Fennel documentation for v0.2.0

Online REPL using Fengari

Summary of user-visible changes

0.2.0 / ???

0.1.1 / 2018-12-05

0.1.0 / 2018-11-29

0.0.1 / 2016-08-14

Fennel Reference

These are all the special forms recognized by the Fennel compiler. It does not include built-in Lua functions; see the Lua reference manual for that.


fn function

Creates a function which binds the arguments given inside the square brackets. Will accept any number of arguments; ones in excess of the declared ones are ignored, and if not enough arguments are supplied to cover the declared ones, the remaining ones are nil.

Example: (fn pxy [x y] (print (+ x y)))

Giving it a name is optional; if one is provided it will be bound to it as a local. Even if you don't use it as an anonymous function, providing a name will cause your stack traces to be more readable, so it's recommended. Providing a name that's a table field will cause it to be inserted in a table instead of bound as a local.

lambda/λ arity-checked function

Creates a function like fn does, but throws an error at runtime if any of the listed arguments are nil, unless its identifier begins with ?.

Example: (lambda [x ?y z] (print (- x (* (or ?y 1) z))))

The λ form is an alias for lambda and behaves identically.

partial partial application

Returns a new function which works like its first argument, but fills the first few arguments in place with the given ones. This is related to currying but different because calling it will call the underlying function instead of waiting till it has the "correct" number of args.

Example: (partial (fn [x y] (print (+ x y))) 2)

This example returns a function which will print a number that is 2 greater than the argument it is passed.


let scoped locals

Introduces a new scope in which a given set of local bindings are used.

Example: (let [x 89] (print (+ x 12)) -> 101

These locals cannot be changed with set but they can be shadowed by an inner let or local. Outside the body of the let, the bindings it introduces are no longer visible.

Any time you bind a local, you can destructure it if the value is a table or a function call which returns multiple values:

Example: (let [[a b c] [1 2 3]] (+ a b c)) -> 6

Example: (let [(x y z) (unpack [10 9 8])] (+ x y z)) -> 27

Example: (let [{:msg msg :val val} (returns-a-table)] (print msg) val)

local declare local

Introduces a new local inside an existing scope. Similar to let but without a body argument. Recommended for use at the top-level of a file for locals which will be used throughout the file.

Example: (local lume (require "lume"))

Supports destructuring and multiple-value binding.

match pattern matching

Evaluates its first argument, then searches thru the subsequent pattern/body clauses to find one where the pattern matches the value, and evaluates the corresponding body. Pattern matching can be thought of as a combination of destructuring and conditionals.


(match mytable
  59      :will-never-match-hopefully
  [9 q 5] (print :q q)
  [1 a b] (+ a b))

In the example above, we have a mytable value followed by three pattern/body clauses. The first clause will only match if mytable is 59. The second clause will match if mytable is a table with 9 as its first element and 5 as its third element; if it matches, then it evaluates (print :q q) with q bound to the second element of mytable. The final clause will only match if mytable has 1 as its first element; if so then it will add up the second and third elements.

Patterns can be tables, literal values, or symbols. If a symbol has already been bound, then the value is checked against the existing local's value, but if it's a new local then the symbol is bound to the value.

Tables can be nested, and they may be either sequential ([] style) or key/value ({} style) tables. Sequential tables will match if they have at least as many elements as the pattern. (To allow an element to be nil, use a symbol like ?this.) Tables will never fail to match due to having too many elements.

(match mytable
  {:subtable [a b ?c] :depth depth} (* b depth)
  _ :unknown)

You can also match against multiple return values using parentheses. (These cannot be nested, but they can contain tables.) This can be useful for error checking.

(match (io.open "/some/file")
  (nil msg) (report-error msg)
  f (read-file f))

Pattern matching performs unification, meaning that if x has an existing binding, clauses which attempt to bind it to a different value will not match:

(let [x 95]
 (match [52 85 95] 
   [b a a] :no ; because a=85 and a=95
   [x y z] :no ; because x=95 and x=52
   [a b x] :yes)) ; a and b are fresh values while x=95 and x=95

There is a special case for _; it is never bound and always acts as a wildcard.

(Note that Lua also has "patterns" which are matched against strings similar to how regular expressions work in other languages; these are two distinct concepts with similar names.)

global set global variable

Sets a global variable to a new value. Note that there is no distinction between introducing a new global and changing the value of an existing one.

Example: (global prettyprint (fn [x] (print (view x))))

Supports destructuring and multiple-value binding.

var declare local variable

Introduces a new local inside an existing scope which may have its value changed. Identical to local apart from allowing set to work on it.

Example: (var x 83)

Supports destructuring and multiple-value binding.

set set local variable or table field

Changes the value of a variable introduced with var. Will not work on globals or let/local-bound locals. Can also be used to change a field of a table, even if the table is bound with let or local, provided the field is given at compile-time.

Example: (set x (+ x 91))

Example: (let [t {:a 4 :b 8}] (set t.a 2) t) -> {:a 2 :b 8}

Supports destructuring and multiple-value binding.

tset set table field

Set the field of a given table to a new value. The field name does not need to be known at compile-time. Works on any table, even those bound with local and let.

