# Ludus prelude documentation
These functions are available in every Ludus script.
The documentation for any function can be found within Ludus by passing the function to `doc!`, 
e.g., running `doc! (add)` will send the documentation for `add` to the console.

•[abs](#abs)    •[add](#add)    •[and](#and)    •[angle](#angle)    •[append](#append)    •[assert!](#assert!)    •[assoc](#assoc)    •[assoc?](#assoc?)    •[at](#at)    •[atan/2](#atan/2)    •[back!](#back!)    •[background!](#background!)    •[bg!](#bg!)    •[bgcolor](#bgcolor)    •[bk!](#bk!)    •[bool](#bool)    •[bool?](#bool?)    •[butlast](#butlast)    •[ceil](#ceil)    •[clear!](#clear!)    •[coll?](#coll?)    •[colors](#colors)    •[concat](#concat)    •[console](#console)    •[cos](#cos)    •[count](#count)    •[dec](#dec)    •[deg/rad](#deg/rad)    •[deg/turn](#deg/turn)    •[deref](#deref)    •[dict](#dict)    •[diff](#diff)    •[dissoc](#dissoc)    •[dist](#dist)    •[div](#div)    •[div/0](#div/0)    •[div/safe](#div/safe)    •[doc!](#doc!)    •[each!](#each!)    •[eq?](#eq?)    •[err](#err)    •[err?](#err?)    •[even?](#even?)    •[false?](#false?)    •[fd!](#fd!)    •[first](#first)    •[floor](#floor)    •[flush!](#flush!)    •[fn?](#fn?)    •[fold](#fold)    •[forward!](#forward!)    •[get](#get)    •[goto!](#goto!)    •[gt?](#gt?)    •[gte?](#gte?)    •[heading](#heading)    •[heading/vector](#heading/vector)    •[home!](#home!)    •[inc](#inc)    •[join](#join)    •[keys](#keys)    •[keyword?](#keyword?)    •[last](#last)    •[left!](#left!)    •[list](#list)    •[lt!](#lt!)    •[lt?](#lt?)    •[lte?](#lte?)    •[make!](#make!)    •[map](#map)    •[mod](#mod)    •[mult](#mult)    •[neg](#neg)    •[neg?](#neg?)    •[nil?](#nil?)    •[not](#not)    •[odd?](#odd?)    •[ok](#ok)    •[ok?](#ok?)    •[or](#or)    •[ordered?](#ordered?)    •[p5_calls](#p5_calls)    •[panic!](#panic!)    •[pc!](#pc!)    •[pd!](#pd!)    •[pencolor](#pencolor)    •[pencolor!](#pencolor!)    •[pendown!](#pendown!)    •[pendown?](#pendown?)    •[penup!](#penup!)    •[penwidth](#penwidth)    •[penwidth!](#penwidth!)    •[pi](#pi)    •[pos?](#pos?)    •[position](#position)    •[print!](#print!)    •[prn!](#prn!)    •[pu!](#pu!)    •[pw!](#pw!)    •[rad/deg](#rad/deg)    •[rad/turn](#rad/turn)    •[random](#random)    •[range](#range)    •[render_turtle!](#render_turtle!)    •[report!](#report!)    •[reset_turtle!](#reset_turtle!)    •[rest](#rest)    •[right!](#right!)    •[round](#round)    •[rt!](#rt!)    •[second](#second)    •[set](#set)    •[show](#show)    •[sin](#sin)    •[slice](#slice)    •[some](#some)    •[some?](#some?)    •[square](#square)    •[string](#string)    •[string?](#string?)    •[sub](#sub)    •[sum_of_squares](#sum_of_squares)    •[tan](#tan)    •[tau](#tau)    •[turn/deg](#turn/deg)    •[turn/rad](#turn/rad)    •[turtle_commands](#turtle_commands)    •[turtle_state](#turtle_state)    •[turtle_states](#turtle_states)    •[type](#type)    •[unwrap!](#unwrap!)    •[unwrap_or](#unwrap_or)    •[update](#update)    •[update!](#update!)    •[values](#values)    •[zero?](#zero?)
## Function documentation
### abs
```
Returns the absolute value of a number.
(0) (n)
```

