runtime/doc/develop.txt

*develop.txt*   For Vim version 7.0aa.  Last change: 2005 Mar 29


		  VIM REFERENCE MANUAL    by Bram Moolenaar


Development of Vim.					*development*

This text is important for those who want to be involved in further developing
Vim.

1. Design goals		|design-goals|
2. Coding style		|coding-style|
3. Design decisions	|design-decisions|
4. Assumptions		|design-assumptions|

See the file README.txt in the "src" directory for an overview of the source
code.

Vim is open source software.  Everybody is encouraged to contribute to help
improving Vim.  For sending patches a context diff "diff -c" is preferred.
Also see http://www.vim.org/tips/tip.php?tip_id=618.

==============================================================================
1. Design goals						*design-goals*

Most important things come first (roughly).

Note that quite a few items are contradicting.  This is intentional.  A
balance must be found between them.


VIM IS... VI COMPATIBLE					*design-compatible*

First of all, it should be possible to use Vim as a drop-in replacement for
Vi.  When the user wants to, he can use Vim in compatible mode and hardly
notice any difference with the original Vi.

Exceptions:
- We don't reproduce obvious Vi bugs in Vim.
- There are different versions of Vi.  I am using Version 3.7 (6/7/85) as a
  reference.  But support for other versions is also included when possible.
  The Vi part of POSIX is not considered a definitive source.
- Vim adds new commands, you cannot rely on some command to fail because it
  didn't exist in Vi.
- Vim will have a lot of features that Vi doesn't have.  Going back from Vim
  to Vi will be a problem, this cannot be avoided.
- Some things are hardly ever used (open mode, sending an e-mail when
  crashing, etc.).  Those will only be included when someone has a good reason
  why it should be included and it's not too much work.
- For some items it is debatable whether Vi compatibility should be
  maintained.  There will be an option flag for these.


VIM IS... IMPROVED					*design-improved*

The IMproved bits of Vim should make it a better Vi, without becoming a
completely different editor.  Extensions are done with a "Vi spirit".
- Use the keyboard as much as feasible.  The mouse requires a third hand,
  which we don't have.  Many terminals don't have a mouse.
- When the mouse is used anyway, avoid the need to switch back to the
  keyboard.  Avoid mixing mouse and keyboard handling.
- Add commands and options in a consistent way.  Otherwise people will have a
  hard time finding and remembering them.  Keep in mind that more commands and
  options will be added later.
- A feature that people do not know about is a useless feature.  Don't add
  obscure features, or at least add hints in documentation that they exists.
- Minimize using CTRL and other modifiers, they are more difficult to type.
- There are many first-time and inexperienced Vim users.  Make it easy for
  them to start using Vim and learn more over time.
- There is no limit to the features that can be added.  Selecting new features
  is one based on (1) what users ask for, (2) how much effort it takes to
  implement and (3) someone actually implementing it.


VIM IS... MULTI PLATFORM				*design-multi-platform*

Vim tries to help as many users on as many platforms as possible.
- Support many kinds of terminals.  The minimal demands are cursor positioning
  and clear-screen.  Commands should only use key strokes that most keyboards
  have.  Support all the keys on the keyboard for mapping.
- Support many platforms.  A condition is that there is someone willing to do
  Vim development on that platform, and it doesn't mean messing up the code.
- Support many compilers and libraries.  Not everybody is able or allowed to
  install another compiler or GUI library.
- People switch from one platform to another, and from GUI to terminal
  version.  Features should be present in all versions, or at least in as many
  as possible with a reasonable effort.  Try to avoid that users must switch
  between platforms to accomplish their work efficiently.
- That a feature is not possible on some platforms, or only possible on one
  platform, does not mean it cannot be implemented.  [This intentionally
  contradicts the previous item, these two must be balanced.]


VIM IS... WELL DOCUMENTED				*design-documented*

- A feature that isn't documented is a useless feature.  A patch for a new
  feature must include the documentation.
- Documentation should be comprehensive and understandable.  Using examples is
  recommended.
- Don't make the text unnecessarily long.  Less documentation means that an
  item is easier to find.


VIM IS... HIGH SPEED AND SMALL IN SIZE			*design-speed-size*

Using Vim must not be a big attack on system resources.  Keep it small and
fast.
- Computers are becoming faster and bigger each year.  Vim can grow too, but
  no faster than computers are growing.  Keep Vim usable on older systems.
- Many users start Vim from a shell very often.  Startup time must be short.
- Commands must work efficiently.  The time they consume must be as small as
  possible.  Useful commands may take longer.
- Don't forget that some people use Vim over a slow connection.  Minimize the
  communication overhead.
- Items that add considerably to the size and are not used by many people
  should be a feature that can be disabled.
- Vim is a component among other components.  Don't turn it into a massive
  application, but have it work well together with other programs.


