Fennel documentation for v1.1.0

Online REPL using Fengari

Summary of user-visible changes

Changes are marked in bold which could result in backwards-incompatibility.

1.1.0 / 2022-04-09

New Forms

New Features

Bug Fixes

Changes and Removals

1.0.0 / 2021-11-14

New Features

Bug Fixes

Changes and Removals

0.10.0 / 2021-08-07

It's Fennel's 5th birthday! We've got the new accumulate macro for reducing over tables, plus a couple new repl commands and more flexibility when using include.

New Forms

New Features

Bug Fixes

Changes and Removals

0.9.2 / 2021-05-02

This release mostly contains small bug fixes.

New Features

Bug Fixes

0.9.1 / 2021-04-10

This release contains one small bug fix.

Bug Fixes

0.9.0 / 2021-04-08

The biggest change in this release is the addition of the :until clause in all iteration forms to end iteration early.

New Forms

New Features

Bug Fixes

0.8.1 / 2021-02-02

This release mostly contains small bug fixes.

Bug Fixes

0.8.0 / 2021-01-18

The highlight of this release is the table comprehension macros which take the place of map in other lisps. The &as clause in destructuring now allows you to access the original table even as you destructure it into its fields. The fennel.view serializer has been completely rewritten to improve indentation.

New Forms

New Features

Bug Fixes

Changes and Removals

0.7.0 / 2020-11-03

This release adds support for reloading modules in the repl, making interactive development more streamlined.

New Features

Bug Fixes

0.6.0 / 2020-09-03

This release introduces the plugin system as well as starting to sandbox the compiler environment for safer code loading. Nothing is blocked yet, but it emits warnings when macros use functionality that is not considered safe; future versions will prevent this.

New Features

Bug Fixes

Changes and Removals

0.5.0 / 2020-08-08

This release features a version of the Fennel compiler that is self-hosted and written entirely in Fennel!

New Features

Bug Fixes

0.4.2 / 2020-07-11

This release mostly includes small bug fixes but also adds the with-open macro for automating closing file handles, etc.

New Features

Bug Fixes

0.4.1 / 2020-05-25

This release mostly includes small bug fixes, but also introduces a very experimental command for compiling standalone executables.

New Features

Bug Fixes

0.4.0 / 2020-05-12

This release adds support for Lua 5.3's bitwise operators as well as a new way of importing macro modules. It also adds pick-values and pick-args for a little more flexibility around function args and return values. The compiler now tries to emit friendlier errors that suggest fixes for problems.

New Forms

New Features

Bug Fixes

0.3.2 / 2020-01-14

This release mostly contains small bug fixes.

Bug Fixes

Changes and Removals

0.3.1 / 2019-12-17

This release mostly contains small bug fixes.

New Features

Bug Fixes

0.3.0 / 2019-09-22

This release introduces docstrings as well as several new features to the macro system and some breaking changes; the most significant being the new unquote syntax and the requirement of auto-gensym for identifiers in backtick.

New Forms

New Features

Bug Fixes

Changes and Removals

0.2.1 / 2019-01-22

This release mostly contains small bug fixes.

New Forms

New Features

Bug Fixes

0.2.0 / 2019-01-17

The second minor release introduces backtick, making macro authoring much more streamlined. Macros may now be defined in the same file, and pattern matching is added.

New Forms

New Features

Bug Fixes

0.1.1 / 2018-12-05

This release contains a few small bug fixes.

Bug Fixes

0.1.0 / 2018-11-29

The first real release sees the addition of several "creature comfort" improvements such as comments, iterator support, line number tracking, accidental global protection, pretty printing, and repl locals. It also introduces the name "Fennel".

New Forms

New Features

Changes and Removals

0.0.1 / 2016-08-14

The initial version (named "fnl") was created in 8 days and then set aside for several years.

Fennel Reference

This document covers the syntax, built-in macros, and special forms recognized by the Fennel compiler. It does not include built-in Lua functions; see the Lua reference manual or the Lua primer for that.

A macro is a function which runs at compile time and transforms some Fennel code into different Fennel. A special form (or special) is a primitive construct which emits Lua code directly. When you are coding, you don't need to care about the difference between built-in macros and special forms; it is an implementation detail.

Remember that Fennel relies completely on Lua for its runtime. Everything Fennel does happens at compile-time, so you will need to familiarize yourself with Lua's standard library functions. Thankfully it's much smaller than almost any other language.

The one exception to this compile-time rule is the fennel.view function which returns a string representation of any Fennel data suitable for printing.

Fennel source code should be UTF-8-encoded text.


(parentheses): used to delimit lists, which are primarily used to denote calls to functions, macros, and specials, but also can be used in binding contexts to bind to multiple values. Lists are a compile-time construct; they are not used at runtime. For example: (print "hello world")

{curly brackets}: used to denote key/value table literals, for example: {:a 1 :b 2}

[square brackets]: used to denote sequential tables, which can be used for literal data structures and also in specials and macros to delimit where new identifiers are introduced, such as argument lists or let bindings. For example: [1 2 3]

The syntax for numbers is the same as Lua's, except that underscores may be used to separate digits for readability. Non-ASCII digits are not yet supported.

The syntax for strings uses double-quotes " around a string. Double quotes inside a string must be escaped with backslashes. The syntax for these is the same as Lua's, except that strings may contain newline characters. Single-quoted or long bracket (aka multi-line) strings are not supported.

