-*- mode: text; mode: fold -*-

This file documents jed's folding mode.

JED will have folding ability only if compiled with the LINE_ATTR feature.
In particlular, the 16 bit jed executables do not support folding.

{{{ What is folding?
--------------------

Folding is a technique that renders specially marked regions of a file
invisible.  Such a region is called a fold.  A fold may contain more
folds, and so on.  Thus, a folded file has a tree-like structure.
Many argue that folding leads to more ``literate'' programming.

As an example of a folded file, look at `site.sl' that is present in the JED
library.  This file contains about 2000 lines.  When folded, the file
appears to contain a far fewer number of lines and the global structure of
the file will be obvious.

In fact, the file that you are currently reading contains special fold
marks that enable it to be folded.  After reading this document,
enable folding mode and re-read it.

}}}

{{{ Fold mode keybindings
-------------------------

JED's folding mode is activated via the function `folding_mode'.  The
following functions/keybindings will be made available by this mode:

    fold_whole_buffer         "^C^W"
    fold_enter_fold           "^C>"
    fold_exit_fold            "^C<"
    fold_open_buffer          "^C^O"
    fold_fold_region          "^C^F"
    fold_open_fold            "^C^S"
    fold_close_fold           "^C^X"
    fold_search_forward       "^Cf"
    fold_search_backward      "^Cb"
    
I realize that these keybindings are sub-optimal.  However, they are
consistent with the Emacs bindings.  Of course, you are always free to
rebind them via a `fold_mode_hook', e.g.,

     define fold_mode_hook ()
     {
        local_setkey ("fold_whole_buffer", "^P");
	 .
	 .
     }

The `fold_search_forward' and `fold_search_backward' functions will
not find matches on hidden buffer lines.  The standard editor search
functions are indifferent to hidden lines.

Double clicking with the mouse left button on a fold will cause the editor
to enter the fold (fold_enter_fold).  Double clicking on any other line will
cause the editor to exit the current fold.

The current fold will be exited when an attempt is made to cross the
boundary of the fold via the the Up/Down arrow keys (more precisely,
the functions `previous_line_cmd' and `next_line_cmd').  This behavior
can be controlled by the variable `Fold_Bob_Eob_Error_Action'.  To
cause an error to be generated when attempting to cross the boundary
of a fold, add the line

    variable Fold_Bob_Eob_Error_Action = 0;
    
to your jedrc startup initialization file.  To automatically enter the
next (previous) fold, use    

    variable Fold_Bob_Eob_Error_Action = 2;
    
The default value of this variable is 1.    

Of course, the easiest way to learn the folding mode is to fold a buffer and
use the folding_mode keybindings to navigate it.  Since this file
contains fold marks, it is a good place to start.

}}}

{{{ Automatically folding files
-------------------------------

By default, folded files are not automatically folded when they are loaded
into the editor.  To automatically fold a file, set the variable,
Fold_Mode_Ok to a non-zero value in your .jedrc file:

    Fold_Mode_Ok = 1;
    
Then any file that contains the line of the form

    -*- bla bla...  mode: fold; -*-

near the top will be folded.  For example, the first line of both site.sl
and folding.sl look like:

    % -*- mode: slang; mode: fold -*-
  
This tells the editor to load slang mode, then load fold mode.  A C file
might contain the line

    /* -*- mode: C; mode: fold -*- */
   
as its first line.

The function `fold_mode', defined in os.sl looks like:

     define fold_mode ()
     {
        if (Fold_Mode_Ok) folding_mode ();
     }
     
That is, it is simply a wrapper around `folding_mode'.
}}}

{{{ Fold Marks
--------------

By now it should be fairly obvious that the curly braces that appear
near the beginning and end of sections of this document have something
to do with folding.

The line that starts a fold must contain a special string of
characters (called a folding mark) either at the beginning of the line
or at the end of the line.  Similarly, the line that denotes end of a
fold is denoted by a mark which must be different from the mark that
starts a fold.

The marks usually vary on a mode-by-mode basis.  For example, this
file uses the characters `{{{' to denote the start of a fold, and the
characters `}}}' denote the end of a fold.  C mode uses `/*{{{ TEXT */' to
start a fold and `/*}}}*/' to end it.  The function
`fold_add_mode_marks' may be used to associate marks with a fold.  See
folding.sl for more information.

The function `fold_fold_region' may be used to facilitate the
placement of folding marks.  To use this function simply mark a region
of text and press `Ctrl-C Ctrl-F' (or whatever key sequence is bound
to the `fold_fold_region' function').  If the beginning of the region
is located anywhere on a non-blank line, the start-fold mark will be
placed at the END of that line; otherwise, the start-fold mark will be
placed at the beginning.

See the folding.sl file for examples of both kinds of start-fold marks.

}}}

{{{ Caveats
-----------

  One should exercise care when editing a folded file to make sure
  that only visible lines are modified.  Most of the interactive
  insert/delete functions are aware of whether or not a line is
  hidden.  However, other functions, e.g., `replace_cmd', will allow a
  hidden line to be modified.

  Fortunately, the interactive cursor movement functions, e.g.,
  `next_line_cmd', avoid leaving the editing point within a fold.
  However, lower level functions such as `down' do not know about
  folds or hidden lines.

  The bottom line is that if you need to modify text within a fold,
  either enter the fold (fold_enter_fold) or unfold the buffer
  (fold_open_buffer).
  
}}}