### add
```
Adds numbers or vectors.
() (x as :number) (x as :number, y as :number) (x, y, ...zs) ((x1, y1), (x2, y2))
```

### and
```
Returns true if all values passed in are truthy.
() (x) (x, y) (x, y, ...zs)
```

### angle
```
Calculates the angle between two vectors.
(v1, v2)
```

### append
```
Adds an element to a list or set.
() (xs as :list) (xs as :list, x) (xs as :set) (xs as :set, x)
```

### assert!
```
Asserts a condition: returns the value if the value is truthy, panics if the value is falsy. Takes an optional message.
(value) (value, message)
```

### assoc
```
Takes a dict, key, and value, and returns a new dict with the key set to value.
() (dict as :dict) (dict as :dict, key as :keyword, value) (dict as :dict, (key as :keyword, value))
```

### assoc?
```
Returns true if a value is an associative collection: a dict, struct, or namespace.
(assoc as :dict) (assoc as :struct) (assoc as :ns) (_)
```

### at
```
Returns the element at index n of a list or tuple. Zero-indexed: the first element is at index 0.
(xs as :list, n as :number) (xs as :tuple, n as :number)
```

### atan/2
```
Returns an angle from a slope. Takes an optional keyword argument to specify units. Takes either two numbers or a vector tuple.
(x as :number, y as :number) (x, y, :turns) (x, y, :radians) (x, y, :degrees) ((x, y)) ((x, y), units as :keyword)
```

### back!
```
Moves the turtle backward by a number of steps. Alias: bk!
(steps as :number)
```

### background!
```
Sets the background color behind the turtle and path. Alias: bg!
(gray as :number) ((r as :number, g as :number, b as :number)) ((r as :number, g as :number, b as :number, a as :number))
```

### bg!
```
Sets the background color behind the turtle and path. Alias: bg!
(gray as :number) ((r as :number, g as :number, b as :number)) ((r as :number, g as :number, b as :number, a as :number))
```

### bgcolor
No documentation available.


### bk!
```
Moves the turtle backward by a number of steps. Alias: bk!
(steps as :number)
```

### bool
```
Returns false if a value is nil or false, otherwise returns true.
(nil) (false) (_)
```

### bool?
```
Returns true if a value is of type :boolean.
(false) (true) (_)
```

### butlast
```
Returns a list, omitting the last element.
(xs as :list)
```

### ceil
```
Truncates a number towards positive infinity. With negative numbers, it returns the integer part. With positive numbers, returns the next more-positive integer.
(n as :number)
```

### clear!
```
Clears the canvas and sends the turtle home.
()
```

### coll?
```
Returns true if a value is a collection: dict, struct, list, tuple, or set.
(coll as :dict) (coll as :struct) (coll as :list) (coll as :tuple) (coll as :set) (coll as :ns) (_)
```

### colors
No documentation available.


### concat
```
Combines two lists, strings, or sets.
(x as :string, y as :string) (xs as :list, ys as :list) (xs as :set, ys as :set) (xs, ys, ...zs)
```

### console
No documentation available.


### cos
```
Returns the cosine of an angle. Default angle measure is turns. An optional keyword argument specifies the units of the angle passed in.
(a as :number) (a as :number, :turns) (a as :number, :degrees) (a as :number, :radians)
```

### count
```
Returns the number of elements in a collection (including string).
(xs as :list) (xs as :tuple) (xs as :dict) (xs as :string) (xs as :set) (xs as :struct)
```

### dec
```
Decrements a number.
(x as :number)
```

### deg/rad
```
Converts an angle in degrees to an angle in radians.
(a as :number)
```

### deg/turn
```
Converts an angle in degrees to an angle in turns.
(a as :number)
```

### deref
```
Resolves a ref into a value.
(r as :ref)
```

### dict
```
Takes a struct or ns, and returns it as a dict. Or, takes a list or tuple of (key, value) tuples and returns it as a dict. Returns dicts unharmed.
(struct as :struct) (ns_ as :ns) (dict as :dict) (list as :list) (tup as :tuple)
```