Example: (let [tbl {:d 32} field :d] (tset tbl field 19) tbl) -> {:d 19}

You can provide multiple successive field names to perform nested sets.

multiple value binding

In any of the above contexts where you can make a new binding, you can use multiple value binding. Otherwise you will only capture the first value.

Example: (let [x (values 1 2 3)] x) => 1

Example: (let [(file-handle message code) (io.open "foo.blah")] message) => "foo.blah: No such file or directory"

Example: (global (x-m x-e) (math.frexp 21)), {:m x-m :e m-e} => {:e 5 :m 0.65625}

Example: (do (local (_ _ z) (unpack [:a :b :c :d :e])), z) => c

Flow Control

if conditional

Checks a condition and evaluates a corresponding body. Accepts any number of condition/body pairs; if an odd number of arguments is given, the last value is treated as a catch-all "else". Similar to cond in other lisps.


(let [x (math.random 64)]
  (if (= 0 (% x 10))
      "multiple of ten"
      (= 0 (% x 2))
      "I dunno, something else"))

All values other than nil or false are treated as true.

when single side-effecting conditional

Takes a single condition and evaluates the rest as a body if it's not nil or false. As it always returns nil; this is intended for side-effects.


(when launch-missiles?

each general iteration

Run the body once for each value provided by the iterator. Commonly used with ipairs (for sequential tables) or pairs (for any table in undefined order) but can be used with any iterator.


(each [key value (pairs mytbl)]
  (print key (f value)))

Most iterators return two values, but each will bind any number.

for numeric loop

Counts a number from a start to stop point (inclusive), evaluating the body once for each value. Accepts an optional step.


(for [i 1 10 2]
  (print i))

This example will print all odd numbers under ten.

do evaluate multiple forms returning last value

Accepts any number of forms and evaluates all of them in order, returning the last value. This is used for inserting side-effects into a form which accepts only a single value, such as in a body of an if when multiple clauses make it so you can't use when. Some lisps call this begin or progn.

(if launch-missiles?
    (promote lt-petrov))



These all work as you would expect, with a few caveats. The ~= operator is used for "not equal", and // for integer division is only available in Lua 5.3 and onward.

They all take any number of arguments, as long as that number is fixed at compile-time. For instance, (= 2 2 (unpack [2 5])) will evaluate to true because the compile-time number of values being compared is 3.

Note that these are all special forms which cannot be used as higher-order functions.

.. string concatenation

Concatenates its arguments into one string. Will coerce numbers into strings, but not other types.

Example: (.. "Hello" " " "world" 7 "!!!") -> "Hello world7!!!"

# string or table length

Returns the length of a string or table. 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. If a table has nils and you want to know the last consecutive numeric index starting at 1, you must calculate it yourself with ipairs; if you want to know the maximum numeric key in a table with nils, you can use table.maxn.

Example: (+ (# [1 2 3 nil 8]) (# "abc")) -> 6 or 8

. table lookup

Looks up a given key in a table. Multiple arguments will perform nested lookup.

Example: (. mytbl myfield)

Example: (let [t {:a [2 3 4]}] (. t :a 2)) -> 3

Note that if the field name is known at compile time, you don't need this and can just use mytbl.field.

: method call

Looks up a function in a table and calls it with the table as its first argument. This is a common idiom in many Lua APIs, including some built-in ones.


(let [f (assert (io.open "hello" "w"))]
  (: f :write "world")
  (: f :close))

Equivalent to:

(let [f (assert (io.open "hello" "w"))]
  (f.write f "world")
  (f.close f))

values multi-valued return

Returns multiple values from a function. Usually used to signal failure by returning nil followed by a message.


(fn [filename]
  (if (valid-file-name? filename)
      (open-file filename)
      (values nil (.. "Invalid filename: " filename))))

while good old while loop

Loops over a body until a condition is met. Uses a native Lua while loop, so is preferable to a lambda function and tail recursion.


  (var done? false)
  (while (not done?)
    (print :not-done)
    (when (> (math.random) 0.95)
      (set done? true))))


->, ->>, -?> and -?>> threading macros

The -> macro takes its first value and splices it into the second form as the first argument. The result of evaluating the second form gets spliced into the first argument of the third form, and so on.


(-> 52
    (+ 91 2) ; (+ 52 91 2)
    (- 8)    ; (- (+ 52 91 2) 8)
    (print "is the answer")) ; (print (- (+ 52 91 2) 8) "is the answer")

The ->> macro works the same, except it splices it into the last position of each form instead of the first.

-?> and -?>>, the thread maybe macros, are similar to -> & ->> but they also do checking after the evaluation of each threaded form. If the result is false or nil then the threading stops and the result is returned. -?> splices the threaded value as the first argument, like ->, and -?>> splices it into the last position, like ->>.

This example shows how to use them to avoid accidentally indexing a nil value:

(-?> {:a {:b {:c 42}}}
     (. :a)
     (. :missing)
     (. :c)) ; -> nil
(-?>> :a
      (. {:a :b})
      (. {:b :missing})
      (. {:c 42})) ; -> nil

Note that these have nothing to do with "threads" used for concurrency; they are named after the thread which is used in sewing. This is similar to the way that |> works in OCaml and Elixir.


