The ctview and ctedit

The ctview is a viewing tool which allows us to manipulate programs through a graphical interface. Clicking on the button in the View pulldown of the Centaur main menu opens a new ctview which contains an empty Centaur editor, called a ctedit. The ctedit is in charge of displaying information and communicating with the X server (thus, it handles mouse events, keyboard events, etc.) The ctedit contains two items: its subject, which is the thing being edited, and its formatter, which determines the subject's visual presentation. In this tutorial, the ctedit's subject is a Centaur variable structure. The formatter will be the Ppml formatter, which displays a syntax tree textually.

Each ctview menu bar has the following pulldown menus:

• File: This menu allows you to read programs from external files into the ctedit, and to write them to files.
• Display: This menu allows you to modify certain pretty printing parameters (e.g., print level, page width), change pretty printers, and refresh the ctedit contents.
• Edit: Allows you to perform generic edition: cut, copy, paste, or send selected subexpressions to a separate GnuEmacs editor for unstructured edition. The undo button of this menu undoes the last editing operation only.
• Selections: Once you have read a program into a ctview, you may select program fragments by clicking and dragging with the mouse. This menu allows you to fine tune the presentation of selected material. You may make shrink it (set its local pretty print level to zero), expand it (the opposite), or clear it.
• Editing-Tools: This menu gives access to guided editing tools. We describe guided editing below.

When we read a program written in L into a ctview, Centaur loads the L environment which typically decorates the ctview with a graphic interface for L tools. If we have described a graphical environment for L, the ctview is transformed by the addition of the environment's buttons, menus, mouse behavior, etc. The enclosed ctedit becomes a structured editor that only allows edition according to L's syntax definition.