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fennel-api(3) Fennel's Lua API fennel-api(3) NAME fennel-api - Fennel's Lua API DESCRIPTION The fennel module provides the following functions for use when embed- ding 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 er- ror upon failure. Any time a function takes an options table argument, that table will usually accept these fields: • allowedGlobals: a sequential table of strings of the names of globals which the compiler will allow references to. Set to false to disable checks. Defaults to the contents of the env table, if provided, or the current environment. • correlate: when this is set, Fennel attempts to emit Lua where the line numbers match up with the Fennel input code; useful for situa- tion where code that isn't under your control will print the stack traces. This is meant as a debugging aid and cannot give exact num- bers in all cases. • useMetadata: enables or disables metadata, allowing use of the ,doc repl command. Intended for development purposes (see performance note); defaults to true for REPL only. • requireAsInclude: Alias any static require calls to the include spe- cial, embedding the module code inline in the compiled output. If the module name isn't a string literal that is resolvable at compile time it falls back to require at runtime. Can be used to embed both Fennel and Lua modules. • toBeClosed: Use Lua 5.4+ to-be-closed variables when compiling with-open in order to avoid interfering with traces. • assertAsRepl: Replace calls to assert with assert-repl to allow for interactive debugging. • lambdaAsFn: Replace lambda function definitions with fn. • env: an environment table in which to run the code; see the Lua man- ual. • compilerEnv: an environment table in which to run compiler-scoped code for macro definitions and eval-compiler calls. Internal Fennel functions such as list, sym, etc. will be exposed in addition to this table. Defaults to a table containing limited known-safe glob- als. Pass _G to disable sandboxing. • unfriendly: disable friendly compiler/parser error messages. • plugins: list of compiler plugins. • error-pinpoint: a list of two strings indicating what to wrap compile errors in • keywords: a table of the form {:keyword1 true :keyword2 true} con- taining symbols that should be treated as reserved Lua keywords. • global-mangle: whether to mangle globals in compiler output; set to false to turn global references that aren't valid Lua into _G['hello-world']. 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. If you want to add additional values to the environment in this case, you can use the extra-env key. You can also use extra-com- piler-env to add fields to the compiler environment used for macros. Note that only the fennel module is part of the public API. The other modules (fennel.utils, fennel.compiler, etc) should be considered com- piler 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 fennel.repl([options]) Takes these additional options: • readChunk(state): a function that when called, returns a line of code to run. This can be an incomplete expression, in which case it will be called again until a complete expression can be constructed. The state argument is a table with a stack-size field which will be zero unless it's reading a continuation of previous input. Strings re- turned should end in newlines. It should return nil when there is no more source, which will exit the repl. • onValues(values): a function which is called for every evaluation with a sequence table containing string representations of each of the values resulting from the input. • onError(errType, err, luaSource): a function that will be called on each error. errType is a string with the type of error: 'parse', 'compile', 'runtime', or 'lua'. err is the error message, and lua- Source is the source of the generated lua code. • pp(x): a pretty-printer function to apply on values (default: fen- nel.view). • view-opts: an options table passed to pp (default: {:depth 4}). • rawValues(...): a function which is passed the raw values from evalu- ation; like onValues but receives the underlying data rather than the string representation. Note that overriding readChunk/onValues will only affect input and out- put initiated by the repl directly. If the repl runs code that calls print, io.write, io.read, etc, those will still use stdio unless over- ridden in env. By default, metadata will be enabled and you can view function signa- tures and docstrings with the ,doc command in the REPL. In Fennel 1.4.1 fennel.repl was changed from a normal function to a callable table. This mostly behaves the same, but it can cause prob- lems with certain functions that are very picky about functions. Un- fortunately this includes coroutine.create. You can pass fen- nel.repl.repl instead. 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 op- tions passed to fennel.repl or assert-repl before defaults are applied, allowing one to customize the default behavior of (fennel.repl): fennel.repl.onError = custom_error_handler function_that_calls_assert_repl_somewhere() -- 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 end EVALUATE A STRING OF FENNEL local result = fennel.eval(str[, options[, ...]]) The options table may also contain: • filename: override the filename that Lua thinks the code came from. Additional arguments beyond options are passed to the code and avail- able as .... EVALUATE A FILE OF FENNEL local result = fennel.dofile(filename[, options[, ...]]) Additional arguments beyond options are passed to the code and avail- able as .... USE LUA'S BUILT-IN REQUIRE FUNCTION require("fennel").install().dofile("main.fnl") 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.make- Searcher(options) to get a searcher function that's equivalent to fen- nel.searcher but overrides the default options table. You can provide a path field to set fennel.path. 