Structured editing is based on an abstract syntax specification. It
ensures syntactic correctness since only complete syntactic elements
can me removed from or added to a program. Two structured editing
techniques are available:
- Generic editing: Certain generic basic editing
strokes -copy, cut, paste, insert, and undo- may be performed for
any abstract syntax tree. These operations are made available through
a graphical interface (the Edit pulldown of a ctview or the
calculator box made visible from the Editing-Tools pulldown).
When you delete or change a subtree, the old subtree is stored in the
global clipboard,which may be made visible by clicking on the
button of the main menu's View pulldown.
Subtrees that you paste are taken from this clipboard. The
editing and transformation manual also describes how to use local
clipboards to accelerate edition.
- Menu mode: The menu mode consists of a number of graphic
tools that together facilitate structural development and program
transformation. Menu mode enables the rapid construction of a program
by selecting and combining templates. A template is a fragment
of abstract syntax interspersed with metavariables. The current
selection in the main program establishes a context to which menus
react, displaying the appropriate templates for that context.
The primary application of menu mode is the guided editing mode.
This tool eliminates syntax errors and takes advantage of information
inherent in the syntax to accelerate program construction.
In this mode, clicking on an applicable template replaces the
currently selected fragment with the menu entry.
To experiment with guided editing, open a new editor by clicking on
the 
button of the View pulldown in the main menu,
then click on 
in the Editing-Tools
menu. A new window opens up, followed by a dialog box asking you for
the formalism name of the program to be developed from scratch. Enter
, in all capital letters. Several things happen:
- The ctedit contains a Metal program fragment. This program is
not whole; it contains metavariable trees, which are place
holders. Guided editing is the process of gradually instantiating
these metavariable trees with more and more detailed program
fragments.
- The second window is the beginner mode menu. One may
select program fragments from this menu (and others) during guiding
editing.
The beginner menu's contents change as the current selection in the
source editor changes. The currently selected subtree establishes a
syntactic context, namely a phylum. This phylum appears in capital
letters in the beginner menu, followed by patterns for operators that
belong to this phylum. Each operator name is followed by its
signature, where descendents are metavariable trees whose names are the
phyla of valid descendents. Clicking on an operator name in this menu
replaces the currently selected subtree by the operator's pattern.
This pattern is in fact a subtree that is pretty printed in the source
ctedit using the current pretty printer. You may go on replacing
non-terminals this way to build your program. The expert menu is just
like the beginner menu except that it displays less pattern information.
To replace atomic values, click on the 
button of
the Editing-Tools menu. This menu contains all atomic values and
tokens that currently appear in your program for the selected context.
Clicking on the <Edit> entry of this menu sends the tree or
metavariable tree text to GnuEmacs for edition. Or, you may click on an
existing atoms menu entry to replace the current selection with that atom.