Fennel has a lot fewer restrictions on identifiers than Lua. Identifiers are represented by symbols, but identifiers are not exactly the same as symbols; some symbols are used by macros for things other than identifiers. Symbols may not begin with digits or a colon, but may have digits anywhere else. Beyond that, any unicode characters are accepted as long as they are not unprintable or whitespace, one of the delimiter characters mentioned above, one of the a prefix characters listed below, or one of these reserved characters:

Underscores are allowed in identifier names, but dashes are preferred as word separators. By convention, identifiers starting with underscores are used to indicate that a local is bound but not meant to be used.

The ampersand character & is allowed in symbols but not in identifiers. This allows it to be reserved for macros, like the behavior of &as in destructuring.

Symbols that contain a dot . or colon : are considered "multi symbols". The part of the symbol before the first dot or colon is used as an identifier, and the part after the dot or colon is a field looked up on the local identified. A colon is only allowed before the final segment of a multi symbol, so x.y:z is valid but a:b.c is not. Colon multi symbols cannot be used for anything but function calls.

Fennel also supports certain kinds of strings that begin with a colon as long as they don't contain any characters which wouldn't be allowed in a symbol, for example :fennel-lang.org is a string.

Spaces, tabs, newlines, vertical tabs, form feeds, and carriage returns are counted as whitespace. Non-ASCII whitespace characters are not yet supported.

Certain prefixes are expanded by the parser into longhand equivalents:

A semicolon and everything following it up to the end of the line is a comment.


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 given values of nil.


(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. Providing a name that's a table field will cause it to be inserted in a table instead of bound as a local:

(local functions {})

(fn functions.p [x y z]
  (print (* x (+ y z))))

Like Lua, functions in Fennel support tail-call optimization, allowing (among other things) functions to recurse indefinitely without overflowing the stack, provided the call is in a tail position.

lambda/λ nil-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 ?.


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

Note that the Lua runtime will fill in missing arguments with nil when they are not provided by the caller, so an explicit nil argument is no different than omitting an argument.

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

Docstrings and metadata

(Since 0.3.0)

The fn, lambda, λ and macro forms accept an optional docstring.

(fn pxy [x y]
  "Print the sum of x and y"
  (print (+ x y)))

(λ pxyz [x ?y z]
  "Print the sum of x, y, and z. If y is not provided, defaults to 0."
  (print (+ x (or ?y 0) z)))

These are ignored by default outside of the REPL, unless metadata is enabled from the CLI (---metadata) or compiler options {useMetadata=true}, in which case they are stored in a metadata table along with the arglist, enabling viewing function docs via the doc macro.

;; this only works in the repl
>> ,doc pxy
(pxy x y)
  Print the sum of x and y

All function metadata will be garbage collected along with the function itself. Docstrings and other metadata can also be accessed via functions on the fennel API with fennel.doc and fennel.metadata.

(Since 1.1.0)

All forms that accept a docstring will also accept a metadata table in the same place:

(fn add [...]
  {:fnl/docstring "Add arbitrary amount of numbers."
   :fnl/arglist [a b & more]}
  (match (values (select :# ...) ...)
    (0) 0
    (1 a) a
    (2 a b) (+ a b)
    (_ a b) (add (+ a b) (select 3 ...))))

Here the arglist is overridden by that in the metadata table (note that the contents of the table are implicitly quoted). Calling ,doc command in the REPL prints specified argument list:

>> ,doc add
(add a b & more)
  Add arbitrary amount of numbers.

Fennel itself uses the metadata keys fnl/docstring and fnl/arglist but third-party code can make use of arbitrary keys.

Hash function literal shorthand

(Since 0.3.0)

It's pretty easy to create function literals, but Fennel provides an even shorter form of functions. Hash functions are anonymous functions of one form, with implicitly named arguments. All of the below functions are functionally equivalent:

(fn [a b] (+ a b))
(hashfn (+ $1 $2)) ; implementation detail; don't use directly
#(+ $1 $2)

This style of anonymous function is useful as a parameter to higher order functions. It's recommended only for simple one-line functions.

The current implementation only allows for hash functions to use up to 9 arguments, each named $1 through $9, or those with varargs, delineated by $... instead of the usual .... A lone $ in a hash function is treated as an alias for $1.

Hash functions are defined with the hashfn macro or special character #, which wraps its single argument in a function literal. For example,

#$3               ; same as (fn [x y z] z)
#[$1 $2 $3]       ; same as (fn [a b c] [a b c])
#{:a $1 :b $2}    ; same as (fn [a b] {:a a :b b})
#$                ; same as (fn [x] x) (aka the identity function)
#val              ; same as (fn [] val)
#[:one :two $...] ; same as (fn [...] ["one" "two" ...])

Hash arguments can also be used as parts of multisyms. For instance, #$.foo is a function which will return the value of the "foo" key in its first argument.

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.


(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.


(let [x 89
      y 198]
  (print (+ x y 12))) ; => 299

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:


(let [(x y z) (unpack [10 9 8])]
  (+ x y z)) ; => 27


(let [[a b c] [1 2 3]]
  (+ a b c)) ; => 6

If a table key is a string with the same name as the local you want to bind to, you can use shorthand of just : for the key name followed by the local name.