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 be- havior 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 iso- lated from the rest of the system, so the above require calls will not work from inside macros. However, there is a separate fen- nel.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 mod- ules. 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 end end) 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 INFORMATION The fennel.traceback function works like Lua's debug.traceback func- tion, except it tracks line numbers from Fennel code correctly. If you are working on an application written in Fennel, you can over- ride 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 af- fected. The fennel.getinfo function works like Lua's debug.getinfo function, except it tracks line numbers from Fennel code correctly. Functions defined from Fennel will have the what field set to "Fennel" instead of "Lua". local mymodule = require("module") print(fennel.getinfo(mymodule.func1).linedefined) COMPILE FENNEL CODE TO LUA Compile a file, AST, or byte iterator local lua = fennel.compile(fennelSource[, options]) The first argument here can be a file, an AST (usually produced by fen- nel.parser), or a stateful iterator function of bytes. Unlike the other functions, the compile functions default to performing no global checks, though you can pass in an allowedGlobals table in op- tions to enable it. Accepts filename in options like fennel.eval for error reporting purposes. Compile a string of Fennel code local lua = fennel.compileString(fennelcode[, options]) Also aliased to fennel.compile-string for convenience calling from Fen- nel. PARSE TEXT INTO AST NODES The fennel.parser function returns a function which you can call re- peatedly to get successive AST nodes from a string. This happens to be an iterator function, so you can use it with Lua's for or Fennel's each. 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 print(fennel.view(ast)) end end 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: • unfriendly: disable enhanced parse error reporting • comments: include comment nodes in AST • plugins: (since 1.2.0) An optional list of compiler plugins. 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 func- tion instead of backtick. AST nodes can be any of these types: list 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 con- structed 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 ex- ist at runtime, except in such cases as the compiler is in use at run- time. 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 "" 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 identi- fied using the fennel.sequence? function and constructed using fen- nel.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. symbol 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 sec- ond. 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 refer- ences like tbl.x for field lookup or access.channel:deny for method in- vocation. The fennel.multi-sym? function will return a table contain- ing 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 al- lowed to be used in the name of a symbol. vararg This is a special type of symbol-like construct (...) indicating func- tions using a variable number of arguments. Its meaning is the same as in Lua. It's identified with fennel.varg? and constructed with fen- nel.varg. number/string/boolean These are literal types defined by Lua. They cannot carry source data. comment 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 us- ing 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 in- line would imbalance these or cause keys to be considered as values and vice versa. So the comments are stored on the comments field of meta- table instead, keyed by the key or value they were attached to. 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. SERIALIZATION (VIEW) The fennel.view function takes any Fennel data and turns it into a rep- resentation 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: • one-line? (default: false) keep the output string as a one-liner • depth (number, default: 128) limit how many levels to go • detect-cycles? (default: true) don't try to traverse a looping table • metamethod? (default: true) use the __fennelview metamethod if found • empty-as-sequence? (default: false) render empty tables as [] • line-length (number, default: 80) length of the line at which multi-line output for tables is forced • byte-escape (function) If present, overrides default behavior of es- caping special characters in decimal format (e.g. <ESC> -> \027). Called with the signature (byte-escape byte view-opts), where byte is the char code for a special character • escape-newlines? (default: false) emit strings with \n instead of newline • prefer-colon? (default: false) emit strings in colon notation when possible • utf8? (default: true) whether to use utf8 module to compute string lengths • max-sparse-gap (number, default: 1) maximum gap to fill in with nils in sparse sequential tables before switching to curly brackets. • preprocess (function) if present, called on x (and recursively on each value in x), and the result is used for pretty printing; takes the same arguments as fennel.view 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 seri- alization 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 intro- duced. 