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---
language: fennel
filename: learnfennel.fnl
contributors:
- ["Jesse Wattenbarger", "https://github.com/jjwatt"]
---
Fennel is a programming language that brings together the simplicity,
speed, and reach of Lua with the flexibility of a lisp syntax and
macro system.
```fennel
;; Comments start with semicolons.
;; Fennel is written in lists of things inside parentheses, separated
;; by whitespace.
;; The first thing in parentheses is a function or macro to call, and
;; the rest are the arguments.
;; ------------------------- ;;
;; 1. Primitives & Operators ;;
;; ------------------------- ;;
;; (local ...) defines a var inside the whole file's scope.
(local s "walternate") ; Immutable strings like Python.
;; local supports destructuring and multiple value binding.
;; (covered later).
;; Strings are utf8 byte arrays
"λx:(μα.α→α).xx" ; can include Unicode characters
;; .. will create a string out of it's arguments.
;; It will coerce numbers but nothing else.
(.. "Hello" " " "World") ; => "Hello World"
;; (print ...) will print all arguments with tabs in between
(print "Hello" "World") ; "Hello World" printed to screen
(local num 42) ;; Numbers can be integer or floating point.
;; Equality is =
(= 1 1) ; => true
(= 2 1) ; => false
;; Nesting forms works as you expect
(+ 1 (- 3 2)) ; = 1 + (3 - 2) => 2
;; Comparisons
(> 1 2) ; => false
(< 1 2) ; => true
(>= 1 1) ; => true
(<= 1 2) ; => true
(not= 1 2) ; => true
;; TODO: find some bitwise operator examples
;; (lshift 1) ; => 2
;; -------- ;;
;; 2. Types ;;
;; -------- ;;
;; Fennel uses Lua's types for booleans, strings & numbers.
;; Use `type` to inspect them.
(type 1) ; => "number"
(type 1.0) ; => "number"
(type "") ; => "string"
(type false) ; => "boolean"
(type nil) ; => "nil"
;; Booleans
true ; for true
false ; for false
(not true) ; => false
(and 0 false) ; => false
(or false 0) ; => 0
;; All values other than nil or false are treated as true.
;;,--------
;;| Binding
;;`--------
;; Use `let` to bind local vars to values.
;; Local binding: `me` is bound to "Bob" only within the (let ...)
(let [me "Bob"]
(print "returning Bob")
me) ; => "Bob"
;; Outside the body of the let, the bindings it introduces are no
;; longer visible. The last form in the body is used as the return
;; value. `set` does not work on `let` bound locals.
;; `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. `set` does not work on `locals`
(local tau-approx 6.28318)
;; `var` 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. `set` works on `vars`
(var x 83)
;; `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.
(set x (+ x 91)) ; var
;; Can also be used to change a field of a table, even if the table is
;; bound with `let` or `local`. If the table field name is static, use
;; `tbl.field`; if the field name is dynamic, use `(. tbl field)`.
(let [t {:a 4 :b 8}] ; static table field
(set t.a 2) t) ; => {:a 2 :b 8}
;; In any context 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
;; `global` set global variable
;; Sets a global variable to a new value. Note that there is no
;; distinction between introducing a new global and changing the value
;; of an existing one. This supports destructuring and multiple-value
;; binding.
(global tbl {:a 4 :b 8})
(global prettyprint (fn [x] (print (fennel.view x))))
;;,--------------------------
;;| Prettyprint and .fennelrc
;;`--------------------------
;; `prettyprint` is a good function to have in your repl
;; Return values in the repl will get pretty-printed, but calling
;; (print tbl) will emit output like table: 0x55a3a8749ef0. If you
;; don't already have one, it's recommended for debugging to define a
;; printer function which calls `fennel.view` on its argument before
;; printing it.
(local fennel (require :fennel))
(fn _G.pp [x] (print
(fennel.view x)))
;; Notice that adding it to `_G` puts the function in the global
;; table, similar to what `global` would do. The fennel documentation
;; says that using `_G` is the preferred method of defining and using
;; globals.
;; If you add this definition to your ~/.fennelrc
;; file it will be available in the standard repl.
(pp tbl)
;;,--------------------------------
;;| Collections & Sequences: Tables
;;`--------------------------------
;; Tables are the only compound data structure in Lua and fennel.
;; Similar to php arrays or js objects, they are
;; hash-lookup dicts that can also be used as lists.
;; tables can be treated as sequential or non-sequential: as hashmaps
;; or lists/arrays.
;; Using tables as dictionaries / maps:
(local t {:key1 "value1" :key2 false})
;; String keys can use dot notation:
(print t.key1) ;; Prints "value1"
;; Setting table keys and values
(tset t :newKey {}) ;; Adds a new key/value pair.