(let [{:msg message : val} {:msg "hello there" :val 19}]
  (print message)
  val) ; prints "hello there" and returns 19

When destructuring a sequential table, you can capture all the remainder of the table in a local by using &:


(let [[a b & c] [1 2 3 4 5 6]]
  (table.concat c ",")) ; => "3,4,5,6"

If a table implements __fennelrest metamethod it is used to capture the remainder of the table. It can be used with custom data structures implemented in terms of tables, which wish to provide custom rest destructuring. The metamethod receives the table as the first argument, and the amount of values it needs to drop from the beginning of the table, much like table.unpack


(local t [1 2 3 4 5 6])
 {:__fennelrest (fn [t k]
                  (let [res {}]
                    (for [i k (length t)]
                      (tset res (tostring (. t i)) (. t i)))
(let [[a b & c] t]
  c) ;; => {:3 3 :4 4 :5 5 :6 6}

When destructuring a non-sequential table, you can capture the original table along with the destructuring by using &as:


(let [{:a a :b b &as all} {:a 1 :b 2 :c 3 :d 4}]
  (+ a b all.c all.d)) ; => 10

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.


(local tau-approx 6.28318)

Supports destructuring and multiple-value binding.

match pattern matching

(Since 0.2.0)

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.

Note: 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.


(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.

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. You can use & to capture all the remaining elements of a sequential table, just like let.

(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 symbols beginning with _; they are never bound and always act as a wildcards.

If no clause matches, the form evaluates to nil.

Sometimes you need to match on something more general than a structure or specific value. In these cases you can use guard clauses:

(match [91 12 53]
  (where [a b c] (= 5 a)) :will-not-match
  (where [a b c] (= 0 (math.fmod (+ a b c) 2)) (= 91 a)) c) ; -> 53

In this case the pattern should be wrapped in parentheses (like when matching against multiple values) but the first thing in the parentheses is the where symbol. Each form after the pattern is a condition; all the conditions must evaluate to true for that pattern to match.

If several patterns share the same body and guards, such patterns can be combined with or special in the where clause:

(match [5 1 2]
  (where (or [a 3 9] [a 1 2]) (= 5 a)) "Either [5 3 9] or [5 1 2]"
  _ "anything else")

This is essentially equivalent to:

(match [5 1 2]
  (where [a 3 9] (= 5 a)) "Either [5 3 9] or [5 1 2]"
  (where [a 1 2] (= 5 a)) "Either [5 3 9] or [5 1 2]"
  _ "anything else")

However, patterns which bind variables should not be combined with or if different variables are bound in different patterns or some variables are missing:

;; bad
(match [1 2 3]
  ;; Will throw an error because `b' is nil for the first
  ;; pattern but the guard still uses it.
  (where (or [a 1 2] [a b 3]) (< a 0) (< b 1))

;; ok
(match [1 2 3]
  (where (or [a b 2] [a b 3]) (< a 0) (<= b 1))

Note: The match macro can be used in place of the if-let macro from Clojure. The reason Fennel doesn't have if-let is that match makes it redundant.

Note 2: Prior to Fennel 0.9.0 the match macro used infix ? operator to test patterns against the guards. While this syntax is still supported, where should be preferred instead:

(match [1 2 3]
  (where [a 2 3] (< 0 a)) "new guard syntax"
  ([a 2 3] ? (< 0 a)) "obsolete guard syntax")

match-try for matching multiple steps

Evaluates a series of pattern matching steps. The value from the first expression is matched against the first pattern. If it matches, the first body is evaluated and its value is matched against the second pattern, etc.

If there is a (catch pat1 body1 pat2 body2 ...) form at the end, any mismatch from the steps will be tried against these patterns in sequence as a fallback just like a normal match. If there is no catch, the mismatched value will be returned as the value of the entire expression.

(fn handle [conn]
  (match-try (conn:receive :*l)
    input (parse input)
    (command-name params) (commands.get command-name)
    command (pcall command (table.unpack params))
     (_ :timeout) nil
     (_ :closed) (pcall disconnect conn "connection closed")
     (_ msg) (print "Error handling input" msg))))

This is useful when you want to perform a series of steps, any of which could fail. The catch clause lets you keep all your error handling in one place. Note that there are two ways to indicate failure in Fennel and Lua; using the assert/error functions or returning nil followed by some data representing the failure. This form only works on the latter, but you can use pcall to transform error calls into values.

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.


(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.


(set x (+ x 91))


(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

Sets 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.


(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.


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


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


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

with-open bind and auto-close file handles

(Since 0.4.2)

While Lua will automatically close an open file handle when it's garbage collected, GC may not run right away; with-open ensures handles are closed immediately, error or no, without boilerplate.

The usage is similar to let, except:

After executing the body, or upon encountering an error, with-open will invoke (value:close) on every bound variable before returning the results.

The body is implicitly wrapped in a function and run with xpcall so that all bound handles are closed before it re-raises the error.


;; Basic usage
(with-open [fout (io.open :output.txt :w) fin (io.open :input.txt)]
  (fout:write "Here is some text!\n")
  ((fin:lines))) ; => first line of input.txt

;; This demonstrates that the file will also be closed upon error.
(var fh nil)
(local (ok err)
  (pcall #(with-open [file (io.open :test.txt :w)]
            (set fh file) ; you would normally never do this
            (error :whoops!))))
(io.type fh) ; => "closed file"
[ok err]     ; => [false "<error message and stacktrace>"]

pick-values emit exactly n values

(Since 0.4.0)

Discards all values after the first n when dealing with multi-values (...) and multiple returns. Useful for composing functions that return multiple values with variadic functions. Expands to a let expression that binds and re-emits exactly n values, e.g.