### diff
```
Takes two associate data structures and returns a dict describing their differences. Does this shallowly, offering diffs only for keys in the original dict.
(d1 as :dict, d2 as :dict)
```

### dissoc
```
Takes a dict and a key, and returns a new dict with the key and associated value omitted.
(dict as :dict) (dict as :dict, key as :keyword)
```

### dist
```
Returns the distance from the origin to a point described by (x, y).
(x as :number, y as :number) ((x, y))
```

### div
```
Divides numbers. Panics on division by zero.
(x as :number) (_, 0) (x as :number, y as :number) (x, y, ...zs)
```

### div/0
```
Divides numbers. Returns 0 on division by zero.
(x as :number) (_, 0) (x as :number, y as :number) (x, y, ...zs)
```

### div/safe
```
Divides a number. Returns a result tuple.
(x as :number) (_, 0) (x, y) (x, y, ...zs)
```

### doc!
```
Prints the documentation of a function to the console.
(f as :fn) (_)
```

### each!
```
Takes a list and applies a function, presumably with side effects, to each element in the list. Returns nil.
(f! as :fn, []) (f! as :fn, [x]) (f! as :fn, [...xs])
```

### eq?
```
Returns true if all arguments have the same value.
(x) (x, y) (x, y, ...zs)
```

### err
```
Takes a value and wraps it in an :err result tuple, presumably as an error message.
(msg)
```

### err?
```
Takes a value and returns true if it is an :err result tuple.
((:err, _)) (_)
```

### even?
```
Returns true if a value is an even number, otherwise returns false.
(x as :number) (_)
```

### false?
```
Returns true if a value is false, otherwise returns false. Useful to distinguish between false and nil.
(false) (_)
```

### fd!
```
Moves the turtle forward by a number of steps. Alias: fd!
(steps as :number)
```

### first
```
Returns the first element of a list or tuple.
(xs)
```

### floor
```
Truncates a number towards negative infinity. With positive numbers, it returns the integer part. With negative numbers, returns the next more-negative integer.
(n as :number)
```

### flush!
```
Clears the console, and returns the messages.
()
```

### fn?
```
Returns true if an argument is a function.
(f as :fn) (_)
```

### fold
```
Folds a list.
(f as :fn, xs as :list) (f as :fn, xs as :list, root)
```

### forward!
```
Moves the turtle forward by a number of steps. Alias: fd!
(steps as :number)
```

### get
```
Takes a dict or struct, key, and optional default value; returns the value at key. If the value is not found, returns nil or the default value. Returns nil or default if the first argument is not a dict or struct.
(key as :keyword) (key as :keyword, coll) (key as :keyword, coll, default)
```

### goto!
```
Sends the turtle to (x, y) coordinates. If the pen is down, the turtle will draw a path to its new location.
(x as :number, y as :number) ((x, y))
```

### gt?
```
Returns true if numbers are in decreasing order.
(x as :number) (x as :number, y as :number) (x, y, ...zs)
```

### gte?
```
Returns true if numbers are in decreasing or flat order.
(x as :number) (x as :number, y as :number) (x, y, ...zs)
```

### heading
```
Returns the turtle's current heading.
()
```

### heading/vector
```
Takes a turtle heading, and returns a unit vector of that heading.
(heading)
```

### home!
```
Sends the turtle home: to the centre of the screen, pointing up. If the pen is down, the turtle will draw a path to home.
()
```

### inc
```
Increments a number.
(x as :number)
```

### join
```
Takes a list of strings, and joins them into a single string, interposing an optional separator.
([]) ([str as :string]) (strs as :list) ([str, ...strs], separator as :string)
```

### keys
```
Takes an associative collection and returns a list of keys in that collection. Returns an empty list on anything other than a collection.
(coll)
```