(tset t :key2 nil) ;; Removes key2 from the table.
;; Literal notation for any (non-nil) value as key
;; length string or table length
(+ (length [1 2 3 nil 8]) (length "abc")) ; => 8
;; . table lookup looks up a given key in a table. Multiple arguments
;; will perform nested lookup.
(. t :key1)
(let [t {:a [2 3 4]}] (. t :a 2)) ; => 3
;; If the field name is a string known at compile time, you don't need
;; this and can just use table.field (dot notation).
;; Nil-safe ?. table lookup
;; Looks up a given key in a table. Multiple arguments will perform
;; nested lookup. If any subsequent keys is not presnet, will
;; short-circuit to nil.
(?. t :key1) ; => "value"
(let [t {:a [2 3 4]}] (?. t :a 4 :b)) ; => nil
(let [t {:a [2 3 4 {:b 42}]}] (?. t :a 4 :b)) ; => 42
;; The table module
(let [t [1 2 3]]
(table.insert t 2 "a") ; t is now [1 "a" 2 3]
(table.insert t "last") ; now [1 "a" 2 3 "last"]
(print (table.remove t)) ; prints "last"
(table.remove t 1) ; t is now ["a" 2 3]
(print (table.concat t ", "))) ; prints "a, 2, 3"
;; The table.sort function sorts a table in-place, as a
;; side-effect. It takes an optional comparator function which should
;; return true when its first argument is less than the second.
;; The table.unpack function returns all the elements in the table as
;; multiple values. Note that table.unpack is just unpack in Lua 5.1.
;; This will make `unpack` work in both.
(var unpack (or _G.unpack table.unpack))
;; See "prettyprint" section about printing tables
;; --------------------- ;;
;; 3. Basic Flow Control ;;
;; --------------------- ;;
;; `if` checks a condition and evaluates the corresponding body.
;; Accepts any number of condition/body pairs. If an odd number of
;; args 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))
"even"
"I dunno, something else"))
;; All values other than nil or false are treated as true.
;; `when` takes a single condition and evalutes the rest as a body if
;; it's truthy. Intended for side-effects. The last form is the return
;; value.
(when launch-missiles?
(power-on)
(open-doors)
(fire))
;;,------------------
;;| Loops & Iteration
;;`------------------
;; each: general iteration
;; `each` runs the body once for each value provided by the iterator.
(each [key value (pairs mytbl)]
(print "executing key")
(print (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))
;; `for` is a numeric loop with start, stop and optional step.
(for [i 1 10 2]
(log-number i)
(print i)) ;; print odd numbers under 10
;; Like each, loops using for can also be terminated early with an
;; &until clause
(var x 0)
(for [i 1 128 &until (maxed-out? x)]
(set x (+ x i)))
;; while loops over a body until a condition is met
;; Returns nil.
(var done? false)
(while (not done?)
(print :not-done)
(when (< 0.95 (math.random))
(set done? true)))
;; while uses the native lua while loop
;; `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?
(do
(power-on)
(open-doors)
(fire))
false-alarm?
(promote lt-petrov))
;; Many functions and macros like fn & let have an implicit do at the
;; start, so you don't have to add it to use multiple forms.
;;,-----------------------
;;| `collect` & `icollect`
;;`-----------------------
;; icollect, collect: table comprehension macros
;; The icollect macro takes a "iterator binding table" in the same
;; format that `each` takes, and returns a sequential table containing
;; all the values produced by each iteration of the macro's body. This
;; is similar to how map works in several other languages, but it is a
;; macro, not a function.
;; If the value is nil, it is omitted from the return table. This is
;; analogous to filter in other languages.
(icollect [_ v (ipairs [1 2 3 4 5 6])]
(if (< 2 v) (* 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 (< 2 v) (* v v))))
tbl)
;; 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= v "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= v "yellow")
(match (values (.. "color-" v) k)
(key value) (tset tbl key value))))
tbl)
;; If the key and value are given directly in the body of collect and
;; not nested in an outer form, then the `values` call can be omitted
;; for brevity
(collect [k v (pairs {:a 85 :b 52 :c 621 :d 44})]
k (* v 5))
;; -> {:a 425 :b 260 :c 3105 :d 220}
;; If the index 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
(icollect [_ x (ipairs [2 3]) &into [9]]
(* x 11))
;; -> [9 22 33]
;; accumulate
;; 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 its 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
;; `faccumulate` range accumulation Identical to accumulate, but
;; instead of taking an iterator and the same bindings as `each`, it
;; accepts the same bindings as `for` and will iterate the numerical
;; range. Accepts `&until` just like `for` and `accumulate`.