(pick-values 2 (func))

expands to

(let [(_0_ _1_) (func)] (values _0_ _1_))


(pick-values 0 :a :b :c :d :e) ; => nil
[(pick-values 2 (table.unpack [:a :b :c]))] ;-> ["a" "b"]

(fn add [x y ...] (let [sum (+ (or x 0) (or y 0))]
                        (if (= (select :# ...) 0) sum (add sum ...))))

(add (pick-values 2 10 10 10 10)) ; => 20
(->> [1 2 3 4 5] (table.unpack) (pick-values 3) (add)) ; => 6

Note: If n is greater than the number of values supplied, n values will still be emitted. This is reflected when using (select "#" ...) to count varargs, but tables [...] ignore trailing nils:

(select :# (pick-values 5 "one" "two")) ; => 5
[(pick-values 5 "one" "two")]           ; => ["one" "two"]

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. This is intended for side-effects.


(when launch-missiles?

each general iteration

Runs 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)))

Any loop can be terminated early by placing an :until clause at the end of the bindings:

(local out [])
(each [_ value (pairs tbl) :until (< max-len (length out))]
  (table.insert out value))

Most iterators return two values, but each will bind any number. See Programming in Lua for details about how iterators work.

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.

Like each, loops using for can also be terminated early with an :until clause. The clause is checked before each iteration of the body; if it is true at the beginning then the body will not run at all.

(var x 0)
(for [i 1 128 :until (maxed-out? x)]
  (set x (+ x i)))

while good old while loop

Loops over a body until a condition is met. Uses a native Lua while loop, so this can be faster than recursion.


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

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 bitwise operators are only available in Lua 5.3+, unless you use the --use-bit-lib flag or the useBitLib flag in the options table, which lets them be used in LuaJIT. The integer division operator (//) is only available in Lua 5.3+.

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.


(.. "Hello" " " "world" 7 "!!!") ; => "Hello world7!!!"

length string or table length

(Changed in 0.3.0: the function was called # before.)

Returns the length of a string or table. Note that the length of a table with gaps (nils) 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.


(+ (length [1 2 3 nil 8]) (length "abc")) ; => 6 or 8

. table lookup

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


(. mytbl myfield)


(let [t {:a [2 3 4]}] (. t :a 2)) ; => 3

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

Nil-safe ?. table lookup

Looks up a given key in a table. Multiple arguments will perform nested lookup. If any of subsequent keys is not present, will short-circuit to nil.


(?. mytbl myfield)


(let [t {:a [2 3 4]}] (?. t :a 4 :b)) ; => nil
(let [t {:a [2 3 4 {:b 42}]}] (?. t :a 4 :b)) ; => 42

icollect, collect table comprehension macros

(Since 0.8.0)

The icollect macro takes a "iterator binding table" in the format that each takes, and returns a sequential table containing all the values produced by each iteration of the macro's body. If the value is nil, it is omitted from the return table. This is similar to how map works in several other languages, but it is a macro, not a function.

(icollect [_ v (ipairs [1 2 3 4 5 6])]
  (if (> v 2) (* v v)))
;; -> [9 16 25 36]

;; equivalent to:
(let [tbl []]
  (each [_ v (ipairs [1 2 3 4 5 6])]
    (tset tbl (+ (length tbl) 1) (if (> v 2) (* v v))))

The collect macro is almost identical, except that the body should return two things: a key and a value.

(collect [k v (pairs {:apple "red" :orange "orange" :lemon "yellow"})]
  (if (not= k "yellow")
      (values (.. "color-" v) k)))
;; -> {:color-orange "orange" :color-red "apple"}

;; equivalent to:
(let [tbl {}]
  (each [k v (pairs {:apple "red" :orange "orange"})]
    (if (not= k "yellow")
      (match (values (.. "color-" v) k)
        (key value) (tset tbl key value))))

If the key and value are given directly in the body of collect and not nested in an outer form, then the values can be omitted for brevity:

(collect [k v (pairs {:a 85 :b 52 :c 621 :d 44})]
  k (* v 5))

Like each and for, the table comprehensions support an :until clause for early termination.

Both icollect and collect take an :into clause which allows you put your results into an existing table instead of starting with an empty one:

(icollect [_ x (ipairs [2 3]) :into [9]]
  (* x 11))
;; -> [9 22 33]

accumulate iterator accumulation

(Since 0.10.0)

Runs through an iterator and performs accumulation, similar to fold and reduce commonly used in functional programming languages. Like collect and icollect, it takes an iterator binding table and an expression as its arguments. The difference is that in accumulate, the first two items in the binding table are used as an "accumulator" variable and its initial value. For each iteration step, it evaluates the given expression and the returned value becomes the next accumulator variable. accumulate returns the final value of the accumulator variable.


(accumulate [sum 0
             i n (ipairs [10 20 30 40])]
    (+ sum n)) ; -> 100

The :until clause is also supported here for early termination.

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))))


: 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.

(Since 0.3.0) Just like Lua, you can perform a method call by calling a function name where : separates the table variable and method name.


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

If the name of the method isn't known at compile time, you can use : followed by the table and then the method's name as a string.


(let [f (assert (io.open "hello" "w"))
      method1 :write
      method2 :close]
  (: f method1 "world")
  (: f method2))

Both of these examples are equivalent to the following:

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

Unlike Lua, there's nothing special about defining functions that get called this way; typically it is given an extra argument called self but this is just a convention; you can name it anything.