VIM IS... MAINTAINABLE					*design-maintain*

- The source code should not become a mess.  It should be reliable code.
- Use the same layout in all files to make it easy to read |coding-style|.
- Use comments in a useful way!
- Porting to another platform should be made easy, without having to change
  too much platform-independent code.
- Use the object-oriented spirit: Put data and code together.  Minimize the
  knowledge spread to other parts of the code.


VIM IS... FLEXIBLE					*design-flexible*

Vim should make it easy for users to work in their preferred styles rather
than coercing its users into particular patterns of work.  This can be for
items with a large impact (e.g., the 'compatible' option) or for details.  The
defaults are carefully chosen such that most users will enjoy using Vim as it
is.  Commands and options can be used to adjust Vim to the desire of the user
and its environment.


VIM IS... NOT						*design-not*

- Vim is not a shell or an Operating System.  You will not be able to run a
  shell inside Vim or use it to control a debugger.  This should work the
  other way around: Use Vim as a component from a shell or in an IDE.
  A satirical way to say this: "Unlike Emacs, Vim does not attempt to include
  everything but the kitchen sink, but some people say that you can clean one
  with it.  ;-)"
- Vim is not a fancy GUI editor that tries to look nice at the cost of
  being less consistent over all platforms.  But functional GUI features are
  welcomed.

==============================================================================
2. Coding style						*coding-style*

These are the rules to use when making changes to the Vim source code.  Please
stick to these rules, to keep the sources readable and maintainable.

This list is not complete.  Look in the source code for more examples.


MAKING CHANGES						*style-changes*

The basic steps to make changes to the code:
1. Adjust the documentation.  Doing this first gives you an impression of how
   your changes affect the user.
2. Make the source code changes.
3. Check ../doc/todo.txt if the change affects any listed item.
4. Make a patch with "diff -c" against the unmodified code and docs.
5. Make a note about what changed and include it with the patch.


USE OF COMMON FUNCTIONS					*style-functions*

Some functions that are common to use, have a special Vim version.  Always
consider using the Vim version, because they were introduced with a reason.

NORMAL NAME	VIM NAME	DIFFERENCE OF VIM VERSION
free()		vim_free()	Checks for freeing NULL
malloc()	alloc()		Checks for out of memory situation
malloc()	lalloc()	Like alloc(), but has long argument
strcpy()	STRCPY()	Includes cast to (char *), for char_u * args
strchr()	vim_strchr()	Accepts special characters
strrchr()	vim_strrchr()	Accepts special characters
isspace()	vim_isspace()	Can handle characters > 128
iswhite()	vim_iswhite()	Only TRUE for Tab and space
memcpy()	vim_memmove()	Handles overlapped copies
bcopy()		vim_memmove()	Handles overlapped copies
memset()	vim_memset()	Uniform for all systems


NAMES							*style-names*

Function names can not be more than 31 characters long (because of VMS).

Don't use "delete" as a variable name, C++ doesn't like it.

Because of the requirement that Vim runs on as many systems as possible, we
need to avoid using names that are already defined by the system.  This is a
list of names that are known to cause trouble.  The name is given as a regexp
pattern.

is.*()		POSIX, ctype.h
to.*()		POSIX, ctype.h

d_.*		POSIX, dirent.h
l_.*		POSIX, fcntl.h
gr_.*		POSIX, grp.h
pw_.*		POSIX, pwd.h
sa_.*		POSIX, signal.h
mem.*		POSIX, string.h
str.*		POSIX, string.h
wcs.*		POSIX, string.h
st_.*		POSIX, stat.h
tms_.*		POSIX, times.h
tm_.*		POSIX, time.h
c_.*		POSIX, termios.h
MAX.*		POSIX, limits.h
__.*		POSIX, system
_[A-Z].*	POSIX, system
E[A-Z0-9]*	POSIX, errno.h

*_t		POSIX, for typedefs.  Use *_T instead.

wait		don't use as argument to a function, conflicts with types.h
index		shadows global declaration
time		shadows global declaration
new		C++ reserved keyword
try		Borland C++ doesn't like it to be used as a variable.

basename()	GNU string function
dirname()	GNU string function
get_env_value()	Linux system function


VARIOUS							*style-various*

Typedef'ed names should end in "_t": >
    typedef int some_t;
Define'ed names should be uppercase: >
    #define SOME_THING
Features always start with "FEAT_": >
    #define FEAT_FOO

Don't use '\"', some compilers can't handle it.  '"' works fine.

