text at point (@pxref{Completion in Buffers}).
@end defopt
+@cindex literate programming
+@cindex multi-mode indentation
+ Some major modes need to support embedded regions of text whose
+syntax belongs to a different major mode. Examples include
+@dfn{literate programming} source files that combine documentation and
+snippets of source code, Yacc/Bison programs that include snippets of
+plain C code, etc. To correctly indent the embedded chunks, the major
+mode needs to delegate the indentation to another mode's indentation
+engine (e.g., call @code{c-indent-defun} for C code or
+@code{python-indent-line} for Python), while providing it with some
+context to guide the indentation. The following facilities support
+such multi-mode indentation.
+
+@defvar prog-indentation-context
+This variable, when non-@code{nil}, holds the indentation context for
+the sub-mode's indentation engine provided by the superior major mode.
+The value should be a list of the form @code{(@var{first-column}
+@w{(@var{start} . @var{end})} @code{prev-chunk})}. The members of the
+list have the following meaning:
+
+@table @var
+@item first-column
+The column to be used for top-level constructs. This replaces the
+default value of the top-level column used by the sub-mode, usually
+zero.
+@item start
+@itemx end
+The region of the code chunk to be indented by the sub-mode. The
+value of @var{end} can be @code{nil}, which stands for the value of
+@code{point-max}.
+@item prev-chunk
+If this is non-@code{nil}, it should provide the sub-mode's
+indentation engine with a virtual context of the code chunk. Valid
+values include:
+
+@itemize @minus
+@item
+A string whose contents is the text the sub-mode's indentation engine
+should consider to precede the code chunk. The sub-mode's indentation
+engine can add text properties to that string, to be reused in
+repeated calls with the same string, thus using it as a cache. An
+example where this is useful is code chunks that need to be indented
+as function bodies, but lack the function's preamble---the string
+could then include that missing preamble.
+@item
+A function. It is expected to be called with the start position of
+the current chunk, and should return a cons cell
+@w{@code{(@var{prev-start} . @var{prev-end})}} that specifies the
+region of the previous code chunk, or @code{nil} if there is no previous
+chunk. This is useful in literate-programming sources, where code is
+split into chunks, and correct indentation needs to access previous
+chunks.
+@end itemize
+@end table
+@end defvar
+
+The following convenience functions should be used by major mode's
+indentation engine in support of invocations as sub-modes of another
+major mode.
+
+@defun prog-first-column
+Call this function instead of using a literal value (usually, zero) of
+the column number for indenting top-level program constructs. The
+function's value is the column number to use for top-level constructs.
+When no superior mode is in effect, this function returns zero.
+@end defun
+
+@defun prog-widen
+Call this function instead of @code{widen} to remove any restrictions
+imposed by the mode's indentation engine and restore the restrictions
+recorded in @code{prog-indentation-context}. This prevents the
+indentation engine of a sub-mode from inadvertently operating on text
+outside of the chunk it was supposed to indent, and preserves the
+restriction imposed by the superior mode. When no superior mode is in
+effect, this function just calls @code{widen}.
+@end defun
+
+
@node Region Indent
@subsection Indenting an Entire Region
"Keymap used for programming modes.")
(defvar prog-indentation-context nil
- "Non-nil while indenting embedded code chunks.
+ "When non-nil, provides context for indenting embedded code chunks.
+
There are languages where part of the code is actually written in
a sub language, e.g., a Yacc/Bison or ANTLR grammar also consists
of plain C code. This variable enables the major mode of the
-main language to use the indentation engine of the sub mode for
-lines in code chunks written in the sub language.
+main language to use the indentation engine of the sub-mode for
+lines in code chunks written in the sub-mode's language.
When a major mode of such a main language decides to delegate the
indentation of a line/region to the indentation engine of the sub
-mode, it is supposed to bind this variable to non-nil around the call.
+mode, it should bind this variable to non-nil around the call.
+
+The non-nil value should be a list of the form:
-The non-nil value looks as follows
(FIRST-COLUMN (START . END) PREVIOUS-CHUNKS)
-FIRST-COLUMN is the column the indentation engine of the sub mode
-should usually choose for top-level language constructs inside
-the code chunk (instead of 0).
+FIRST-COLUMN is the column the indentation engine of the sub-mode
+should use for top-level language constructs inside the code
+chunk (instead of 0).
-START to END is the region of the code chunk. See function
-`prog-widen' for additional info.
+START and END specify the region of the code chunk. END can be
+nil, which stands for the value of `point-max'. The function
+`prog-widen' uses this to restore restrictions imposed by the
+sub-mode's indentation engine.
PREVIOUS-CHUNKS, if non-nil, provides the indentation engine of
-the sub mode with the virtual context of the code chunk. Valid
+the sub-mode with the virtual context of the code chunk. Valid
values are:
- - A string containing code which the indentation engine can
+ - A string containing text which the indentation engine can
consider as standing in front of the code chunk. To cache the
string's calculated syntactic information for repeated calls
- with the same string, it is valid and expected for the inner
- mode to add text-properties to the string.
+ with the same string, the sub-mode can add text-properties to
+ the string.
A typical use case is for grammars with code chunks which are
- to be indented like function bodies - the string would contain
- a corresponding function header.
+ to be indented like function bodies -- the string would contain
+ the corresponding function preamble.
- - A function called with the start position of the current
- chunk. It will return either the region of the previous chunk
- as (PREV-START . PREV-END) or nil if there is no further
- previous chunk.
+ - A function, to be called with the start position of the current
+ chunk. It should return either the region of the previous chunk
+ as (PREV-START . PREV-END), or nil if there is no previous chunk.
- A typical use case are literate programming sources - the
- function would successively return the code chunks of the
- previous macro definitions for the same name.")
+ A typical use case are literate programming sources -- the
+ function would successively return the previous code chunks.")
(defun prog-indent-sexp (&optional defun)
"Indent the expression after point.
(defun prog-widen ()
"Remove restrictions (narrowing) from current code chunk or buffer.
-This function can be used instead of `widen' in any function used
-by the indentation engine to make it respect the value
+This function should be used instead of `widen' in any function used
+by the indentation engine to make it respect the value of
`prog-indentation-context'.
This function (like `widen') is useful inside a
narrowing is in effect."
(let ((chunk (cadr prog-indentation-context)))
(if chunk
- ;; no widen necessary here, as narrow-to-region changes (not
- ;; just narrows) existing restrictions
+ ;; No call to `widen' is necessary here, as narrow-to-region
+ ;; changes (not just narrows) the existing restrictions
(narrow-to-region (car chunk) (or (cdr chunk) (point-max)))
(widen))))
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