(local t {})

(fn t.enable [self]
  (set self.enabled? 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

While -> and ->> pass multiple values thru without any trouble, the checks in -?> and -?>> prevent the same from happening there without performance overhead, so these pipelines are limited to a single value.

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.


(since 0.3.0)

(include :my.embedded.module)

Loads Fennel/Lua module code at compile time and embeds it in the compiled output. The module name must resolve to a string literal during compilation. The bundled code will be wrapped in a function invocation in the emitted Lua and set on package.preload[modulename]; a normal require is then emitted where include was used to load it on demand as a normal module.

In most cases it's better to use require in your code and use the requireAsInclude option in the API documentation and the --require-as-include CLI flag (fennel --help) to accomplish this.

The require function is not part of Fennel; it comes from Lua. However, it works to load Fennel code. See the Modules and multiple files section in the tutorial and Programming in Lua for details about require.

Starting from version 0.10.0 include and hence --require-as-include support semi-dynamic compile-time resolution of module paths similarly to import-macros. See the relative require section in the tutorial for more information.


All forms which introduce macros do so inside the current scope. This is usually the top level for a given file, but you can introduce macros into smaller scopes as well. Note that macros are a compile-time construct; they do not exist at runtime. As such macros cannot be exported at the bottom of a module like functions and other values.

import-macros load macros from a separate module

(Since 0.4.0)

Loads a module at compile-time and binds its functions as local macros.

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 as code. The module calling import-macros gets whatever functions have been exported to use as macros. For instance, here is a macro module which implements when2 in terms of if and do:

(fn when2 [condition body1 ...]
  (assert body1 "expected body")
  `(if ,condition
     (do ,body1 ,...)))

{:when2 when2}

For a full explanation of how this works see the macro guide. All forms in Fennel are normal tables you can use table.insert, ipairs, destructuring, etc on. The backtick on the third line creates a template list for the code emitted by the macro, and the comma serves as "unquote" which splices values into the template. (Changed in 0.3.0: @ was used instead of , before.)

Assuming the code above is in the file "my-macros.fnl" then it turns this input:

(import-macros {: when2} :my-macros)

(when2 (= 3 (+ 2 a))
  (print "yes")

and transforms it into this code at compile time by splicing the arguments into the backtick template:

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

The import-macros macro can take any number of binding/module-name pairs. It can also bind the entire macro module to a single name rather than destructuring it. In this case you can use a dot to call the individual macros inside the module:

(import-macros mine :my-macros)

(mine.when2 (= 3 (+ 2 a))
  (print "yes")

Note that all macro code runs at compile time, which happens before runtime. Locals which are in scope at runtime are not visible during compile-time. So this code will not work:

(local (module-name file-name) ...)
(import-macros mymacros (.. module-name ".macros"))

However, this code will work, provided the module in question exists:

(import-macros mymacros (.. ... ".macros"))

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

Macro module searching

By default, Fennel will search for macro modules similarly to how it searches for normal runtime modules: by walking thru entries on fennel.macro-path and checking the filesystem for matches. However, in some cases this might not be suitable, for instance if your Fennel program is packaged in some kind of archive file and the modules do not exist as distinct files on disk.

To support this case you can add your own searcher function to the fennel.macro-searchers table. For example, assuming find-in-archive is a function which can look up strings from the archive given a path:

(local fennel (require :fennel))

(fn my-searcher [module-name]
  (let [filename (.. "src/" module-name ".fnl")]
    (match (find-in-archive filename)
      code (values (partial fennel.eval code {:env :_COMPILER})

(table.insert fennel.macro-searchers my-searcher)

The searcher function should take a module name as a string and return two values if it can find the macro module: a loader function which will return the macro table when called, and an optional filename. The loader function will receive the module name and the filename as arguments.

macros define several macros

(Since 0.3.0)

Defines a table of macros. Note that inside the macro definitions, you cannot access variables and bindings from the surrounding code. The macros are essentially compiled in their own compiler environment. Again, see the "Compiler API" section for more details about the functions available here.

(macros {:my-max (fn [x y]
                   `(let [x# ,x y# ,y]
                      (if (< x# y#) y# x#)))})

(print (my-max 10 20))
(print (my-max 20 10))
(print (my-max 20 20))

macro define a single macro

(macro my-max [x y]
  `(let [x# ,x y# ,y]
     (if (< x# y#) y# x#)))

If you are only defining a single macro, this is equivalent to the previous example. The syntax mimics fn.

macrodebug print the expansion of a macro

(macrodebug (-> abc
                (+ 99)
                (< 0)
                (when (os.exit))))
; -> (if (< (+ abc 99) 0) (do (os.exit)))

Call the macrodebug macro with a form and it will repeatedly expand top-level macros in that form and print out the resulting form. Note that the resulting form will usually not be sensibly indented, so you might need to copy it and reformat it into something more readable.

Note that this prints at compile-time since macrodebug is a macro.

Macro gotchas

It's easy to make macros which accidentally evaluate their arguments more than once. This is fine if they are passed literal values, but if they are passed a form which has side-effects, the result will be unexpected:

(var v 1)
(macros {:my-max (fn [x y]
                   `(if (< ,x ,y) ,y ,x))})

(fn f [] (set v (+ v 1)) v)

(print (my-max (f) 2)) ; -> 3 since (f) is called twice in the macro body above

(Since 0.3.0) In order to prevent accidental symbol capture, you may not bind a bare symbol inside a backtick as an identifier. Appending a # on the end of the identifier name as above invokes "auto gensym" which guarantees the local name is unique.

