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diff --git a/doc/coding_style.md b/doc/coding_style.md new file mode 100644 index 00000000..6f34d371 --- /dev/null +++ b/doc/coding_style.md @@ -0,0 +1,805 @@ + +# Prosody Coding Style Guide + +This style guides lists the coding conventions used in the +[Prosody](https://prosody.im/) project. It is based heavily on the [style guide used by the LuaRocks project](https://github.com/luarocks/lua-style-guide). + +## Indentation and formatting + +* Prosody code is indented with tabs at the start of the line, a single + tab per logical indent level: + +```lua +for i, pkg in ipairs(packages) do + for name, version in pairs(pkg) do + if name == searched then + print(version) + end + end +end +``` + +Tab width is configurable in editors, so never assume a particular width. +Specically this means you should not mix tabs and spaces, or use tabs for +alignment of items at different indentation levels. + +* Use LF (Unix) line endings. + +## Comments + +* Comments are encouraged where necessary to explain non-obvious code. + +* In general comments should be used to explain 'why', not 'how' + +### Comment tags + +A comment may be prefixed with one of the following tags: + +* **FIXME**: Indicates a serious problem with the code that should be addressed +* **TODO**: Indicates an open task, feature request or code restructuring that + is primarily of interest to developers (otherwise it should be in the + issue tracker). +* **COMPAT**: Must be used on all code that is present only for backwards-compatibility, + and may be removed one day. For example code that is added to support old + or buggy third-party software or dependencies. + +**Example:** + +```lua +-- TODO: implement method +local function something() + -- FIXME: check conditions +end + +``` + +## Variable names + +* Variable names with larger scope should be more descriptive than those with +smaller scope. One-letter variable names should be avoided except for very +small scopes (less than ten lines) or for iterators. + +* `i` should be used only as a counter variable in for loops (either numeric for +or `ipairs`). + +* Prefer more descriptive names than `k` and `v` when iterating with `pairs`, +unless you are writing a function that operates on generic tables. + +* Use `_` for ignored variables (e.g. in for loops:) + +```lua +for _, item in ipairs(items) do + do_something_with_item(item) +end +``` + +* Generally all identifiers (variables and function names) should use `snake_case`, + i.e. lowercase words joined by `_`. + +```lua +-- bad +local OBJEcttsssss = {} +local thisIsMyObject = {} +local c = function() + -- ...stuff... +end + +-- good +local this_is_my_object = {} + +local function do_that_thing() + -- ...stuff... +end +``` + +> **Rationale:** The standard library uses lowercase APIs, with `joinedlowercase` +names, but this does not scale too well for more complex APIs. `snake_case` +tends to look good enough and not too out-of-place along side the standard +APIs. + +```lua +for _, name in pairs(names) do + -- ...stuff... +end +``` + +* Prefer using `is_` when naming boolean functions: + +```lua +-- bad +local function evil(alignment) + return alignment < 100 +end + +-- good +local function is_evil(alignment) + return alignment < 100 +end +``` + +* `UPPER_CASE` is to be used sparingly, with "constants" only. + +> **Rationale:** "Sparingly", since Lua does not have real constants. This +notation is most useful in libraries that bind C libraries, when bringing over +constants from C. + +* Do not use uppercase names starting with `_`, they are reserved by Lua. + +## Tables + +* When creating a table, prefer populating its fields all at once, if possible: + +```lua +local player = { name = "Jack", class = "Rogue" } +} +``` + +* Items should be separated by commas. If there are many items, put each + key/value on a separate line and use a semi-colon after each item (including + the last one): + +```lua +local player = { + name = "Jack"; + class = "Rogue"; +} +``` + +> **Rationale:** This makes the structure of your tables more evident at a glance. +Trailing commas make it quicker to add new fields and produces shorter diffs. + +* Use plain `key` syntax whenever possible, use `["key"]` syntax when using names +that can't be represented as identifiers and avoid mixing representations in +a declaration: + +```lua +local mytable = { + ["1394-E"] = val1, + ["UTF-8"] = val2, + ["and"] = val2, +} +``` + +## Strings + +* Use `"double quotes"` for strings; use `'single quotes'` when writing strings +that contain double quotes. + +```lua +local name = "Prosody" +local sentence = 'The name of the program is "Prosody"' +``` + +> **Rationale:** Double quotes are used as string delimiters in a larger number of +programming languages. Single quotes are useful for avoiding escaping when +using double quotes in literals. + +## Line lengths + +* There are no hard or soft limits on line lengths. Line lengths are naturally +limited by using one statement per line. If that still produces lines that are +too long (e.g. an expression that produces a line over 256-characters long, +for example), this means the expression is too complex and would do better +split into subexpressions with reasonable names. + +> **Rationale:** No one works on VT100 terminals anymore. If line lengths are a proxy +for code complexity, we should address code complexity instead of using line +breaks to fit mind-bending statements over multiple lines. + +## Function declaration syntax + +* Prefer function syntax over variable syntax. This helps differentiate between +named and anonymous functions. + +```lua +-- bad +local nope = function(name, options) + -- ...stuff... +end + +-- good +local function yup(name, options) + -- ...stuff... +end +``` + +* Perform validation early and return as early as possible. + +```lua +-- bad +local function is_good_name(name, options, arg) + local is_good = #name > 3 + is_good = is_good and #name < 30 + + -- ...stuff... + + return is_good +end + +-- good +local function is_good_name(name, options, args) + if #name < 3 or #name > 30 then + return false + end + + -- ...stuff... + + return true +end +``` + +## Function calls + +* Even though Lua allows it, generally you should not omit parentheses + for functions that take a unique string literal argument. + +```lua +-- bad +local data = get_data"KRP"..tostring(area_number) +-- good +local data = get_data("KRP"..tostring(area_number)) +local data = get_data("KRP")..tostring(area_number) +``` + +> **Rationale:** It is not obvious at a glace what the precedence rules are +when omitting the parentheses in a function call. Can you quickly tell which +of the two "good" examples in equivalent to the "bad" one? (It's the second +one). + +* You should not omit parenthesis for functions that take a unique table +argument on a single line. You may do so for table arguments that span several +lines. + +```lua +local an_instance = a_module.new { + a_parameter = 42, + another_parameter = "yay", +} +``` + +> **Rationale:** The use as in `a_module.new` above occurs alone in a statement, +so there are no precedence issues. + +## Table attributes + +* Use dot notation when accessing known properties. + +```lua +local luke = { + jedi = true, + age = 28, +} + +-- bad +local is_jedi = luke["jedi"] + +-- good +local is_jedi = luke.jedi +``` + +* Use subscript notation `[]` when accessing properties with a variable or if using a table as a list. + +```lua +local vehicles = load_vehicles_from_disk("vehicles.dat") + +if vehicles["Porsche"] then + porsche_handler(vehicles["Porsche"]) + vehicles["Porsche"] = nil +end +for name, cars in pairs(vehicles) do + regular_handler(cars) +end +``` + +> **Rationale:** Using dot notation makes it clearer that the given key is meant +to be used as a record/object field. + +## Functions in tables + +* When declaring modules and classes, declare functions external to the table definition: + +```lua +local my_module = {} + +function my_module.a_function(x) + -- code +end +``` + +* When declaring metatables, declare function internal to the table definition. + +```lua +local version_mt = { + __eq = function(a, b) + -- code + end; + __lt = function(a, b) + -- code + end; +} +``` + +> **Rationale:** Metatables contain special behavior that affect the tables +they're assigned (and are used implicitly at the call site), so it's good to +be able to get a view of the complete behavior of the metatable at a glance. + +This is not as important for objects and modules, which usually have way more +code, and which don't fit in a single screen anyway, so nesting them inside +the table does not gain much: when scrolling a longer file, it is more evident +that `check_version` is a method of `Api` if it says `function Api:check_version()` +than if it says `check_version = function()` under some indentation level. + +## Variable declaration + +* Always use `local` to declare variables. + +```lua +-- bad +superpower = get_superpower() + +-- good +local superpower = get_superpower() +``` + +> **Rationale:** Not doing so will result in global variables to avoid polluting +the global namespace. + +## Variable scope + +* Assign variables with the smallest possible scope. + +```lua +-- bad +local function good() + local name = get_name() + + test() + print("doing stuff..") + + --...other stuff... + + if name == "test" then + return false + end + + return name +end + +-- good +local bad = function() + test() + print("doing stuff..") + + --...other stuff... + + local name = get_name() + + if name == "test" then + return false + end + + return name +end +``` + +> **Rationale:** Lua has proper lexical scoping. Declaring the function later means that its +scope is smaller, so this makes it easier to check for the effects of a variable. + +## Conditional expressions + +* False and nil are falsy in conditional expressions. Use shortcuts when you +can, unless you need to know the difference between false and nil. + +```lua +-- bad +if name ~= nil then + -- ...stuff... +end + +-- good +if name then + -- ...stuff... +end +``` + +* Avoid designing APIs which depend on