elisp: Format Conversion Round-Trip
24.12.2 Round-Trip Specification
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The most general of the two facilities is controlled by the variable
‘format-alist’, a list of “file format” specifications, which describe
textual representations used in files for the data in an Emacs buffer.
The descriptions for reading and writing are paired, which is why we
call this “round-trip” specification (
Format Conversion
Piecemeal, for non-paired specification).
-- Variable: format-alist
This list contains one format definition for each defined file
format. Each format definition is a list of this form:
(NAME DOC-STRING REGEXP FROM-FN TO-FN MODIFY MODE-FN PRESERVE)
Here is what the elements in a format definition mean:
NAME
The name of this format.
DOC-STRING
A documentation string for the format.
REGEXP
A regular expression which is used to recognize files represented
in this format. If ‘nil’, the format is never applied
automatically.
FROM-FN
A shell command or function to decode data in this format (to
convert file data into the usual Emacs data representation).
A shell command is represented as a string; Emacs runs the command
as a filter to perform the conversion.
If FROM-FN is a function, it is called with two arguments, BEGIN
and END, which specify the part of the buffer it should convert.
It should convert the text by editing it in place. Since this can
change the length of the text, FROM-FN should return the modified
end position.
One responsibility of FROM-FN is to make sure that the beginning of
the file no longer matches REGEXP. Otherwise it is likely to get
called again.
TO-FN
A shell command or function to encode data in this format—that is,
to convert the usual Emacs data representation into this format.
If TO-FN is a string, it is a shell command; Emacs runs the command
as a filter to perform the conversion.
If TO-FN is a function, it is called with three arguments: BEGIN
and END, which specify the part of the buffer it should convert,
and BUFFER, which specifies which buffer. There are two ways it
can do the conversion:
• By editing the buffer in place. In this case, TO-FN should
return the end-position of the range of text, as modified.
• By returning a list of annotations. This is a list of
elements of the form ‘(POSITION . STRING)’, where POSITION is
an integer specifying the relative position in the text to be
written, and STRING is the annotation to add there. The list
must be sorted in order of position when TO-FN returns it.
When ‘write-region’ actually writes the text from the buffer
to the file, it intermixes the specified annotations at the
corresponding positions. All this takes place without
modifying the buffer.
MODIFY
A flag, ‘t’ if the encoding function modifies the buffer, and ‘nil’
if it works by returning a list of annotations.
MODE-FN
A minor-mode function to call after visiting a file converted from
this format. The function is called with one argument, the integer
1; that tells a minor-mode function to enable the mode.
PRESERVE
A flag, ‘t’ if ‘format-write-file’ should not remove this format
from ‘buffer-file-format’.
The function ‘insert-file-contents’ automatically recognizes file
formats when it reads the specified file. It checks the text of the
beginning of the file against the regular expressions of the format
definitions, and if it finds a match, it calls the decoding function for
that format. Then it checks all the known formats over again. It keeps
checking them until none of them is applicable.
Visiting a file, with ‘find-file-noselect’ or the commands that use
it, performs conversion likewise (because it calls
‘insert-file-contents’); it also calls the mode function for each format
that it decodes. It stores a list of the format names in the
buffer-local variable ‘buffer-file-format’.
-- Variable: buffer-file-format
This variable states the format of the visited file. More
precisely, this is a list of the file format names that were
decoded in the course of visiting the current buffer’s file. It is
always buffer-local in all buffers.
When ‘write-region’ writes data into a file, it first calls the
encoding functions for the formats listed in ‘buffer-file-format’, in
the order of appearance in the list.
-- Command: format-write-file file format &optional confirm
This command writes the current buffer contents into the file FILE
in a format based on FORMAT, which is a list of format names. It
constructs the actual format starting from FORMAT, then appending
any elements from the value of ‘buffer-file-format’ with a
non-‘nil’ PRESERVE flag (see above), if they are not already
present in FORMAT. It then updates ‘buffer-file-format’ with this
format, making it the default for future saves. Except for the
FORMAT argument, this command is similar to ‘write-file’. In
particular, CONFIRM has the same meaning and interactive treatment
as the corresponding argument to ‘write-file’. Definition of
write-file.
-- Command: format-find-file file format
This command finds the file FILE, converting it according to format
FORMAT. It also makes FORMAT the default if the buffer is saved
later.
The argument FORMAT is a list of format names. If FORMAT is ‘nil’,
no conversion takes place. Interactively, typing just <RET> for
FORMAT specifies ‘nil’.
-- Command: format-insert-file file format &optional beg end
This command inserts the contents of file FILE, converting it
according to format FORMAT. If BEG and END are non-‘nil’, they
specify which part of the file to read, as in
‘insert-file-contents’ (Reading from Files).
The return value is like what ‘insert-file-contents’ returns: a
list of the absolute file name and the length of the data inserted
(after conversion).
The argument FORMAT is a list of format names. If FORMAT is ‘nil’,
no conversion takes place. Interactively, typing just <RET> for
FORMAT specifies ‘nil’.
-- Variable: buffer-auto-save-file-format
This variable specifies the format to use for auto-saving. Its
value is a list of format names, just like the value of
‘buffer-file-format’; however, it is used instead of
‘buffer-file-format’ for writing auto-save files. If the value is
‘t’, the default, auto-saving uses the same format as a regular
save in the same buffer. This variable is always buffer-local in
all buffers.
Info Catalog
elisp: Format Conversion Overview
elisp: Format Conversion
elisp: Format Conversion Piecemeal