(macros {:my-max (fn [x y]
                   `(let [x2 ,x y2 ,y]
                      (if (< x2 y2) y2 x2)))})

(print (my-max 10 20))
; Compile error in 'x2' unknown:?: macro tried to bind x2 without gensym; try x2# instead

macros is useful for one-off, quick macros, or even some more complicated macros, but be careful. It may be tempting to try and use some function you have previously defined, but if you need such functionality, you should probably use import-macros.

For example, this will not compile in strict mode! Even when it does allow the macro to be called, it will fail trying to call a global my-fn when the code is run:

(fn my-fn [] (print "hi!"))

(macros {:my-max (fn [x y]
                   `(let [x# ,x y# ,y]
                      (if (< x# y#) y# x#)))})
; Compile error in 'my-max': attempt to call global '__fnl_global__my_2dfn' (a nil value)


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.


  (each [name (pairs _G)]
    (print name)))

This prints all the functions available in compiler scope.

Compiler Environment

Inside eval-compiler, macros, or macro blocks, as well as import-macros modules, the functions listed below are visible to your code.

These functions can be used from within macros only, not from any eval-compiler call:

Note that lists are compile-time concepts that don't exist at runtime; they are implemented as 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 marker metatable. You can use tostring to get the name of a symbol.

As of 1.0.0 the compiler will not allow access to the outside world (os, io, etc) from macros. The one exception is print which is included for debugging purposes. You can disable this by providing the command-line argument --no-compiler-sandbox or by passing {:compiler-env _G} in the options table when using the compiler API to get full access.

Please note that the sandbox is not suitable to be used as a robust security mechanism. It has not been audited and should not be relied upon to protect you from running untrusted code.

Note that other internals of the compiler exposed in compiler scope but not listed above are subject to change.

lua Escape Hatch

There are some cases when you need to emit Lua output from Fennel in ways that don't match Fennel's semantics. For instance, if you are porting an algorithm from Lua that uses early returns, you may want to do the port as literally as possible first, and then come back to it later to make it idiomatic. You can use the lua special form to accomplish this:

(fn find [tbl pred]
  (each [key val (pairs tbl)]
    (when (pred val)
      (lua "return key"))))

Lua code inside the string can refer to locals which are in scope; however note that it must refer to the names after mangling has been done, because the identifiers must be valid Lua. The Fennel compiler will emit foo-bar as foo_bar in the Lua output in order for it to be valid. When in doubt, inspect the compiler output to see what it looks like. For example the following Fennel code:

(let [foo-bar :hello]
  (lua "print(foo_bar .. \" world\")"))

will produce this Lua code:

local foo_bar = "hello"
print(foo_bar .. " world")
return nil

Normally in these cases you would want to emit a statement, in which case you would pass a string of Lua code as the first argument. But you can also use it to emit an expression if you pass in a string as the second argument.

Note that this should only be used in exceptional circumstances, and if you are able to avoid it, you should.

Deprecated Forms

require-macros load macros with less flexibility

(Since 0.1.0, deprecated in 0.4.0)

The require-macros form is like import-macros, except it does not give you any control over the naming of the macros being imported. It is strongly recommended to use import-macros instead.

pick-args create a function of fixed arity

(Since 0.4.0, deprecated 0.10.0)

Like pick-values, but takes an integer n and a function/operator f, and creates a new function that applies exactly n arguments to f.

Example, using the add function created above:

(pick-args 2 add) ; expands to `(fn [_0_ _1_] (add _0_ _1_))`
(-> [1 2 3 4 5] (table.unpack) ((pick-args 3 add))) ; => 6

(local count-args (partial select "#"))
((pick-args 3 count-args) "still three args, but 2nd and 3rd are nil") ; => 3

global set global variable

(Since 0.1.0, deprecated in 1.1.0)

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. This supports destructuring and multiple-value binding.


(global prettyprint (fn [x] (print (fennel.view x))))

Using global adds the identifier in question to the list of allowed globals so that referring to it later on will not cause a compiler error. However, globals are also available in the _G table, and accessing them that way instead is recommended for clarity.

Fennel's Lua API

The fennel module provides the following functions for use when embedding Fennel in a Lua program. If you're writing a pure Fennel program or working on a system that already has Fennel support, you probably don't need this.

Only the fennel module is part of the public API. The other modules are implementation details subject to change. Most functions will error upon failure.

Any time a function takes an options table argument, that table will usually accept these fields:

You can pass the string "_COMPILER" as the value for env; it will cause the code to be run/compiled in a context which has all compiler-scoped values available. This can be useful for macro modules or compiler plugins.

Note that only the fennel module is part of the public API. The other modules (fennel.utils, fennel.compiler, etc) should be considered compiler internals subject to change.

Start a configurable repl


Takes these additional options:

By default, metadata will be enabled and you can view function signatures and docstrings with the ,doc command in the REPL.

Evaulate a string of Fennel

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

The options table may also contain:

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

Evaluate a file of Fennel

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

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

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.searchers (or in Lua 5.1 package.loaders) to teach it how to load Fennel code.

If you would rather change some of the options you can use fennel.makeSearcher(options) to get a searcher function that's equivalent to fennel.searcher but overrides the default options table.

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. There is also a fennel.macro-path which is used to look up macro modules.