the difference between `nil` and `false`. + +* Use the `and`/`or` idiom for the pseudo-ternary operator when it results in +more straightforward code. When nesting expressions, use parentheses to make it +easier to scan visually: + +```lua +local function default_name(name) + -- return the default "Waldo" if name is nil + return name or "Waldo" +end + +local function brew_coffee(machine) + return (machine and machine.is_loaded) and "coffee brewing" or "fill your water" +end +``` + +Note that the `x and y or z` as a substitute for `x ? y : z` does not work if +`y` may be `nil` or `false` so avoid it altogether for returning booleans or +values which may be nil. + +## Blocks + +* Use single-line blocks only for `then return`, `then break` and `function return` (a.k.a "lambda") constructs: + +```lua +-- good +if test then break end + +-- good +if not ok then return nil, "this failed for this reason: " .. reason end + +-- good +use_callback(x, function(k) return k.last end) + +-- good +if test then + return false +end + +-- bad +if test < 1 and do_complicated_function(test) == false or seven == 8 and nine == 10 then do_other_complicated_function() end + +-- good +if test < 1 and do_complicated_function(test) == false or seven == 8 and nine == 10 then + do_other_complicated_function() + return false +end +``` + +* Separate statements onto multiple lines. Do not use semicolons as statement terminators. + +```lua +-- bad +local whatever = "sure"; +a = 1; b = 2 + +-- good +local whatever = "sure" +a = 1 +b = 2 +``` + +## Spacing + +* Use a space after `--`. + +```lua +--bad +-- good +``` + +* Always put a space after commas and between operators and assignment signs: + +```lua +-- bad +local x = y*9 +local numbers={1,2,3} +numbers={1 , 2 , 3} +numbers={1 ,2 ,3} +local strings = { "hello" + , "Lua" + , "world" + } +dog.set( "attr",{ + age="1 year", + breed="Bernese Mountain Dog" +}) + +-- good +local x = y * 9 +local numbers = {1, 2, 3} +local strings = { + "hello"; + "Lua"; + "world"; +} +dog.set("attr", { + age = "1 year", + breed = "Bernese Mountain Dog", +}) +``` + +* Indent tables and functions according to the start of the line, not the construct: + +```lua +-- bad +local my_table = { + "hello", + "world", + } +using_a_callback(x, function(...) + print("hello") + end) + +-- good +local my_table = { + "hello"; + "world"; +} +using_a_callback(x, function(...) + print("hello") +end) +``` + +> **Rationale:** This keep indentation levels aligned at predictable places. You don't +need to realign the entire block if something in the first line changes (such as +replacing `x` with `xy` in the `using_a_callback` example above). + +* The concatenation operator gets a pass for avoiding spaces: + +```lua +-- okay +local message = "Hello, "..user.."! This is your day # "..day.." in our platform!" +``` + +> **Rationale:** Being at the baseline, the dots already provide some visual spacing. + +* No spaces after the name of a function in a declaration or in its arguments: + +```lua +-- bad +local function hello ( name, language ) + -- code +end + +-- good +local function hello(name, language) + -- code +end +``` + +* Add blank lines between functions: + +```lua +-- bad +local function foo() + -- code +end +local function bar() + -- code +end + +-- good +local function foo() + -- code +end + +local function bar() + -- code +end +``` + +* Avoid aligning variable declarations: + +```lua +-- bad +local a = 1 +local long_identifier = 2 + +-- good +local a = 1 +local long_identifier = 2 +``` + +> **Rationale:** This produces extra diffs which add noise to `git blame`. + +* Alignment is occasionally useful when logical correspondence is to be highlighted: + +```lua +-- okay +sys_command(form, UI_FORM_UPDATE_NODE, "a", FORM_NODE_HIDDEN, false) +sys_command(form, UI_FORM_UPDATE_NODE, "sample", FORM_NODE_VISIBLE, false) +``` + +## Typing + +* In non-performance critical code, it can be useful to add type-checking assertions +for function arguments: + +```lua +function manif.load_manifest(repo_url, lua_version) + assert(type(repo_url) == "string") + assert(type(lua_version) == "string" or not lua_version) + + -- ... +end +``` + +* Use the standard functions for type conversion, avoid relying on coercion: + +```lua +-- bad +local total_score = review_score .. "" + +-- good +local total_score = tostring(review_score) +``` + +## Errors + +* Functions that can fail for reasons that are expected (e.g. I/O) should +return `nil` and a (string) error message on error, possibly followed by other +return values such as an error code. + +* On errors such as API misuse, an error should be thrown, either with `error()` +or `assert()`. + +## Modules + +Follow [these guidelines](http://hisham.hm/2014/01/02/how-to-write-lua-modules-in-a-post-module-world/) for writing modules. In short: + +* Always require a module into a local variable named after the last component of the module’s full name. + +```lua +local bar = require("foo.bar") -- requiring the module + +bar.say("hello") -- using the module +``` + +* Don’t rename modules arbitrarily: + +```lua +-- bad +local skt = require("socket") +``` + +> **Rationale:** Code is much harder to read if we have to keep going back to the top +to check how you chose to call a module. + +* Start a module by declaring its table using the same all-lowercase local +name that will be used to require it. You may use an LDoc comment to identify +the whole module path. + +```lua +--- @module foo.bar +local bar = {} +``` + +* Try to use names that won't clash with your local variables. For instance, don't +name your module something like “size”. + +* Use `local function` to declare _local_ functions only: that is, functions +that won’t be accessible from outside the module. + +That is, `local function helper_foo()` means that `helper_foo` is really local. + +* Public functions are declared in the module table, with dot syntax: + +```lua +function bar.say(greeting) + print(greeting) +end +``` + +> **Rationale:** Visibility rules are made explicit through syntax. + +* Do not set any globals in your module and always return a table in the end. + +* If you would like your module to be used as a function, you may set the +`__call` metamethod on the module table instead. + +> **Rationale:** Modules should return tables in order to be amenable to have their +contents inspected via the Lua interactive interpreter or other tools. + +* Requiring a module should cause no side-effect other than loading other +modules and returning the module table. + +* A module should not have state. If a module needs configuration, turn + it into a factory. For example, do not make something like this: + +```lua +-- bad +local mp = require "MessagePack" +mp.set_integer("unsigned") +``` + +and do something like this instead: + +```lua +-- good +local messagepack = require("messagepack") +local mpack = messagepack.new({integer = "unsigned"}) +``` + +* The invocation of require may omit parentheses around the module name: + +```lua +local bla = require "bla" +``` + +## Metatables, classes and objects + +If creating a new type of object that has a metatable and methods, the +metatable and methods table should be separate, and the metatable name +should end with `_mt`. + +```lua +local mytype_methods = {}; +local mytype_mt = { __index = mytype_methods }; + +function mytype_methods:add_new_thing(thing) +end + +local function new() + return setmetatable({}, mytype_mt); +end + +return { new = new }; +``` + +* Use the method notation when invoking methods: + +``` +-- bad +my_object.my_method(my_object) + +-- good +my_object:my_method() +``` + +> **Rationale:** This makes it explicit that the intent is to use the function as a method. + +* Do not rely on the `__gc` metamethod to release resources other than memory. +If your object manage resources such as files, add a `close` method to their +APIs and do not auto-close via `__gc`. Auto-closing via `__gc` would entice +users of your module to not close resources as soon as possible. (Note that +the standard `io` library does not follow this recommendation, and users often +forget that not closing files immediately can lead to "too many open files" +errors when the program runs for a while.) + +> **Rationale:** The garbage collector performs automatic *memory* management, +dealing with memory only. There is no guarantees as to when the garbage +collector will be invoked, and memory pressure does not correlate to pressure +on other resources. + +## File structure + +* Lua files should be named in all lowercase. + +* Tests should be in a top-level `spec` directory. Prosody uses +[Busted](http://olivinelabs.com/busted/) for testing. + +## Static checking + +All code should pass [luacheck](https://github.com/mpeterv/luacheck) using +the `.luacheckrc` provided in the Prosody repository, and using miminal +inline exceptions. + +* luacheck warnings of class 211, 212, 213 (unused variable, argument or loop +variable) may be ignored, if the unused variable was added explicitly: for +example, sometimes it is useful, for code understandability, to spell out what +the keys and values in a table are, even if you're only using one of them. +Another example is a function that needs to follow a given signature for API +reasons (e.g. a callback that follows a given format) but doesn't use some of +its arguments; it's better to spell out in the argument what the API the +function implements is, instead of adding `_` variables. + +``` +local foo, bar = some_function() --luacheck: ignore 212/foo +print(bar) +``` + +* luacheck warning 542 (empty if branch) can also be ignored, when a sequence +of `if`/`elseif`/`else` blocks implements a "switch/case"-style list of cases, +and one of the cases is meant to mean "pass". For example: + +```lua +if warning >= 600 and warning <= 699 then + print("no whitespace warnings") +elseif warning == 542 then --luacheck: ignore 542 + -- pass +else + print("got a warning: "..warning) +end +``` + +> **Rationale:** This avoids writing negated conditions in the final fallback +case, and it's easy to add another case to the construct without having to +edit the fallback. + |