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develop.txt
*develop.txt* For Vim version 8.0. Last change: 2018 Apr 18 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://vim.wikia.com/wiki/How_to_make_and_submit_a_patch. ============================================================================== 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 exist. - 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! Quoting the function name and argument names is NOT useful. Do explain what they are for. - 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. It does provide a terminal window, in which you can run a shell or debugger. E.g. to be able to do this over an ssh connection. But if you don't need a text editor with that it is out of scope (use something like screen or tmux instead). 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. ;-)" To use Vim with gdb see: http://www.agide.org and http://clewn.sf.net. - 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. Get the code from github. That makes it easier to keep your changed version in sync with the main code base (it may be a while before your changes will be included). You do need to spend some time learning git, it's not the most user friendly tool. 2. Adjust the documentation. Doing this first gives you an impression of how your changes affect the user. 3. Make the source code changes. 4. Check ../doc/todo.txt if the change affects any listed item. 5. Make a patch with "git diff". You can also create a pull request on github, but it's the diff that matters. 6. Make a note about what changed, preferably mentioning the problem and the solution. Send an email to the |vim-dev| maillist with an explanation and include the diff. Or create a pull request on github. C COMPILER *style-compiler* The minimal C compiler version supported is C89, also known as ANSI C. Later standards, such as C99, are not widely supported, or at least not 100% supported. Therefore we use only some of the C99 features and disallow some (at least for now). Please don't make changes everywhere to use the C99 features, it causes merge problems for existing patches. Only use them for new and changed code. Comments ~ Traditionally Vim uses /* comments */. We intend to keep it that way, especially for file and function headers. For new code or lines of code that change, it is allowed to use // comments. Especially when it comes after code: int some_var; // single line comment useful here Enums ~ The last item in an enum may have a trailing comma. C89 didn't allow this. Types ~ "long long" is allowed and can be expected to be 64 bits. Use %lld in printf formats. Also "long long unsigned" with %llu. Not to be used ~ These C99 features are not to be used, because not enough compilers support them: - Declaration after Statements (MSVC 2012 does not support it). All declarations need to be at the start of the block. - Variable length arrays (even in C11 this is an optional feature). - _Bool and _Complex types. - "inline" (it's hardly ever needed, let the optimizer do its work) - flexible array members: Not supported by HP-UX C compiler (John Marriott) 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() mch_memmove() Handles overlapped copies bcopy() mch_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" or "this" 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. clear Mac curses.h echo Mac curses.h instr Mac curses.h meta Mac curses.h newwin Mac curses.h nl Mac curses.h overwrite Mac curses.h refresh Mac curses.h scroll Mac curses.h typeahead Mac curses.h 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); Wrong: if (cond) { cmd; cmd; } else { cmd; cmd; } OK: if (cond) { cmd; cmd; } else { cmd; cmd; } Use ANSI (new style) function declarations with the return type on a separate indented line. Wrong: int function_name(int arg1, int arg2) OK: /* * Explanation of what this function is used for. * * Return value explanation. */ int function_name( int arg1, /* short comment about arg1 */ int arg2) /* short comment about arg2 */ { int local; /* comment about local */ local = arg1 * arg2; 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 easier 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. Doing on-the-fly conversion is not always possible (would require iconv support). - For the programs and libraries: Using them as-is would require installing them separately from Vim. That's mostly 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 hashtable, for example. The affix compression that most spell checkers use makes it slower too. - For using an external 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. - For making suggestions the speed is less important and requiring to install another program or library would be acceptable. But the word lists probably differ, the suggestions may be wrong words. Spelling suggestions *develop-spell-suggestions* For making suggestions there are two basic mechanisms: 1. Try changing the bad word a little bit and check for a match with a good word. Or go through the list of good words, change them a little bit and check for a match with the bad word. The changes are deleting a character, inserting a character, swapping two characters, etc. 2. Perform soundfolding on both the bad word and the good words and then find matches, possibly with a few changes like with the first mechanism. The first is good for finding typing mistakes. After experimenting with hashtables and looking at solutions from other spell checkers the conclusion was that a trie (a kind of tree structure) is ideal for this. Both for reducing memory use and being able to try sensible changes. For example, when inserting a character only characters that lead to good words need to be tried. Other mechanisms (with hashtables) need to try all possible letters at every position in the word. Also, a hashtable has the requirement that word boundaries are identified separately, while a trie does not require this. That makes the mechanism a lot simpler. Soundfolding is useful when someone knows how the words sounds but doesn't know how it is spelled. For example, the word "dictionary" might be written as "daktonerie". The number of changes that the first method would need to try is very big, it's hard to find the good word that way. After soundfolding the words become "tktnr" and "tkxnry", these differ by only two letters. To find words by their soundfolded equivalent (soundalike word) we need a list of all soundfolded words. A few experiments have been done to find out what the best method is. Alternatives: 1. Do the sound folding on the fly when looking for suggestions. This means walking through the trie of good words, soundfolding each word and checking how different it is from the bad word. This is very efficient for memory use, but takes a long time. On a fast PC it takes a couple of seconds for English, which can be acceptable for interactive use. But for some languages it takes more than ten seconds (e.g., German, Catalan), which is unacceptable slow. For batch processing (automatic corrections) it's too slow for all languages. 2. Use a trie for the soundfolded words, so that searching can be done just like how it works without soundfolding. This requires remembering a list of good words for each soundfolded word. This makes finding matches very fast but requires quite a lot of memory, in the order of 1 to 10 Mbyte. For some languages more than the original word list. 3. Like the second alternative, but reduce the amount of memory by using affix compression and store only the soundfolded basic word. This is what Aspell does. Disadvantage is that affixes need to be stripped from the bad word before soundfolding it, which means that mistakes at the start and/or end of the word will cause the mechanism to fail. Also, this becomes slow when the bad word is quite different from the good word. The choice made is to use the second mechanism and use a separate file. This way a user with sufficient memory can get very good suggestions while a user who is short of memory or just wants the spell checking and no suggestions doesn't use so much memory. Word frequency For sorting suggestions it helps to know which words are common. In theory we could store a word frequency with the word in the dictionary. However, this requires storing a count per word. That degrades word tree compression a lot. And maintaining the word frequency for all languages will be a heavy task. Also, it would be nice to prefer words that are already in the text. This way the words that appear in the specific text are preferred for suggestions. What has been implemented is to count words that have been seen during displaying. A hashtable is used to quickly find the word count. The count is initialized from words listed in COMMON items in the affix file, so that it also works when starting a new file. This isn't ideal, because the longer Vim is running the higher the counts become. But in practice it is a noticeable improvement over not using the word count. ============================================================================== 4. Assumptions *design-assumptions* Size of variables: char 8 bit signed char_u 8 bit unsigned int 32 or 64 bit signed (16 might be possible with limited features) unsigned 32 or 64 bit unsigned (16 as with ints) 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:
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