### keyword?
```
Returns true if a value is a keyword, otherwise returns false.
(kw as :keyword) (_)
```

### last
```
Returns the last element of a list or tuple.
(xs)
```

### left!
```
Rotates the turtle left, measured in turns. Alias: lt!
(turns as :number)
```

### list
```
Takes a value and returns it as a list. For values, it simply wraps them in a list. For collections, conversions are as follows. A tuple->list conversion preservers order and length. Unordered collections do not preserve order. Associative collections return lists of (key, value) tuples.
(x)
```

### lt!
```
Rotates the turtle left, measured in turns. Alias: lt!
(turns as :number)
```

### lt?
```
Returns true if numbers are in increasing order.
(x as :number) (x as :number, y as :number) (x, y, ...zs)
```

### lte?
```
Returns true if numbers are in increasing or flat order.
(x as :number) (x as :number, y as :number) (x, y, ...zs)
```

### make!
```
Sets the value of a ref.
(r as :ref, value)
```

### map
```
Maps over a list.
(f as :fn, xs) (kw as :keyword, xs)
```

### mod
```
Returns the modulus of num and div. Truncates towards negative infinity.
(num as :number, y as :number)
```

### mult
```
Multiplies numbers or vectors.
() (x as :number) (x as :number, y as :number) (x, y, ...zs) (scalar as :number, (x, y)) ((x, y), scalar as :number)
```

### neg
```
Multiplies a number by -1, negating it.
(n as :number)
```

### neg?
```
Returns true if a value is a negative number, otherwise returns false.
(x as :number) (_)
```

### nil?
```
Returns true if a value is nil.
(nil) (_)
```

### not
```
Returns false if a value is truthy, true if a value is falsy.
(nil) (false) (_)
```

### odd?
```
Returns true if a value is an odd number, otherwise returns false.
(x as :number) (_)
```

### ok
```
Takes a value and wraps it in an :ok result tuple.
(value)
```

### ok?
```
Takes a value and returns true if it is an :ok result tuple.
((:ok, _)) (_)
```

### or
```
Returns true if any value passed in is truthy.
() (x) (x, y) (x, y, ...zs)
```

### ordered?
```
Returns true if a value is an indexed collection: list or tuple.
(coll as :list) (coll as :tuple) (_)
```

### p5_calls
No documentation available.


### panic!
```
Causes Ludus to panic, outputting any arguments as messages.
() (...args)
```

### pc!
```
Changes the turtle's pen color. Takes a single grayscale value, an rgb tuple, or an rgba tuple. Alias: pc!
(gray as :number) ((r as :number, g as :number, b as :number)) ((r as :number, g as :number, b as :number, a as :number))
```

### pd!
```
Lowers the turtle's pen, causing it to draw. Alias: pd!
()
```

### pencolor
```
Returns the turtle's pen color as an (r, g, b, a) tuple.
()
```

### pencolor!
```
Changes the turtle's pen color. Takes a single grayscale value, an rgb tuple, or an rgba tuple. Alias: pc!
(gray as :number) ((r as :number, g as :number, b as :number)) ((r as :number, g as :number, b as :number, a as :number))
```

### pendown!
```
Lowers the turtle's pen, causing it to draw. Alias: pd!
()
```

### pendown?
```
Returns the turtle's pen state: true if the pen is down.
()
```

### penup!
```
Lifts the turtle's pen, stopping it from drawing. Alias: pu!
()
```

### penwidth
```
Returns the turtle's pen width in pixels.
()
```

### penwidth!
```
Sets the width of the turtle's pen, measured in pixels. Alias: pw!
(width as :number)
```

### pi
No documentation available.


### pos?
```
Returns true if a value is a positive number, otherwise returns false.
(x as :number) (_)
```

### position
```
Returns the turtle's current position.
()
```

### print!
```
Sends a text representation of Ludus values to the console.
(...args)
```

### prn!
```
Prints the underlying Clojure data structure of a Ludus value.
(x)
```

### pu!
```
Lifts the turtle's pen, stopping it from drawing. Alias: pu!
()
```

### pw!
```
Sets the width of the turtle's pen, measured in pixels. Alias: pw!
(width as :number)
```

### rad/deg
```
Converts an angle in radians to an angle in degrees.
(a as :number)
```

### rad/turn
```
Converts an angle in radians to an angle in turns.
(a as :number)
```

### random
```
Returns a random number. With zero arguments, returns a random number between 0 (inclusive) and 1 (exclusive). With one argument, returns a random number between 0 and n. With two arguments, returns a random number between m and n.
() (n as :number) (m as :number, n as :number)
```

