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fennel-tutorial(7) Getting Started with Fennel fennel-tutorial(7) NAME fennel-tutorial - Getting Started with Fennel DESCRIPTION A programming language is made up of syntax and semantics. The semantics of Fennel vary only in small ways from Lua (all noted below). The syntax of Fennel comes from the lisp family of languages. Lisps have syntax which is very uniform and predictable, which makes it easier to write code that operates on code as well as structured editing. If you know Lua and a lisp already, you'll feel right at home in Fennel. Even if not, Lua is one of the simplest programming languages in existence, so if you've programmed before you should be able to pick it up without too much trouble, especially if you've used another dynamically-typed impera- tive language with closures. The Lua reference manual is a fine place to look for details, but Fennel's own Lua Primer is shorter and covers the highlights. If you've already got some Lua example code and you just want to see how it would look in Fennel, you can learn a lot from putting it in antifennel. OK, SO HOW DO YOU DO THINGS? Functions and lambdas Use fn to make functions. If you provide an optional name, the function will be bound to that name in local scope; otherwise it is simply an anony- mous value. A brief note on naming: identifiers are typically lowercase sepa- rated by dashes (aka "kebab-case"). They may contain digits too, as long as they're not at the start. You can also use the question mark (typically for functions that return a true or false, ex., at-max-velocity?). Underscores (_) are often used to name a vari- able that we don't plan on using. The argument list is provided in square brackets. The final value in the body is returned. (If you've never used a lisp before, the main thing to note is that the function or macro being called goes inside the parens, not outside.) (fn print-and-add [a b c] (print a) (+ b c)) Functions can take an optional docstring in the form of a string that imme- diately follows the argument list. Under normal compilation, this is re- moved from the emitted Lua, but during development in the REPL the doc- string and function usage can be viewed with the ,doc command: (fn print-sep [sep ...] "Prints args as a string, delimited by sep" (print (table.concat [...] sep))) ,doc print-sep ; -> outputs: ;; (print-sep sep ...) ;; Prints args as a string, delimited by sep Like other lisps, Fennel uses semicolons for comments. Functions defined with fn are fast; they have no runtime overhead compared to Lua. However, they also have no arity checking. (That is, calling a function with the wrong number of arguments does not cause an error.) For safer code you can use lambda which ensures you will get at least as many arguments as you define, unless you signify that one may be omitted by be- ginning its name with a ?: (lambda print-calculation [x ?y z] (print (- x (* (or ?y 1) z)))) (print-calculation 5) ; -> error: Missing argument z Note that the second argument ?y is allowed to be nil, but z is not: (print-calculation 5 nil 3) ; -> 2 Like fn, lambdas accept an optional docstring after the argument list. Locals and variables Locals are introduced using let with the names and values wrapped in a sin- gle set of square brackets: (let [x (+ 89 5.2) f (fn [abc] (print (* 2 abc)))] (f x)) Here x is bound to the result of adding 89 and 5.2, while f is bound to a function that prints twice its argument. These bindings are only valid in- side the body of the let call. All values are dynamically typed. You can also introduce locals with local, which is nice when they'll be used across the whole file, but in general let is preferred inside func- tions because it's clearer at a glance where the value can be used: (local tau-approx 6.28318) Locals set this way cannot be given new values, but you can introduce new locals that shadow the outer names: (let [x 19] ;; (set x 88) <- not allowed! (let [x 88] (print (+ x 2))) ; -> 90 (print x)) ; -> 19 If you need to change the value of a local, you can use var which works like local except it allows set to work on it. There is no nested let-like equivalent of var. (var x 19) (set x (+ x 8)) (print x) ; -> 27 Note that introducing a table with local or let does not prevent its fields from being changed using set. Numbers and strings Of course, all our standard arithmetic operators like +, -, *, and / work here in prefix form. Note that numbers are double-precision floats in all Lua versions prior to 5.3, which introduced integers. On 5.3 and up, inte- ger division uses // and bitwise operations use lshift, rshift, bor, band, bnot and xor. Bitwise operators and integer division will not work if the host Lua environment is older than version 5.3. You may also use underscores to separate sections of long numbers. The un- derscores have no effect on the value. (let [x (+ 1 99) y (- x 12) z 100_000] (+ z (/ y 10))) Strings are essentially immutable byte arrays. UTF-8 support is provided in the utf8 table in Lua 5.3+ or from a 3rd-party library in earlier ver- sions. Strings are concatenated with ..: (.. "hello" " world") Tables In Lua (and thus in Fennel), tables are the only data structure. The main syntax for tables uses curly braces with key/value pairs in them: {"key" value "number" 531 "f" (fn [x] (+ x 2))} You can use . to get data out of tables: (let [tbl (function-which-returns-a-table) key "a certain key"] (. tbl key)) And tset to put them in: (let [tbl {} key1 "a long string" key2 12] (tset tbl key1 "the first value") (tset tbl key2 "the second one") tbl) ; -> {"a long string" "the first value" 12 "the second one"} Sequential Tables Some tables are used to store data that's used sequentially; the keys in this case are just numbers starting with 1 and going up. Fennel provides alternate syntax for these tables with square brackets: ["abc" "def" "xyz"] ; equivalent to {1 "abc" 2 "def" 3 "xyz"} Lua's built-in table.insert function is meant to be used with sequential tables; all elements after the inserted value are shifted up by one index: If you don't provide an index to table.insert it will append to the end of the table. The table.remove function works similarly; it takes a table and an index (which defaults to the end of the table) and removes the value at that in- dex, returning it. (local ltrs ["a" "b" "c" "d"]) (table.remove ltrs) ; Removes "d" (table.remove ltrs 1) ; Removes "a" (table.insert ltrs "d") ; Appends "d" (table.insert ltrs 1 "a") ; Prepends "a" (. ltrs 2) ; -> "b" ;; ltrs is back to its original value ["a" "b" "c" "d"] The length form returns the length of sequential tables and strings: (let [tbl ["abc" "def" "xyz"]] (+ (length tbl) (length (. tbl 1)))) ; -> 6 Note that the length of a table with gaps in it is undefined; it can return a number corresponding to any of the table's "boundary" positions between nil and non-nil values. Lua's standard library is very small, and thus several functions you might expect to be included, such map, reduce, and filter are absent. In Fennel macros are used for this instead; see icollect, collect, and accumulate. Iteration Looping over table elements is done with each and an iterator like pairs (used for general tables) or ipairs (for sequential tables): (each [key value (pairs {"key1" 52 "key2" 99})] (print key value)) (each [index value (ipairs ["abc" "def" "xyz"])] (print index value)) Note that whether a table is sequential or not is not an inherent property of the table but depends on which iterator is used with it. You can call ipairs on any table, and it will only iterate over numeric keys starting with 1 until it hits a nil. You can use any Lua iterator with each, but these are the most common. Here's an example that walks through matches in a string: (var sum 0) (each [digits (string.gmatch "244 127 163" "%d+")] (set sum (+ sum (tonumber digits)))) If you want to get a table back, try icollect to get a sequential table or collect to get a key/value one. A body which returns nil will cause that to be omitted from the resulting table. (icollect [_ s (ipairs [:greetings :my :darling])] (if (not= :my s) (s:upper))) ;; -> ["GREETINGS" "DARLING"] (collect [_ s (ipairs [:greetings :my :darling])] s (length s)) ;; -> {:darling 7 :greetings 9 :my 2} A lower-level iteration construct is for which iterates numerically from the provided start value to the inclusive finish value: (for [i 1 10] (print i)) You can specify an optional step value; this loop will only print odd num- bers under ten: (for [i 1 10 2] (print i)) Looping If you need to loop but don't know how many times, you can use while: (while (keep-looping?) (do-something)) Conditionals Finally we have conditionals. The if form in Fennel can be used the same way as in other lisp languages, but it can also be used as cond for multi- ple conditions compiling into elseif branches: (let [x (math.random 64)] (if (= 0 (% x 2)) "even" (= 0 (% x 9)) "multiple of nine" "I dunno, something else")) With an odd number of arguments, the final clause is interpreted as "else". Being a lisp, Fennel has no statements, so if returns a value as an expres- sion. Lua programmers will be glad to know there is no need to construct precarious chains of and/or just to get a value! The other conditional is when, which is used for an arbitrary number of side-effects and has no else clause: (when (currently-raining?) (wear "boots") (deploy-umbrella)) BACK TO TABLES JUST FOR A BIT Strings that don't have spaces or reserved characters in them can use the :shorthand syntax instead, which is often used for table keys: {:key value :number 531} If a table has string keys like this, you can pull fields out of it easily with a dot if the keys are known up front: (let [tbl {:x 52 :y 91}] (+ tbl.x tbl.y)) ; -> 143 You can also use this syntax with set: (let [tbl {}] (set tbl.one 1) (set