(s) == typeid(Circle))
        DrawCircle(static_cast<Circle&>(s));
}

VG>void DrawShape(const Shape& s)
VG>{
VG>    if (typeid(s) == typeid(Square))
VG>        DrawSquare(static_cast<Square&>(s));
VG>    else if (typeid(s) == typeid(Circle))
VG>        DrawCircle(static_cast<Circle&>(s));
VG>}
VG>

DrawShape(shape : Shape) : void
{
  | square is Square => DrawSquare(square);
  | circle is Circle => DrawCircle(circle);
    | _                => DoDefaultAction(shape);
}

Subtypes must be substitutable for their base types

VD>Тут уже пробегала цитата про паттерны GoF и динамические языки

AD>Расскажите, плиз, только попроще и по русски

Если подстановка типа A, вместо типа B не приводит к изменению поведения программы P, то тип A, явялется подтипом B.

СГ>вроде аналогичный, а ничего не нарушает

Oberon-2 introduced limited reflection, methods ("type bound procedures"), and single inheritance ("type extension") without interfaces or mixins. Method calls were resolved at run-time, and limited polymorphism was possible by either overloading methods or by explicit typecase ("with" statement).

VG>вместо одного "msg.DoSomething()"

  TYPE
    CursorMessage = ABSTRACT RECORD (RequestMessage)
      x, y: INTEGER
    END;

    DropMsg = RECORD (TransferMessage)
      view: Views.View;
      isSingle: BOOLEAN;
      w, h, rx, ry: INTEGER
    END;

    EditMsg = RECORD (RequestMessage)
      op: INTEGER;
      modifiers: SET;
      char: CHAR;
      view: Views.View;
      w, h: INTEGER;
      isSingle, clipboard: BOOLEAN
    END;

    MarkMsg = RECORD (Views.CtrlMessage)
      show, focus: BOOLEAN
    END;

    Message = Views.CtrlMessage;

    PageMsg = RECORD (Views.CtrlMessage)
      op, pageX, pageY: INTEGER;
      done, eox, eoy: BOOLEAN
    END;

    PollCursorMsg = RECORD (CursorMessage)
      cursor: INTEGER;
      modifiers: SET
    END;

    PollDropMsg = RECORD (TransferMessage)
      mark, show: BOOLEAN;
      type: Stores.TypeName;
      isSingle: BOOLEAN;
      w, h, rx, ry: INTEGER;
      dest: Views.Frame
    END;

    PollFocusMsg = EXTENSIBLE RECORD (Views.CtrlMessage)
      focus: Views.Frame
    END;

    PollOpsMsg = RECORD (Views.CtrlMessage)
      type, pasteType: Stores.TypeName;
      singleton: Views.View;
      selectable: BOOLEAN;
      valid: SET
    END;

    PollSectionMsg = RECORD (Views.CtrlMessage)
      focus, vertical: BOOLEAN;
      wholeSize, partSize, partPos: INTEGER;
      valid, done: BOOLEAN
    END;

    ReplaceViewMsg = RECORD (RequestMessage)
      old, new: Views.View
    END;

    RequestMessage = ABSTRACT RECORD (Views.CtrlMessage)
      requestFocus: BOOLEAN
    END;

    ScrollMsg = RECORD (Views.CtrlMessage)
      focus, vertical: BOOLEAN;
      op, pos: INTEGER;
      done: BOOLEAN
    END;

    SelectMsg = RECORD (Views.CtrlMessage)
      set: BOOLEAN
    END;

    TickMsg = RECORD (Views.CtrlMessage)
      tick: INTEGER
    END;

    TrackMsg = RECORD (CursorMessage)
      modifiers: SET
    END;

    TransferMessage = ABSTRACT RECORD (CursorMessage)
      source: Views.Frame;
      sourceX, sourceY: INTEGER
    END;

    WheelMsg = RECORD (CursorMessage)
      done: BOOLEAN;
      op, nofLines: INTEGER
    END;

TYPE WheelMsg (CursorMessage)
Это сообщение посылается, когда колесо на мыши с колесом поворачивается.

done: BOOLEAN
Если отображение обрабатывает это сообщение, оно должно установить флаг done в TRUE.

op: INTEGER
Показывает, какой тип события колеса мыши произошел. Используются те же константы, что и для прокрутки, но допустимы только следующие из них: incPage, decPage, incLine and decLine.

nofLines: INTEGER nofLines >= 1
Если op или icnLine, или decLine, то nofLines показывает, сколько строк должно быть прокручено. Для incPage и decPage это значение не определено.