Similarly, the doto macro splices the first value into subsequent forms. However, it keeps the same value and continually splices the same thing in rather than using the value from the previous form for the next form.

(doto (io.open "/tmp/err.log)
  (: :write contents)
  (: :close))

;; equivalent to:
(let [x (io.open "/tmp/err.log")]
  (: x :write contents)
  (: x :close)

The first form becomes the return value for the whole expression, and subsequent forms are evaluated solely for side-effects.


Requires a module at compile-time and binds its fields locally as macros.

Macros currently must be defined in separate modules. A macro module exports any number of functions which take code forms as arguments at compile time and emit lists which are fed back into the compiler. For instance, here is a macro function which implements when in terms of if and do:

(fn [condition body1 ...]
  (assert body1 "expected body")
  `(if @condition
     (do @body1 @...)))

A full explanation of how macros work is out of scope for this document, but you can think of it as a compile-time template function. The backtick on the third line creates a template for the code emitted by the macro. The @ serves as "unquote" (other lisps use , or ~ for this purpose) which splices values into the template.

In effect it turns this input:

(when (= 3 (+ 2 a)) 
  (print "yes")

and transforms it into this code at compile time:

(if (= 3 (+ 2 a))
    (print "yes")

See "Compiler API" below for details about additional functions visible inside compiler scope which macros run in.

Note that the macro interface is still preliminary and is subject to change over time.


Evaluate a block of code during compile-time with access to compiler scope. This gives you a superset of the features you can get with macros, but you should use macros if you can.


  (tset _SPECIALS "local" (. _SPECIALS "global")))

Compiler API

Inside eval-compiler blocks or require-macros modules, these functions are visible to your code.

Note that lists are compile-time concepts that don't exist at runtime; they are implemented as regular tables which have a special metatable to distinguish them from regular tables defined with square or curly brackets. Similarly symbols are tables with a string entry for their name and a metatable that the compiler uses to distinguish them. You can use tostring to get the name of a symbol.

Note that other internals of the compiler exposed in compiler scope are subject to change.

Fennel's Lua API

The fennel module provides the following functions.

Start a configurable repl


Takes these additional options:

The pretty-printer defaults to loading fennelview.fnl if present and falls back to tostring otherwise. fennelview.fnl will produce output that can be fed back into Fennel (other than functions, coroutines, etc) but you can use a 3rd-party pretty-printer that produces output in Lua format if you prefer.

If you don't provide allowedGlobals then it defaults to being all the globals in the environment under which the code will run. Passing in false here will disable global checking entirely.

Evaulate a string of Fennel

local result = fennel.eval(str[, options[, ...]])

The options table may contain:

Additional arguments beyond options are passed to the code and available as ....

Evaluate a file of Fennel

local result = fennel.dofile(filename[, options[, ...]])

The env key in options and the additional arguments after it work the same as with eval above.

Use Lua's built-in require function

table.insert(package.loaders or package.searchers, fennel.searcher)
local mylib = require("mylib") -- will compile and load code in mylib.fnl

Normally Lua's require function only loads modules written in Lua, but you can install fennel.searcher into package.loaders (or in Lua 5.3+ package.searchers) to teach it how to load Fennel code.

The require function is different from fennel.dofile in that it searches the directories in fennel.path for .fnl files matching the module name, and also in that it caches the loaded value to return on subsequent calls, while fennel.dofile will reload each time. The behavior of fennel.path mirrors that of Lua's package.path.

If you install Fennel into package.loaders then you can use the 3rd-party lume.hotswap function to reload modules that have been loaded with require.

Compile a string into Lua (can throw errors)

local lua = fennel.compileString(str[, options])

Accepts indent as a string in options causing output to be indented using that string, which should contain only whitespace if provided. Unlike the other functions, the compile functions default to performing no global checks, though you can pass in an allowedGlobals table in options to enable it.

Compile an iterator of bytes into a string of Lua (can throw errors)

local lua = fennel.compileStream(strm[, options])

Accepts indent in options as per above.

Compile a data structure (AST) into Lua source code (can throw errors)

The code can be loaded via dostring or other methods. Will error on bad input.

local lua = fennel.compile(ast[, options])

Accepts indent in options as per above.

Get an iterator over the bytes in a string

local stream = fennel.stringStream(str)

Converts an iterator for strings into an iterator over their bytes

Useful for the REPL or reading files in chunks. This will NOT insert newlines or other whitespace between chunks, so be careful when using with io.read(). Returns a second function, clearstream, which will clear the current buffered chunk when called. Useful for implementing a repl.

local bytestream, clearstream = fennel.granulate(chunks)

Converts a stream of bytes to a stream of values

Valuestream gets the next top level value parsed. Returns true in the first return value if a value was read, and returns nil if and end of file was reached without error. Will error on bad input or unexpected end of source.

local valuestream = fennel.parser(strm)
local ok, value = valuestream()

-- Or use in a for loop
for ok, value in valuestream do
    print(ok, value)


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