### range
```
Returns the set of integers between start (inclusive) and end (exclusive) as a list. With one argument, starts at 0. If end is less than start, returns an empty list.
(end as :number) (start as :number, end as :number)
```

### render_turtle!
```

()
```

### report!
```
Prints a value, then returns it.
(x) (msg as :string, x)
```

### reset_turtle!
```
Resets the turtle to its original state.
()
```

### rest
```
Returns all but the first element of a list or tuple, as a list.
(xs as :list) (xs as :tuple)
```

### right!
```
Rotates the turtle right, measured in turns. Alias: rt!
(turns as :number)
```

### round
```
Rounds a number to the nearest integer.
(n as :number)
```

### rt!
```
Rotates the turtle right, measured in turns. Alias: rt!
(turns as :number)
```

### second
```
Returns the second element of a list or tuple.
(xs)
```

### set
```
Takes an ordered collection--list or tuple--and turns it into a set.
(xs as :list) (xs as :tuple)
```

### show
```
Returns a text representation of a Ludus value as a string.
(x)
```

### sin
```
Returns the sine of an angle. Default angle measure is turns. An optional keyword argument specifies the units of the angle passed in.
(a as :number) (a as :number, :turns) (a as :number, :degrees) (a as :number, :radians)
```

### slice
```
Returns a slice of a list, representing a sub-list.
(xs as :list, end as :number) (xs as :list, start as :number, end as :number)
```

### some
```
Takes a possibly nil value and a default value. Returns the value if it's not nil, returns the default if it's nil.
(nil, default) (value, _)
```

### some?
```
Returns true if a value is not nil.
(nil) (_)
```

### square
```
Squares a number.
(x as :number)
```

### string
```
Converts a value to a string by using `show`. If it is a string, returns it unharmed. Use this to build up strings of differen kinds of values.
(x as :string) (x) (x, ...xs)
```

### string?
```
Returns true if a value is a string.
(x as :string) (_)
```

### sub
```
Subtracts numbers or vectors.
() (x as :number) (x as :number, y as :number) (x, y, ...zs) ((x1, y1), (x2, y2))
```

### sum_of_squares
```
Returns the sum of squares of numbers.
() (x as :number) (x as :number, y as :number) (x, y, ...zs)
```

### tan
```
Returns the sine of an angle. Default angle measure is turns. An optional keyword argument specifies the units of the angle passed in.
(a as :number) (a as :number, :turns) (a as :number, :degrees) (a as :number, :radians)
```

### tau
No documentation available.


### turn/deg
```
Converts an angle in turns to an angle in degrees.
(a as :number)
```

### turn/rad
```
Converts an angle in turns to an angle in radians.
(a as :number)
```

### turtle_commands
No documentation available.


### turtle_state
```
Returns the turtle's current state.
()
```

### turtle_states
No documentation available.


### type
```
Returns a keyword representing the type of the value passed in.
(x)
```

### unwrap!
```
Takes a result tuple. If it's :ok, then returns the value. If it's not :ok, then it panics. If it's not a result tuple, it also panics.
((:ok, value)) ((:err, msg)) (_)
```

### unwrap_or
```
Takes a value that is a result tuple and a default value. If it's :ok, then it returns the value. If it's :err, returns the default value.
((:ok, value), _) ((:err, _), default)
```

### update
```
Takes a dict, key, and function, and returns a new dict with the key set to the result of applying the function to original value held at the key.
(dict as :dict) (dict as :dict, key as :keyword, updater as :fn)
```

### update!
```
Updates a ref by applying a function to its value. Returns the new value.
(r as :ref, f as :fn)
```

### values
```
Takes an associative collection and returns a list of values in that collection. Returns an empty list on anything other than a collection.
(coll)
```

### zero?
```
Returns true if a number is 0.
(0) (_)
```