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? op- tion. options table contains options described above, and also visible-cycle? function, that takes a table being serialized, detects and saves infor- mation about possible reachable cycle. Should be used in __fennelview to implement cycle detection. __fennelview metamethod should always return a table of correctly in- dented 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 gen- eral 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 re- sults 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 struc- ture. For example, if you want to print a multi-line table, like this: @my-table[1 2 3] __fennelview should return a sequence of lines: ["@my-table[1" " 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 se- quential 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 ta- ble, and inner tables also remain indented correctly. When using the :preprocess option or __fennelview method, avoid modify- ing 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 con- sumed 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 es- capes: (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 /lambda will use the created function as a key on fennel.metadata to store the function's arglist and (if pro- vided) docstring. The metadata table is weakly-referenced by key, so each function's metadata will be garbage collected along with the func- tion 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: • fennel.metadata:get(func, key): get a value from a function's meta- data • fennel.metadata:set(func, key, val): set a metadata value • fennel.metadata:setall(func, key1, val1, key2, val2, ...): set pairs • fennel.doc(func, fnName): print formatted documentation for function using name. Utilized by the ,doc command, name is whatever symbol you operate on that's bound to the function. local greet = fennel.eval('( greet [name] "Say hello" (print "Hello," name))', {useMetadata = true}) fennel.metadata[greet] -- > {"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 ex- tra 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() print(fennel.view(syntax["icollect"])) --> {: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 spe- cials 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.runtime-version) print(fennel.runtimeVersion()) -- > Fennel 1.0.0 on PUC Lua 5.4 The fennel.version field will give you the version of just Fennel it- self. (since 1.3.1) If an optional argument is given, returns version information as a ta- ble: (fennel.runtime-version :as-table) ;; > {:fennel "1.3.1" :lua "PUC Lua 5.4"} PLUGINS 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 ver- sions are likely to break plugins, and each plugin should only be as- sumed 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 com- piler 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 nor- mally do side effects and return nil or error out. If a function re- turns non-nil, it will cause the rest of the plugins for a given event to be skipped. • symbol-to-expression • call • do • fn • destructure • parse-error • assert-compile 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 be- ing destructured and a to AST which is the AST for the form being de- structured 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 <https://clojure.github.io/clojure/clojure.edn-api.html>-style tagging, or manipulating external s-expression-based syntax, such as tree-sitter queries <https://tree-sitter.github.io/tree-sitter/using-parsers#query- syntax>. 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 func- tion which will read the next form from stdin (ignoring newlines and other whitespace), 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 chars opts] "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) nil >> (local abc :xyz) nil >> ,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. Activation 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 field which either contains a list of strings indicating every version of Fennel which you have tested it with, or a string containing a pattern which is checked against Fennel's version with string.find. If your plugin is used with a version of Fennel that doesn't match :versions it will emit a warn- ing. You should also have a :name field with the plugin's name. AUTHORS Fennel Maintainers. fennel 1.6.0 2025-10-13 fennel-api(3)
NAME | DESCRIPTION | START A CONFIGURABLE REPL | EVALUATE A STRING OF FENNEL | EVALUATE A FILE OF FENNEL | USE LUA'S BUILT-IN REQUIRE FUNCTION | MACRO SEARCHERS | GET FENNEL-AWARE STACK INFORMATION | COMPILE FENNEL CODE TO LUA | PARSE TEXT INTO AST NODES | AST NODE DEFINITION | SEARCH THE PATH FOR A MODULE WITHOUT LOADING IT | SERIALIZATION (VIEW) | WORK WITH DOCSTRINGS AND METADATA | DESCRIBE FENNEL SYNTAX | LOAD LUA CODE IN A PORTABLE WAY | DETECT LUA VM RUNTIME VERSION | PLUGINS | AUTHORS
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