If you install Fennel into package.searchers then you can use the repl's ,reload mod command to reload modules that have been loaded with require.

Macro Searchers

The compiler sandbox makes it so that the module system is also isolated from the rest of the system, so the above require calls will not work from inside macros. However, there is a separate fennel.macro-searchers table which can be used to allow different modules to be loaded inside macros. By default it includes a searcher to load sandboxed Fennel modules and a searcher to load sandboxed Lua modules, but if you disable the compiler sandbox you may want to replace these with searchers which can load arbitrary modules.

The default fennel.macro-searchers functions also cannot load C modules. Here's an example of some code which would allow that to work:

table.insert(fennel["macro-searchers"], function(module_name)
  local filename = fennel["search-module"](module_name, package.cpath)
  if filename then
    local func = "luaopen_" .. module_name
    return function() return package.loadlib(filename, func) end, filename

Macro searchers store loaded macro modules in the fennel.macro-loaded table which works the same as package.loaded but for macro modules.

Get Fennel-aware stack traces.

The fennel.traceback function works like Lua's debug.traceback function, except it tracks line numbers from Fennel code correctly.

If you are working on an application written in Fennel, you can override the default traceback function to replace it with Fennel's:

debug.traceback = fennel.traceback

Note that some systems print stack traces from C, which will not be affected.

Search the path for a module without loading it

print(fennel.searchModule("my.mod", package.path))

If you just want to find the file path that a module would resolve to without actually loading it, you can use fennel.searchModule. The first argument is the module name, and the second argument is the path string to search. If none is provided, it defaults to Fennel's own path.

Returns nil if the module is not found on the path.

Compile a string into Lua

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.

Accepts filename in options as in fennel.eval.

Compile an iterator of bytes into a string of Lua

This is useful when streaming data into the compiler to allow you to avoid loading all the code into a single string in one go.

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

Accepts indent and filename in options as per above.

Compile an AST data structure into Lua source code

The ast here can be gotten from fennel.parser.

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

Accepts indent and filename in options as per above.

Convert text into AST node(s)

The fennel.parser function returns a stateful iterator function. If a form was successfully read, it returns true followed by the AST node. Returns nil when it reaches the end. Raises an error if it can't parse the input.

local parse = fennel.parser(text)
local ok, ast = assert(parse()) -- just get the first form

-- Or use in a for loop
for ok, ast in parse do
  if ok then

The first argument can either be a string or a function that returns one byte at a time. It takes two optional arguments; a filename and a table of options. Supported options are both booleans that default to false:

The list of common options at the top of this document do not apply here.

AST node definition

The AST returned by the parser consists of data structures representing the code. Passing AST nodes to the fennel.view function will give you a string which should round-trip thru the parser to give you the same data back. The same is true with tostring, except it does not work with non-sequence tables.

AST nodes can be any of these types:


A list represents a call to function/macro, or destructuring multiple return values in a binding context. It's represented as a table which can be identified using the fennel.list? predicate function or constructed using fennel.list which takes any number of arguments for the contents of the list.

Note that lists are compile-time constructs in Fennel. They do not exist at runtime, except in such cases as the compiler is in use at runtime.

The list also contains these keys indicating where it was defined: filename, line, bytestart, and byteend. This data is used for stack traces and for pinpointing compiler error messages.

sequence/kv table

These are table literals in Fennel code produced by square brackets (sequences) or curly brackets (kv tables). Sequences can be identified using the fennel.sequence? function and constructed using fennel.sequence. There is no predicate or constructor for kv tables; any table which is not one of the other types is assumed to be one of these.

At runtime there is no difference between sequences and kv tables which use monotonically increasing integer keys, but the parser is able to distinguish between them to improve error reporting.

Sequences have their source data in filename, line, etc keys just like lists. But kv tables cannot have this, because adding arbitrary keys to the table would change its contents, so these fields are instead stored on the metatable. The metatable for kv tables also includes a keys sequence which tells you which order the keys appeared originally, since kv tables are unordered and there would otherwise be no way to reconstruct this information.


Symbols typically represent identifiers in Fennel code. Symbols can be identified with fennel.sym? and constructed with fennel.sym which takes a string name as its first argument and a source data table as the second. Symbols are represented as tables which store their source data in fields on themselves. Unlike the other tables in the AST, they do not represent collections; they are used as scalar types.

Note: nil is not a valid AST; code that references nil will have the symbol named "nil" which unfortunately prints in a way that is visually indistinguishable from actual nil.

The fennel.sym-char? function will tell you if a given character is allowed to be used in the name of a symbol.


This is a special type of symbol-like construct (...) indicating functions using a variable number of arguments. Its meaning is the same as in Lua. It's identified with fennel.varg? and constructed with fennel.varg.


These are literal types defined by Lua. They cannot carry source data.


By default, ASTs will omit comments. However, when the :comment field is set in the parser options, comments will be included in the parsed values. They are identified using fennel.comment? and constructed using the fennel.comment function. They are represented as tables that have source data as fields inside them.

In most data context, comments just get included inline in a list or sequence. However, in a kv table, this cannot be done, because kv tables must have balanced key/value pairs, and including comments inline would imbalance these or cause keys to be considered as values and vice versa. So the comments are stored on the comments field of metatable instead, keyed by the key or value they were attached to.


