[Oberon] On Oberon Commands

[Srinivas Nayak]

Dear All,

 From Project Oberon Book,

    2.2.2. Commands

    This tremendous flexibility seems to open Pandora's box, and indeed
    it does when misused. The reason is that global variables' states
    may completely determine and alter the effect of a command. The
    variables represent hidden states, hidden in the sense that the user
    is in general unaware of them and has no easy way to determine their
    value. The positive aspect of using global variables as interfaces
    between commands is that some of them may well be visible on the
    display. All viewers - and with them also their contents - are
    organized in a data structure that is rooted in a global variable
    (in module Viewers). Parts of this variable therefore constitute
    visible states, and it is highly appropriate to refer to them as
    command parameters.

This is good, I am able to understand it.


    One of the rules of what may be called the Oberon Programming Style
    is therefore to avoid hidden states, and to reduce the introduction
    of global variables. We do not, however, raise this rule to the rank
    of a dogma. There exist genuinely useful exceptions, even if the
    variables have no visible parts.

As I understood, we are using global variables, but these have visible 
states. So better for using them as parameters to user commands.


    There remains the question of how to denote visible objects as
    command parameters. An obvious case is the use of the most recent
    selection as parameter. A procedure for locating that selection is
    provided by module Oberon. (It is restricted to text selections).
    Another possibility is the use of the caret position in a text. This
    is used in the case of inserting new text; the pressing of a key on
    the keyboard is also considered to be a command, and it causes the
    character's insertion at the caret position.

This says, caret position (which is seen on screen texts after pressing 
left mouse key) can be interpreted as starting of a parameter list. Also 
the text under selection (which we are able to make using right mouse 
button on screen texts) can be interpreted as starting of a parameter list.


    A special facility is introduced for designating viewers as
    operands: the star marker. It is placed at the cursor position when
    the keyboard's mark key (SETUP) is pressed. The procedure
    Oberon.MarkedViewer identifies the viewer in whose area the star
    lies. Commands which take it as their parameter are typically
    followed by an asterisk in the text. Whether the text contained in a
    text viewer, or a graph contained in a graphic viewer, or any other
    part of the marked viewer is taken as the actual parameter depends
    on how the command procedure is programmed.

Huh, here I couldn't understand, what is a marked viewer. Which one is 
the equivalent of "keyboard's mark key" in IBM 102 style keyboard? How a 
graph be input to some command? I am not able to understand this paragraph!


With thanks and best regards,

Yours sincerely,
Srinivas Nayak

Home: http://www.mathmeth.com/sn/
Blog: http://srinivas-nayak.blogspot.in/