tbl.two 2) tbl) ; -> {:one 1 :two 2} If a table key has the same name as the variable you're binding it to, you can omit the key name and use : instead: (let [one 1 two 2 tbl {: one : two}] tbl) ; -> {:one 1 :two 2} Finally, let can destructure a table into multiple locals. There is positional destructuring: (let [data [1 2 3] [fst snd thrd] data] (print fst snd thrd)) ; -> 1 2 3 And destructuring of tables via key: (let [pos {:x 23 :y 42} {:x x-pos :y y-pos} pos] (print x-pos y-pos)) ; -> 23 42 As above, if a table key has the same name as the variable you're destruc- turing it to, you can omit the key name and use : instead: (let [pos {:x 23 :y 42} {: x : y} pos] (print x y)) ; -> 23 42 This can nest and mix and match: (let [f (fn [] ["abc" "def" {:x "xyz" :y "abc"}]) [a d {:x x : y}] (f)] (print a d) (print x y)) If the size of the table doesn't match the number of binding locals, miss- ing values are filled with nil and extra values are discarded. Note that unlike many languages, nil in Lua actually represents the absence of a value, and thus tables cannot contain nil. It is an error to try to use nil as a key, and using nil as a value removes whatever entry was at that key before. ERROR HANDLING Errors in Lua have two forms they can take. Functions in Lua can return any number of values, and most functions which can fail will indicate fail- ure by using two return values: nil followed by a failure message string. You can interact with this style of function in Fennel by placing the func- tion call into a table and pattern matching against the table: (case [(io.open "file")] ;; when io.open succeeds, it will return a file, but if it fails ;; it will return nil and an err-msg string describing why [f] (do (use-file-contents (f:read :*all)) (f:close)) [nil err-msg] (report-error "Could not open file:" err-msg)) You can write your own function which returns multiple values by calling values. (fn use-file [filename] (if (valid-file-name? filename) (open-file filename) (values nil (.. "Invalid filename: " filename)))) Note: while errors are the most common reason to return multiple values from a function, it can be used in other cases as well. This is the most complicated feature of Lua, and a full discussion is out of scope for this tutorial, but it's https://benaiah.me/posts/everything-you-didnt-want-to- know-about-lua-multi- vals/ covered well elsewhere. The problem with this type of error is that it does not compose well; the error status must be propagated all the way along the call chain from inner to outer. To address this, you can use error. This will terminate the whole process unless it's within a protected call, similar to the way in other languages where throwing an exception will stop the program unless it is within a try/catch. You can make a protected call with pcall: (let [(ok? val-or-msg) (pcall potentially-disastrous-call filename)] (if ok? (print "Got value" val-or-msg) (print "Could not get value:" val-or-msg))) The pcall invocation there means you are running (potentially-disas- trous-call filename) in protected mode. pcall takes an arbitrary number of arguments which are passed on to the function. You can see that pcall re- turns a boolean (ok? here) to let you know if the call succeeded or not, and a second value (val-or-msg) which is the actual value if it succeeded or an error message if it didn't. The assert function takes a value and an error message; it calls error if the value is nil and returns it otherwise. This can be used to turn multi- ple-value failures into errors (kind of the inverse of pcall which turns errors into multiple-value failures): (let [f (assert (io.open filename)) contents (f.read f "*all")] (f.close f) contents) In this example because io.open returns nil and an error message upon fail- ure, a failure will trigger an error and halt execution. VARIADIC FUNCTIONS Fennel supports variadic functions (in other words, functions which take any number of arguments) like many languages. The syntax for taking a variable number of arguments to a function is the ... symbol, which must be the last parameter to a function. This syntax is inherited from Lua rather than Lisp. The ... form is not a list or first class value, it expands to multiple values inline. To access individual elements of the vararg, you can de- structure with parentheses, or first wrap it in a table literal ([...]) and index like a normal table, or use the select function from Lua's core li- brary. Often, the vararg can be passed directly to another function such as print without needing to bind it. (fn print-each [...] (each [i v (ipairs [...])] (print (.. "Argument " i " is " v)))) (print-each :a :b :c) (fn myprint [prefix ...] (io.write prefix) (io.write (.. (select "#" ...) " arguments given: ")) (print ...)) (myprint ":D " :d :e :f) Varargs are scoped differently than other variables as well - they are only accessible to the function in which they are created. Unlike