(faccumulate [n 0 i 1 5] (+ n i)) ; => 15
;; `fcollect` range comprehension Similarly to `icollect`, `fcollect`
;; provides a way of building a sequential table. Unlike `icollect`,
;; instead of an iterator it traverses a range, as accepted by the
;; `for` special. The `&into` and `&until` clauses work the same as in
;; `icollect.`
(fcollect [i 0 10 2]
(if (> i 2) (* i i)))
;; -> [16 36 64 100]
;; equivalent to:
(let [tbl {}]
(for [i 0 10 2]
(if (> i 2)
(table.insert tbl (* i i))))
tbl)
;; `values` 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))))
;; See the Destructuring and Matching sections for more advanced Flow
;; Control.
;; ------------ ;;
;; 4. Functions ;;
;; ------------ ;;
;; Use fn to create new functions. A function always returns its last
;; statement.
(fn [] "Hello World") ; => #<function: 0x55630f9d7f20>
; (You need extra parens to call it)
((fn [] "Hello World")) ; => "Hello World"
;; Assign a function to a local variable
(local hello-world (fn [] "Hello World"))
(hello-world) ; => "Hello World"
;; You can use fn and name a function.
(fn hello-world [] "Hello World") ; => "Hello World"
;; The [] is the list of arguments to the function.
(fn hello [name]
(.. "Hello " name))
(hello "Steve") ; => "Hello Steve"
;; 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.
;; 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))))
;; equivalent to:
(set functions.p (fn [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.
(fn factorial [x acc]
(if (= 0 x)
acc
(factorial (- x 1) (* x acc))))
(factorial 5 1) ;; -> 120
;; The final form in this and all other function forms is used as the
;; return value.
;; (lambda [...])
;; 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.
;; Docstrings and metadata
;; 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)))
;; Hash function literal shorthand
;; hashfn is a special function that you can abbreviate as #
;; #foo expands to (hashfn foo)
;; Hash functions are anonymous functions of one form, with implicitly
;; named arguments.
#(+ $1 $2) ;; same as
(hashfn (+ $1 $2)) ; implementation detail; don't use directly
;; same as
(fn [a b] (+a b))
;; A lone $ in a hash function is treated as ana alias for $1.
#(+ $ 1)
#$ ; same as (fn [x] x) (aka the identity function
#val ; same as (fn [] val)
#[$1 $2 $3] ; same as (fn [a b c] [a b c])
;; ---------------------------------------------;;
;; 5. Destructuring, Binding & Pattern Matching ;;
;; ---------------------------------------------;;
;; 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. This works for both creating tables and
;; destructuring them.
(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"
;; 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
;;,-----------------------
;;| Multiple value binding
;;`-----------------------
;; In most contexts where you can make a new binding, you can use
;; multiple value binding.
(let [x (values 1 2 3)] x) ; = > 1
(let [(file-handle message code) (io.open "fooblah.blah")]
message) ; => "fooblah.blah: No such file or directory"
(do
(local (_ _ z) (table.unpack [:a :b :c :d :e]))
z) ; => c
;; tset sets the field of a given table to a new value.
(let [tbl {:d 32}
field :d]
(tset tbl field 19) tbl) ; => {:d 19}
;; You can provide multiple successive field names to perform
;; nested sets.
(let [tbl {:a
{:b {}}}
field :c]
(tset tbl :a :b field "d") tbl) ; => .. .. {:a {:b {:c "d"}}}
;;,------------------------
;;| `case` pattern matching
;;`------------------------
;; Evaluates its first argument, then searches thru the subsequent
;; pattern/body clauses to find one where the pattern matches the
;; value, and evaluates the corresponding body. Pattern matching can
;; be thought of as a combination of destructuring and conditionals.
(case mytable
59 :will-never-match-hopefully
[9 q 5] (print :q q)
[1 a b] (+ a b))q
;; Patterns can be tables, literal values, or symbols. Any symbol is
;; implicitly checked to be not nil. Symbols can be repeated in an
;; expression to check for the same value.
(case mytable
;; the first and second values of mytable are not nil and are the same value
[a a] (* a 2)
;; the first and second values are not nil and are not the same value
[a b] (+ a b))
;; It's important to note that expressions are checked in order! In
;; the above example, since [a a] is checked first
;; You may allow a symbol to optionally be nil by prefixing it with ?.
