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.

If you are embedding Fennel in a context where ANSI escape codes are not interpreted, you can set error-pinpoint to false to disable the highlighting of compiler and parse errors.

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.

Customize REPL default options

Any fields set on fennel.repl, which is actually a table with a __call metamethod rather than a function, will used as a fallback for any options passed to (fennel.repl) before defaults are applied, allowing one to customize the default behavior of (fennel.repl):

fennel.repl.onError = custom_error_handler
-- In rare cases this needs to be temporary, overrides
-- can be cleared by simply clearing the entire table
for k in pairs(fennel.repl) do
  fennel.repl[k] = nil

Evaluate 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


This is the equivalent of this code:

local fennel = require("fennel")
table.insert(package.loaders or package.searchers, fennel.searcher)
fennel.dofile("main.fnl") -- require calls in main.fnl can load fennel modules

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 like 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. The first argument should be a stateful iterator; in other words a function which returns one byte at a time.

local lua = fennel.compileStream(stream[, 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.

The fennel.ast-source function takes an AST node and returns a table with source data around filename, line number, et in it, if possible. Some AST nodes cannot provide this data, for instance numbers, strings, and booleans, or symbols constructed within macros using the sym function instead of backtick.

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, col, endcol, bytestart, and byteend. This data is used for stack traces and for pinpointing compiler error messages. Note that column numbers are based on character count, which does not always correspond to visual columns; for instance "วัด" is three characters but only two visual columns.

sequence/key-value table

These are table literals in Fennel code produced by square brackets (sequences) or curly brackets (k/v tables). Sequences can be identified using the fennel.sequence? function and constructed using fennel.sequence. There is no predicate or constructor for k/v 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 k/v tables which use monotonically increasing integer keys, but the parser is able to distinguish between them to improve error reporting.

Sequences and k/v tables have their source data in filename, line, etc keys of their metatable. The metatable for k/v tables also includes a keys sequence which tells you which order the keys appeared originally, since k/v 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 (filename, line, col, etc) in fields on themselves. Unlike the other tables in the AST, they do not represent collections; they are used as scalar types.

Symbols can refer not just directly to locals, but also to table references like tbl.x for field lookup or access.channel:deny for method invocation. The fennel.multi-sym? function will return a table containing the segments if the symbol if it is one of these, or nil otherwise.

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 contexts, comments just get included inline in a list or sequence. However, in a k/v table, this cannot be done, because k/v 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.

Serialization (view)

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, which cannot be understood by the parser.

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:

All options can be set to {:once some-value} to force their value to be some-value but only for the current level. After that, such option is reset to its default value. Alternatively, {:once value :after other-value} can be used, with the difference that after first use, the options will be set to other-value instead of the default value.

You can set a __fennelview metamethod on a table to override its serialization behavior. It should take the table being serialized as its first argument, a function as its second argument, options table as third argument, and current amount of indentation as its last argument:

(fn [t view options indent] ...)

view function contains a pretty printer that can be used to serialize elements stored within the table being serialized. If your metamethod produces indented representation, you should pass indent parameter to view increased by the amount of additional indentation you've introduced. This function has the same interface as __fennelview metamethod, but in addition accepts colon-string? as last argument. If colon? is true, strings will be printed as colon-strings when possible, and if its value is false, strings will be always printed in double quotes. If omitted or nil will default to value of :prefer-colon? option.

options table contains options described above, and also visible-cycle? function, that takes a table being serialized, detects and saves information about possible reachable cycle. Should be used in __fennelview to implement cycle detection.

__fennelview metamethod should always return a table of correctly indented lines when producing multi-line output, or a string when always returning single-line item. fennel.view will transform your data structure to correct multi-line representation when needed. There's no need to concatenate table manually ever - fennel.view will apply general rules for your data structure, depending on current options. By default multiline output is produced only when inner data structures contains newlines, or when returning table of lines as single line results in width greater than line-size option.

Multi-line representation can be forced by returning two values from __fennelview - a table of indented lines as first value, and true as second value, indicating that multi-line representation should be forced.

There's no need to incorporate indentation beyond needed to correctly align elements within the printed representation of your data structure. For example, if you want to print a multi-line table, like this:


__fennelview should return a sequence of lines:

 "          2"
 "          3]"]

Note, since we've introduced inner indent string of length 10, when calling view function from within __fennelview metamethod, in order to keep inner tables indented correctly, indent must be increased by this amount of extra indentation.

Here's an implementation of such pretty-printer for an arbitrary sequential table:

(fn pp-doc-example [t view options indent]
  (let [lines (icollect [i v (ipairs t)]
                (let [v (view v options (+ 10 indent))]
                  (if (= i 1) v
                      (.. "          " v))))]
    (doto lines
      (tset 1 (.. "@my-table[" (or (. lines 1) "")))
      (tset (length lines) (.. (. lines (length lines)) "]")))))

Setting table's __fennelview metamethod to this function will provide correct results regardless of nesting:

>> {:my-table (setmetatable [[1 2 3 4 5]
                             {:smalls [6 7 8 9 10 11 12]
                              :bigs [500 1000 2000 3000 4000]}]
                            {:__fennelview pp-doc-example})
    :normal-table [{:c [1 2 3] :d :some-data} 4]}
{:my-table @my-table[[1 2 3 4 5]
                     {:bigs [500 1000 2000 3000 4000]
                      :smalls [6 7 8 9 10 11 12]}]
 :normal-table [{:c [1 2 3] :d "some-data"} 4]}

Note that even though we've only indented inner elements of our table with 10 spaces, the result is correctly indented in terms of outer table, and inner tables also remain indented correctly.

When using the :preprocess option or __fennelview method, avoid modifying any tables in-place in the passed function. Since Lua tables are mutable and passed in without copying, any modification done in these functions will be visible outside of fennel.view.

Using :byte-escape to override the special character escape format is intended for use-cases where it's known that the output will be consumed by something other than Lua/Fennel, and may result in output that Fennel can no longer parse. For example, to force the use of hex escapes:

(print (fennel.view {:clear-screen "\027[H\027[2J"}
                    {:byte-escape #(: "\\x%2x" :format $)}))
;; > {:clear-screen "\x1b[H\x1b[2J"}

While Lua 5.2+ supports hex escapes, PUC Lua 5.1 does not, so compiling this with Fennel later would result in an incorrect escape code in Lua 5.1.

Work with docstrings and metadata

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.

(since 1.3.1)

If an optional argument is given, returns version information as a table:

(fennel.runtime-version :as-table)
;; > {:fennel "1.3.1" :lua "PUC Lua 5.4"}


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. If your plugin is used with a version of Fennel that isn't in the list, it will emit a warning.