normal val- ues, functions cannot close over them. This means that the following code will NOT work, as the varargs in the inner function are out of scope. (fn badcode [...] (fn [] (print ...))) STRICT GLOBAL CHECKING If you get an error that says unknown global in strict mode it means that you're trying compile code that uses a global which the Fennel compiler doesn't know about. Most of the time, this is due to a coding mistake. However, in some cases you may get this error with a legitimate global ref- erence. If this happens, it may be due to an inherent limitation of Fen- nel's strategy. You can use _G.myglobal to refer to it in a way that works around this check and calls attention to the fact that this is in fact a global. Another possible cause for this error is a modified function environment. The solution depends on how you're using Fennel: * Embedded Fennel can have its searcher modified to ignore certain (or all) globals via the allowedGlobals parameter. See the Lua API page for in- structions. * Fennel's CLI has the --globals parameter, which accepts a comma-separated list of globals to ignore. For example, to disable strict mode for glob- als x, y, and z: fennel --globals x,y,z yourfennelscript.fnl GOTCHAS There are a few surprises that might bite seasoned lispers. Most of these result necessarily from Fennel's insistence upon imposing zero runtime overhead over Lua. * The arithmetic, comparison, and boolean operators are not first-class functions. They can behave in surprising ways with multiple-return-val- ued functions, because the number of arguments to them must be known at compile-time. * There is no apply function; instead use table.unpack or unpack depending on your Lua version: (f 1 3 (table.unpack [4 9])) does what (apply f 1 3 [4 9]) would do in other lisps. * Tables are compared for equality by identity, not based on the value of their contents, as per Baker. * Return values in the repl will get pretty-printed, but calling (print tbl) will emit output like table: 0x55a3a8749ef0. If you don't already have one, it's recommended for debugging to define a printer function which calls fennel.view on its argument before printing it: (local fennel (require :fennel)) (fn _G.pp [x] (print (fennel.view x))). If you add this definition to your ~/.fennelrc file it will be available in the standard repl. * Lua programmers should note Fennel functions cannot do early returns. OTHER STUFF JUST WORKS Note that built-in functions in Lua's standard library like math.random above can be called with no fuss and no overhead. This includes features like coroutines, which are often implemented using special syntax in other languages. Coroutines let you express non-blocking operations without callbacks. Tables in Lua may seem a bit limited, but metatables allow a great deal more flexibility. All the features of metatables are accessible from Fen- nel code just the same as they would be from Lua. MODULES AND MULTIPLE FILES You can use the require function to load code from other files. (let [lume (require :lume) tbl [52 99 412 654] plus (fn [x y] (+ x y))] (lume.map tbl (partial plus 2))) ; -> [54 101 414 656] Modules in Fennel and Lua are simply tables which contain functions and other values. The last value in a Fennel file will be used as the value of the whole module. Technically this can be any value, not just a table, but using a table is most common for good reason. To require a module that's in a subdirectory, take the file name, replace the slashes with dots, and remove the extension, then pass that to require. For instance, a file called lib/ui/menu.lua would be read when loading the module lib.ui.menu. When you run your program with the fennel command, you can call require to load Fennel or Lua modules. But in other contexts (such as compiling to Lua and then using the lua command, or in programs that embed Lua) it will not know about Fennel modules. You need to install the searcher that knows how to find .fnl files: require("fennel").install() local mylib = require("mylib") -- will compile and load code in mylib.fnl Once you add this, require will work on Fennel files just like it does with Lua; for instance (require :mylib.parser) will look in "mylib/parser.fnl" on Fennel's search path (stored in fennel.path which is distinct from pack- age.path used to find Lua modules). The path usually includes an entry to let you load things relative to the current directory by default. RELATIVE REQUIRE There are several ways to write a library which uses modules. One of these is to rely on something like LuaRocks, to manage library installation and availability of it and its modules. Another way is to use the relative re- quire style for loading nested modules. With relative require, libraries don't depend on the root directory name or its location when resolving in- ner module paths. For example, here's a small example library, which contains an init.fnl file, and