(case mytable
;; not-nil, maybe-nil
[a ?b] :maybe-one-maybe-two-values
;; maybe-nil == maybe-nil, both are nil or both are the same value
[?a ?a] :maybe-none-maybe-two-same-values
;; maybe-nil, maybe-nil
[?a ?b] :maybe-none-maybe-one-maybe-two-values)
;; Symbols prefixed by an _ are ignored and may stand in as positional
;; placeholders or markers for "any" value - including a nil value. A
;; single _ is also often used at the end of a case expression to
;; define an "else" style fall-through value.
(case mytable
;; not-nil, anything
[a _b] :maybe-one-maybe-two-values
;; anything, anything (different to the previous ?a example!)
;; note this is effectively the same as []
[_a _a] :maybe-none-maybe-one-maybe-two-values
;; anything, anything
;; this is identical to [_a _a] and in this example would never actually match.
[_a _b] :maybe-none-maybe-one-maybe-two-values
;; when no other clause matched, in this case any non-table value
_ :no-match)
;; You can match with multiple return values with
;; parenthesis, like you can with destructuring `let`
(case (io.open "/some/file")
(nil msg) (report-error msg)
f (read-file f))
;;,--------------
;;| Guard Clauses
;;`--------------
;; If you need to match on something more general than
;; a structure, use guard clauses:
(case [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
;; Each form after the pattern is a condition. All conditions must
;; evaluate to true for the pattern to match.
;; If several patterns share the same body & guards, such patterns can
;; be with the `or` special in the `where` clause.
(case [5 1 2]
(where (or [a 3 9] [a 1 2]) (= 5 a))
"Either [5 3 9] or [5 1 2]"
_ "anything else")
;; Symbols bound inside a case pattern are independent from any
;; existing symbols in the current scope, Sometimes it may be
;; desirable to match against an existing value in the outer scope. To
;; do this we can "pin" a binding inside the pattern with an existing
;; outer binding with the unary (= binding-name) form. The unary (=
;; binding-name) form is only valid in a case pattern and must be
;; inside a (where) guard.
(let [x 1]
(case [1]
;; 1 == 1
(where [(= x)]) x
_ :no-match)) ; -> 1
;; Pinning is only required inside the pattern. Outer bindings are
;; automatically available inside guards and bodies as long as the
;; name has not been rebound in the pattern.
;; Note: The case macro can be used in place of the if-let macro from
;; Clojure. The reason Fennel doesn't have if-let is that case makes
;; it redundant.
;;,-------------------------
;;| `match` pattern matching
;;`-------------------------
;; match is conceptually equivalent to case, except symbols in the
;; patterns are always pinned with outer-scope symbols if they exist.
(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
;;,-----------------------------------------------------
;;| `case-try` & `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 case. If no catch pattern
;; matches, nil is returned.
;;
;; If there is no catch, the mismatched value will be returned as the
;; value of the entire expression.
(fn handle [conn token]
(case-try (conn:receive :*l)
input (parse input)
(command-name params (= token)) (commands.get command-name)
command (pcall command (table.unpack params))
(catch
(_ :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.
;; `match-try` for matching multiple steps
;; Unlike case-try, match-try will pin values in a given catch block
;; with those in the original steps.
(fn handle [conn token]
(match-try (conn:receive :*l)
input (parse input)
(command-name params token) (commands.get command-name)
command (pcall command (table.unpack params))
(catch
(_ :timeout) nil
(_ :closed) (pcall disconnect conn "connection closed")
(_ msg) (print "Error handling input" msg))))
;; ---------;;
;; 6. Other ;;
;; ---------;;
;; The `:` 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. 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")
(f:close))
;; If the name of the method or the table containing it isn't fixed,
;; you can use : followed by the table and then the method's name to
;; allow it to be a dynamic string instead
(let [f (assert (io.open "hello" "w"))
method1 :write
method2 :close]
(: f method1 "world")
(: f method2))
;; 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))
(t:enable)
;; ->, ->>, -?> 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.
;; doto
;; 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)
x)
;; tail!
;; the tail! form asserts that its argument is called in a tail
;; position. You can use this when the code depends on tail call
;; optimization; that way if the code is changed so that the recursive
;; call is no longer in the tail position, it will cause a compile
;; error instead of overflowing the stack later on large data sets.
(fn process-all [data i]
(case (process (. data i))
:done (print "Process completed.")
:next (process-all data (+ i 1))
:skip (do (tail! (process-all data (+ i 2)))
;; ^^^^^ Compile error: Must be in tail position
(print "Skipped" (+ i 1)))))
```
### Further Reading
The [fennel website] (https://fennel-lang.org/) is the best resource
on fennel. It links to the [fennel setup guide]
(https://fennel-lang.org/setup) and to the [fennel reference manual]
(https://fennel-lang.org/reference). This docuement borrows heavily in
parts from the fennel reference manual.