These are all the special forms recognized by the Fennel compiler. It does not include built-in Lua functions; see the Lua reference manual for that.
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 given to
match the declared ones, the remaining ones are nil.
Example: (fn [x y] (print (+ x y)))
Creates a function like fn does, but throws an error at runtime if
any of the listed arguments are nil, unless its identifier begins with ?.
Example: (lambda [x ?y z] (print (- x (* (or ?y 1) z))))
The λ form is an alias for lambda and behaves identically.
Returns a new function which works like its first argument, but fills the first few arguments in place with the given ones. This is related to currying but different because calling it will call the underlying function instead of waiting till it has the "correct" number of args.
Example: (partial (fn [x y] (print (+ x y))) 2)
This example returns a function which will print a number that is 2 greater than the argument it is passed.
Creates a function and binds it to a given name. Can either be used to
create a local function or put a function in a table, depending on
whether the argument has a . in it or not.
Example: (defn plus [x y] (+ x y))
Example: (defn love.update [dt] (set total-time (+ total-time dt)))
Cannot be used to create a global function, but can be used to add a function to a table that is global.
Introduces a new scope in which a given set of local bindings are used.
Example: (let [x 89] (print (+ x 12)) -> 101
These locals cannot be changed with set but they can be shadowed by
an inner let or local. Outside the body of the let, the bindings
it introduces are no longer visible.
Any time you bind a local, you can destructure it if the value is a sequential table or a function call which returns multiple values:
Example: (let [[a b c] [1 2 3]] (+ a b c)) -> 6
Example: (let [(x y z) (unpack [10 9 8])] (+ x y z)) -> 27
Introduces a new local inside an existing scope. Similar to let but
without a body argument. Recommended for use at the top-level of a
file for locals which will be used throughout the file.
Example: (local lume (require "lume"))
Supports destructuring and multiple-value binding.
Sets a global variable to a new value. Note that there is no distinction between introducing a new global and changing the value of an existing one.
Example: (global prettyprint (fn [x] (print (view x))))
Supports destructuring and multiple-value binding.
Introduces a new local inside an existing scope which may have its
value changed. Identical to local apart from allowing set to work
on it.
Example: (var x 83)
Supports destructuring and multiple-value binding.
Changes the value of a variable introduced with var. Will not work
on globals or let/local-bound locals. Can also be used to change a
field of a table, even if the table is bound with let or local,
provided the field is given at compile-time.
Example: (set x (+ x 91))
Example: (let [t {:a 4 :b 8}] (set t.a 2) t) -> {:a 2 :b 8}
Supports destructuring and multiple-value binding.
Set the field of a given table to a new value. The field name does not
need to be known at compile-time. Works on any table, even those bound
with local and let.
Example: (let [tbl {:d 32} field :d] (tset tbl field 19) tbl) -> {:d 19}
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.
Example:
(let [x (math.random 64)]
(if (= 0 (% x 2))
"even"
(= 0 (% x 10))
"multiple of ten"
"I dunno, something else"))
All values other than nil or false are treated as true.
Takes a single condition and evaluates the rest as a body if it's not nil or false. As it always returns nil; this is intended for side-effects.
Example:
(when launch-missiles?
(power-on)
(open-doors)
(fire))
Run the body once for each value provided by the iterator. Nearly
always used with ipairs (for sequential tables) or pairs (for any
table in undefined order) but can be used with any iterator.
Example:
(each [key value (pairs mytbl)]
(print key (f value)))
Most iterators return two values, but each will bind any number.
Counts a number from a start to stop point, evaluating the body once for each value. Accepts an optional step.
Example:
(for [i 1 10 2]
(print i))
This example will print all odd numbers under ten.
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.
(if launch-missiles?
(do
(power-on)
(open-doors)
(fire))
false-alarm?
(promote lt-petrov))
and,or,notboolean+,-,*,/,//,%,^arithmetic>,<,>=,<=,=,~=comparison
These all work as you would expect, with a few caveats. The ~=
operator is used for "not equal", and // for integer division is
only available in Lua 5.3 and onward.
They all take any number of arguments, as long as that number is fixed
at compile-time. For instance, (= 2 2 (unpack [2 5])) will evaluate
to true because the compile-time number of values being compared is 3.
Note that these are all special forms which cannot be used as higher-order functions.
Concatenates its arguments into one string. Will coerce numbers into strings, but not other types.
Example: (.. "Hello" " " "world" 7 "!!!") -> "Hello world7!!!"
Returns the length of a string or table. Note that the length of a
table with gaps in it is undefined; it can return a number
corresponding to any of the table's "boundary" positions between nil
and non-nil values. If a table has nils and you want to know the last
consecutive numeric index starting at 1, you must calculate it
yourself with ipairs; if you want to know the maximum numeric key in
a table with nils, you can use table.maxn.
Example: (+ (# [1 2 3 nil 8]) (# "abc")) -> 6 or 8
Looks up a given key in a table. Multiple arguments will index repeatedly.
Example: (. mytbl myfield)
Example: (let [t {:a [2 3 4]}] (. t :a 2)) -> 3
Note that if the field name is known at compile time, you don't need
this and can just use mytbl.field.
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.
Example:
(let [f (assert (io.open "hello" "w"))]
(: f :write "world")
(: f :close))
Equivalent to:
(let [f (assert (io.open "hello" "w"))]
(f.write f "world")
(f.close f))
Returns multiple values from a function. Usually used to signal failure by returning nil followed by a message.
Example:
(fn [filename]
(if (valid-file-name? filename)
(open-file filename)
(values nil (.. "Invalid filename: " filename))))
Loops over a body until a condition is met. Uses a native Lua while loop, so is preferable to a lambda function and tail recursion.
Example:
(do
(var done? false)
(while (not done?)
(print :not-done)
(when (> (math.random) 0.95)
(set done? true))))
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.
Example:
(-> 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.
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.
Requires a module and binds its fields locally as macros.
TODO: document; see Fennel's own test cases for now.
See "Compiler API" below for details about extra functions and tables visible inside compiler scope which macros run in.
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.
Example:
(eval-compiler
(tset _SPECIALS "local" (. _SPECIALS "global")))
Inside eval-compiler blocks or require-macros modules, this extra
functionality is visible to your Fennel code:
listsymlist?sym?multi-sym?table?varg?
Note that other internals of the compiler exposed in compiler scope are subject to change.