a module at the root directory: ;; file example/init.fnl: (local a (require :example.module-a)) {:hello-a a.hello} Here, the main module requires additional example.module-a module, which holds the implementation: ;; file example/module-a.fnl (fn hello [] (print "hello from a")) {:hello hello} The main issue here is that the path to the library must be exactly exam- ple, e.g. library must be required as (require :example) for it to work, which can't be enforced on the library user. For example, if the library were moved into libs directory of the project to avoid cluttering, and re- quired as (require :libs.example), there will be a runtime error. This happens because library itself will try to require :example.module-a and not :libs.example.module-a, which is now the correct module path: runtime error: module 'example.module-a' not found: no field package.preload['example.module-a'] ... no file './example/module-a.lua' ... stack traceback: [C]: in function 'require' ./libs/example/init.fnl:2: in main chunk LuaRocks addresses this problem by enforcing both the directory name and installation path, populating the LUA_PATH environment variable to make the library available. This, of course, can be done manually by setting LUA_PATH per project in the build pipeline, pointing it to the right direc- tory. But this is not very transparent, and when requiring a project local library it's better to see the full path, that directly maps to the project's file structure, rather than looking up where the LUA_PATH is mod- ified. In the Fennel ecosystem we encourage a simpler way of managing project de- pendencies. Simply dropping a library into your project's tree or using git submodule is usually enough, and the require paths should be handled by the library itself. Here's how a relative require path can be specified in the libs/exam- ple/init.fnl to make it name/path agnostic, assuming that we've moved our example library there: ;; file libs/example/init.fnl: (local a (require (.. ... :.module-a))) {:hello-a a.hello} Now, it doesn't matter how library is named or where we put it - we can re- quire it from anywhere. It works because when requiring the library with (require :lib.example), the first value in ... will hold the "lib.example" string. This string is then concatenated with the ".module-a", and require will properly find and load the nested module at runtime under the "lib.ex- ample.module-a" path. It's a Lua feature, and not something Fennel spe- cific, and it will work the same when the library is AOT compiled to Lua. Compile-time relative include Since Fennel v0.10.0 this also works at compile-time, when using the in- clude special or the --require-as-include flag, with the constraint that the expression can be computed at compile time. This means that the ex- pression must be self-contained, i.e. doesn't refer to locals or globals, but embeds all values directly. In other words, the following code will only work at runtime, but not with include or --require-as-include because current-module is not known at compile time: (local current-module ...) (require (.. current-module :.other-module)) This, on the other hand, will work both at runtime and at compile time: (require (.. ... :.other-module)) The ... module args are propagated during compilation, so when the applica- tion which uses this library is compiled, all library code is correctly in- cluded into the self-contained Lua file. Compiling a project that uses this example library with --require-as-in- clude will include the following section in the resulting Lua code: package.preload["libs.example.module-a"] = package.preload["libs.example.module-a"] or function(...) local function hello() return print("hello from a") end return {hello = hello} end Note that the package.preload entry contains a fully qualified path "libs.example.module-a", which was resolved at compile time. Requiring modules from modules other than init.fnl To require a module from a module other than init module, we must keep the path up to the current module, but remove the module name. For example, let's add a greet module in libs/example/utils/greet.fnl, and require it from libs/example/module-a.fnl: ;; file libs/example/utils/greet.fnl: (fn greet [who] (print (.. "hello " who))) This module can be required as follows: ;; file libs/example/module-a.fnl (local greet (require (.. (: ... :match "(.+)%.[^.]+") :.utils.greet))) (fn hello [] (print "hello from a")) {:hello hello :greet greet} The parent module name is determined via calling the match method on the current module name string (...). AUTHORS Fennel Maintainers. fennel 1.6.1 2025-12-30 fennel-tutorial(7)
NAME | DESCRIPTION | OK, SO HOW DO YOU DO THINGS? | BACK TO TABLES JUST FOR A BIT | ERROR HANDLING | VARIADIC FUNCTIONS | STRICT GLOBAL CHECKING | GOTCHAS | OTHER STUFF JUST WORKS | MODULES AND MULTIPLE FILES | RELATIVE REQUIRE | AUTHORS
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