# Functions

Janet is a functional language - that means that one of the basic building blocks of your program will be defining functions (the other is using data structures). Because janet is a Lisp, functions are values just like numbers or strings - they can be passed around and created as needed.

Functions can be defined with the `defn` macro, like so:

``````(defn triangle-area
"Calculates the area of a triangle."
[base height]
(print "calculating area of a triangle...")
(* base height 0.5))``````

A function defined with `defn` consists of a name, a number of optional flags for def, and finally a function body. The example above is named triangle-area and takes two parameters named base and height. The body of the function will print a message and then evaluate to the area of the triangle.

Once a function like the above one is defined, the programmer can use the `triangle-area` function just like any other, say `print` or `+`.

``````# Prints "calculating area of a triangle..." and then "25"
(print (triangle-area 5 10))``````

Note that when nesting function calls in other function calls like above (a call to triangle-area is nested inside a call to print), the inner function calls are evaluated first. Also, arguments to a function call are evaluated in order, from first argument to last argument).

Because functions are first-class values like numbers or strings, they can be passed as arguments to other functions as well.

``(print triangle-area)``

This prints the location in memory of the function triangle area.

Functions don't need to have names. The `fn` keyword can be used to introduce function literals without binding them to a symbol.

``````# Evaluates to 40
((fn [x y] (+ x x y)) 10 20)
# Also evaluates to 40
((fn [x y &] (+ x x y)) 10 20)

# Will throw an error about the wrong arity
((fn [x] x) 1 2)
# Will not throw an error about the wrong arity
((fn [x &] x) 1 2)``````

The first expression creates an anonymous function that adds twice the first argument to the second, and then calls that function with arguments 10 and 20. This will return (10 + 10 + 20) = 40.

There is a common macro `defn` that can be used for creating functions and immediately binding them to a name. `defn` works as expected at both the top level and inside another form. There is also the corresponding macro `defmacro` that does the same kind of wrapping for macros.

``````(defn myfun [x y]
(+ x x y))

# You can think of defn as a shorthand for def and fn together
(def myfun-same (fn [x y]
(+ x x Y)))

(myfun 3 4) # -> 10``````

Janet has many macros provided for you (and you can write your own). Macros are just functions that take your source code and transform it into some other source code, usually automating some repetitive pattern for you.

## Optional Arguments

Most Janet functions will raise an error at runtime if not passed exactly the right number of arguments. Sometimes, you want to define a function with optional arguments, where the arguments take a default value if not supplied by the caller. Janet has support for this via the `&opt` symbol in parameter lists, where all parameters after `&opt` are `nil` if not supplied.

``````(defn my-opt-function
"A dumb function with optional arguments."
[a b c &opt d e f]
(default d 10)
(default e 11)
(default f 12)
(+ a b c d e f))``````

The `(default)` macro is a useful macro for setting default values for parameters. If a parameter is nil, `(default)` will redefine it to a default value.

Janet also provides first-class support for variadic functions. Variadic functions can take any number of parameters, and gather them up into a tuple. To define a variadic function, use the `&` symbol as the second to last item if the parameter list. Parameters defined before the last variadic argument are not optional, unless specified as so with the `&opt` symbol.

``````(defn my-adder
"Adds numbers in a dubious way."
[& xs]
(var accum 0)
(each x xs
(+= accum (- (* 2 x) x)))
accum)

(my-adder 1 2 3) # -> 6``````

## Ignoring Extra Arguments

Sometimes you may want to have a function that can take a number of extra arguments but not use them. This happens because a function may be part of an interface, but the function itself doesn't need those arguments. You can write a function that will simply drop extra parameters by adding `&` as the last parameter in the function.

``````(defn ignore-extra
[x &]
(+ x 1))

(ignore-extra 1 2 3 4 5) # -> 2``````

## Keyword Style Arguments

Sometimes, you want a function to have many arguments, and calling such a function can get confusing without naming the arguments in the call. One solution to this problem is passing a table or struct with all of the arguments you want to use. This is in general a good approach, as now your original arguments can be identified by the keys in the struct.

``````(defn make-recipe
"Create some kind of cake recipe..."
[args]
(def dry [(args :flour) (args :sugar) (args :baking-soda)])
(def wet [(args :water) (args :eggs) (args :vanilla-extract)])
{:name "underspecified-cake" :wet wet :dry dry})

# Call with an argument struct
(make-recipe
{:flour 1
:sugar 1
:baking-soda 0.5
:water 2
:eggs 2
:vanilla-extract 0.5})``````

This is often good enough, but there are a couple downsides. The first is that our semantic arguments are not documented, the docstring will just have a single argument "args" which is not helpful. The docstring should have some indication of they keys that we expect in the struct. We also need to write out an extra pair of brackets for the struct - this isn't a huge deal, but it would be nice if we didn't need to write this out explicitly.

We can solve this first problem by using destructuring in the argument.

``````(defn make-recipe-2
"Create some kind of cake recipe with destructuring..."
[{:flour flour
:sugar sugar
:baking-soda soda
:water water
:eggs eggs
:vanilla-extract vanilla}]
(def dry [flour sugar soda])
(def wet [water eggs vanilla])
{:name "underspecified-cake" :wet wet :dry dry})

# We can call the function in the same manner as before.
(make-recipe-2
{:flour 1
:sugar 1
:baking-soda 0.5
:water 2
:eggs 2
:vanilla-extract 0.5})``````

The docstring of the improved function will contain a list of arguments that our function takes. To fix the second issue, we can use the &keys symbol in a function parameter list to automatically create our big argument struct for use on invocation.

``````(defn make-recipe-3
"Create some kind of recipe using &keys..."
[&keys {:flour flour
:sugar sugar
:baking-soda soda
:water water
:eggs eggs
:vanilla-extract vanilla}]
(def dry [flour sugar soda])
(def wet [water eggs vanilla])
{:name "underspecified-cake" :wet wet :dry dry})

# Calling this function is a bit different now - no struct
(make-recipe-3
:flour 1
:sugar 1
:baking-soda 0.5
:water 2
:eggs 2
:vanilla-extract 0.5)``````

Usage of this last variant looks cleaner than the previous two recipe making functions, but there is a caveat. Having all of the arguments packaged as a struct is often useful, as the struct can be passed around and threaded through an application efficiently. Functions that require many arguments often pass those arguments to subroutines, so in practice this issue is quite common. The `&keys` syntax is therefor most useful where terseness and visual clarity is more important.

It is not difficult to convert between the two calling styles, if need be. To pass a struct to a function that expects `&keys`-style arguments, the `kvs` function works well. To convert arguments in the other direction, from `&keys`-style to a single struct argument, is trivial - just wrap your arguments in curly brackets!

``````(def args
{:flour 1
:sugar 1
:baking-soda 0.5
:water 2
:eggs 2
:vanilla-extract 0.5})

# Turn struct into an array key,value,key,value,... and splice into a call
(make-recipe-3 ;(kvs args))``````