The fennel.view function takes any Fennel data and turns it into a representation suitable for feeding back to Fennel's parser. In addition to tables, strings, numbers, and booleans, it can produce reasonable output from ASTs that come from the parser. It will emit an unreadable placeholder for coroutines, compiled functions, and userdata though.

print(fennel.view({abc=123}[, options])
{:abc 123}

The list of common options at the top of this document do not apply here; instead these options are accepted:

Work with docstrings and metadata

(Since 0.3.0)

When running a REPL or using compile/eval with metadata enabled, each function declared with fn or λ/lambda will use the created function as a key on fennel.metadata to store the function's arglist and (if provided) docstring. The metadata table is weakly-referenced by key, so each function's metadata will be garbage collected along with the function itself.

You can work with the API to view or modify this metadata yourself, or use the ,doc repl command to view function documentation.

In addition to direct access to the metadata tables, you can use the following methods:

local greet = fennel.eval('(λ greet [name] "Say hello" (print "Hello," name))',
                          {useMetadata = true})

-- > {"fnl/docstring" = "Say hello", "fnl/arglist" = ["name"]}

fennel.doc(greet, "greet")
-- > (greet name)
-- >   Say hello

fennel.metadata:set(greet, "fnl/docstring", "Say hello!!!")
fennel.doc(greet, "greet!")
--> (greet! name)
-->   Say hello!!!

Metadata performance note

Enabling metadata in the compiler/eval/REPL will cause every function to store a new table containing the function's arglist and docstring in the metadata table, weakly referenced by the function itself as a key.

This may have a performance impact in some applications due to the extra allocations and garbage collection associated with dynamic function creation. The impact hasn't been benchmarked, but enabling metadata is currently recommended for development purposes only.

Describe Fennel syntax

If you're writing a tool which performs syntax highlighting or some other operations on Fennel code, the fennel.syntax function can provide you with data about what forms and keywords to treat specially.

local syntax = fennel.syntax()
--> {:binding-form? true :body-form? true :macro? true}

The table has string keys and table values. Each entry will have one of "macro?", "global?", or "special?" set to true indicating what type it is. Globals can also have "function?" set to true. Macros and specials can have "binding-form?" set to true indicating it accepts a [] argument which introduces new locals, and/or a "body-form?" indicating whether it should be indented with two spaces instead of being indented like a function call. They can also have a "define?" key indicating whether it introduces a new top-level identifier like local or fn.

Load Lua code in a portable way

This isn't Fennel-specific, but the loadCode function takes a string of Lua code along with an optional environment table and filename string, and returns a function for the loaded code which will run inside that environment, in a way that's portable across any Lua 5.1+ version.

local f = fennel.loadCode(luaCode, { x = y }, "myfile.lua")

Detect Lua VM runtime version

This function does a best effort detection of the Lua VM environment hosting Fennel. Useful for displaying an "About" dialog in your Fennel app that matches the REPL and --version CLI flag.

-- > Fennel 1.0.0 on PUC Lua 5.4

The fennel.version field will give you the version of just Fennel itself.


Fennel's plugin system is extremely experimental and exposes internals of the compiler in ways that no other part of the compiler does. It should be considered unstable; changes to the compiler in future versions are likely to break plugins, and each plugin should only be assumed to work with specific versions of the compiler that they're tested against. The backwards-compatibility guarantees of the rest of Fennel do not apply to plugins.

Compiler plugins allow the functionality of the compiler to be extended in various ways. A plugin is a module containing various functions in fields named after different compiler extension points. When the compiler hits an extension point, it will call each plugin's function for that extension point, if provided, with various arguments; usually the AST in question and the scope table. Each plugin function should normally do side effects and return nil or error out. If a function returns non-nil, it will cause the rest of the plugins for a given event to be skipped.

The destructure extension point is different because instead of just taking ast and scope it takes a from which is the AST for the value being destructured and a to AST which is the AST for the form being destructured to. This is most commonly a symbol but can be a list or a table.

The parse-error and assert-compile hooks can be used to override how fennel behaves down to the parser and compiler levels. Possible use-cases include building atop fennel.view to serialize data with EDN-style tagging, or manipulating external s-expression-based syntax, such as tree-sitter queries.

The scope argument is a table containing all the compiler's information about the current scope. Most of the tables here look up values in their parent scopes if they do not contain a key.

Plugins can also contain repl commands. If your plugin module has a field with a name beginning with "repl-command-" then that function will be available as a comma command from within a repl session. It will be called with a table for the repl session's environment, a function which will read the next form from stdin, a function which is used to print normal values, and one which is used to print errors.

(local fennel (require :fennel)
(fn locals [env read on-values on-error scope]
  "Print all locals in repl session scope."
  (on-values [(fennel.view env.___replLocals___)]))

{:repl-command-locals locals}
$ fennel --plugin locals-plugin.fnl
Welcome to Fennel 0.8.0 on Lua 5.4!
Use ,help to see available commands.
>> (local x 4)
>> (local abc :xyz)
>> ,locals
  :abc "xyz"
  :x 4

The docstring of the function will be used as its summary in the ",help" command listing. Unlike other plugin hook fields, only the first plugin to provide a repl command will be used.


Plugins are activated by passing the --plugin argument on the command line, which should be a path to a Fennel file containing a module that has some of the functions listed above. If you're using the compiler programmatically, you can include a :plugins table in the options table to most compiler entry point functions.

Your plugin should contain a :versions table which contains a list of strings indicating every version of Fennel which you have tested it with. You should also have a :name field with the plugin's name.