Don't use:
    #if HAVE_SOME
Some compilers can't handle that and complain that "HAVE_SOME" is not defined.
Use
    #ifdef HAVE_SOME
or
    #if defined(HAVE_SOME)


STYLE							*style-examples*

General rule: One statement per line.

Wrong:	    if (cond) a = 1;

OK:	    if (cond)
		a = 1;

Wrong:	    while (cond);

OK:	    while (cond)
		;

Wrong:	    do a = 1; while (cond);

OK:	    do
		a = 1;
	    while (cond);


Functions start with:

Wrong:	int function_name(int arg1, int arg2)

OK:	/*
	 * Explanation of what this function is used for.
	 *
	 * Return value explanation.
	 */
	    int
	function_name(arg1, arg2)
	    int		arg1;		/* short comment about arg1 */
	    int		arg2;		/* short comment about arg2 */
	{
	    int		local;		/* comment about local */

	    local = arg1 * arg2;

NOTE: Don't use ANSI style function declarations.  A few people still have to
use a compiler that doesn't support it.


SPACES AND PUNCTUATION					*style-spaces*

No space between a function name and the bracket:

Wrong:  func (arg);
OK:	func(arg);

Do use a space after if, while, switch, etc.

Wrong:	if(arg)		for(;;)
OK:	if (arg)	for (;;)

Use a space after a comma and semicolon:

Wrong:  func(arg1,arg2);	for (i = 0;i < 2;++i)
OK:	func(arg1, arg2);	for (i = 0; i < 2; ++i)

Use a space before and after '=', '+', '/', etc.

Wrong:	var=a*5;
OK:	var = a * 5;

In general: Use empty lines to group lines of code together.  Put a comment
just above the group of lines.  This makes it more easy to quickly see what is
being done.

OK:	/* Prepare for building the table. */
	get_first_item();
	table_idx = 0;

	/* Build the table */
	while (has_item())
	    table[table_idx++] = next_item();

	/* Finish up. */
	cleanup_items();
	generate_hash(table);

==============================================================================
3. Design decisions					*design-decisions*

Folding

Several forms of folding should be possible for the same buffer.  For example,
have one window that shows the text with function bodies folded, another
window that shows a function body.

Folding is a way to display the text.  It should not change the text itself.
Therefore the folding has been implemented as a filter between the text stored
in a buffer (buffer lines) and the text displayed in a window (logical lines).


Naming the window

The word "window" is commonly used for several things: A window on the screen,
the xterm window, a window inside Vim to view a buffer.
To avoid confusion, other items that are sometimes called window have been
given another name.  Here is an overview of the related items:

screen		The whole display.  For the GUI it's something like 1024x768
		pixels.  The Vim shell can use the whole screen or part of it.
shell		The Vim application.  This can cover the whole screen (e.g.,
		when running in a console) or part of it (xterm or GUI).
window		View on a buffer.  There can be several windows in Vim,
		together with the command line, menubar, toolbar, etc. they
		fit in the shell.


Spell checking						*develop-spell*

When spell checking was going to be added to Vim a survey was done over the
available spell checking libraries and programs.  Unfortunately, the result
was that none of them provided sufficient capabilities to be used as the spell
checking engine in Vim, for various reasons:

- Missing support for multi-byte encodings.  At least UTF-8 must be supported,
  so that more than one language can be used in the same file.
- For the programs and libraries: Using them as-is would require installing
  them separately from Vim.  That's not impossible, but a drawback.
- Performance: A few tests showed that it's possible to check spelling on the
  fly (while redrawing), just like syntax highlighting.  But the mechanisms
  used by other code are much slower.  Myspell uses a simplistic hashtable,
  for example.
- For a program like aspell a communication mechanism would have to be setup.
  That's complicated to do in a portable way (Unix-only would be relatively
  simple, but that's not good enough).  And performance will become a problem
  (lots of process switching involved).
- Missing support for words with non-word characters, such as "Etten-Leur" and
  "et al.", would require marking the pieces of them OK, lowering the
  reliability.
- Missing support for regions or dialects.  Makes it difficult to accept
  all English words and highlight non-Canadian words differently.
- Missing support for rare words.  Many words are correct but hardly ever used
  and could be a misspelled often-used word.


==============================================================================
4. Assumptions						*design-assumptions*

Size of variables:
char	    8 bit signed
char_u	    8 bit unsigned
int	    16, 32 or 64 bit signed
unsigned    16, 32 or 64 bit unsigned
long	    32 or 64 bit signed, can hold a pointer

Note that some compilers cannot handle long lines or strings.  The C89
standard specifies a limit of 509 characters.

 vim:tw=78:ts=8:ft=help:norl: