& this file, uniquely, gets `base` loaded as context. See src/base.janet for exports

& some forward declarations
& TODO: fix this so that we don't need (as many of) them
fn and
fn append
fn apply_command
fn assoc
fn atan/2
fn deg/rad
fn dict
fn first
fn floor
fn get
fn join
fn mod
fn neg?
fn print!
fn some?
fn store!
fn string
fn turn/rad
fn unbox
fn update!

& the very base: know something's type
fn type {
	"Returns a keyword representing the type of the value passed in."
	(x) -> base :type (x)
}

& some helper type functions
fn coll? {
	"Returns true if a value is a collection: dict, list, tuple, or set."
	(coll as :dict) -> true
	(coll as :list) -> true
	(coll as :tuple) -> true
	(coll as :set) -> true
	(_) -> false
}

fn ordered? {
	"Returns true if a value is an indexed collection: list or tuple."
	(coll as :list) -> true
	(coll as :tuple) -> true
	(coll as :string) -> true
	(_) -> false
}

fn assoc? {
	"Returns true if a value is an associative collection: a dict or a pkg."
	(d as :dict) -> true
	(p as :pkg) -> true
	(_) -> false
}

&&& nil: working with nothing

fn nil? {
	"Returns true if a value is nil."
	(nil) -> true
	(_) -> false
}

fn some? {
	"Returns true if a value is not nil."
	(nil) -> false
	(_) -> true
}

fn 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) -> default
	(value, _) -> value 
}

& ...and if two things are the same
fn eq? {
	"Returns true if all arguments have the same value."
	(x) -> true
	(x, y) -> base :eq? (x, y)
	(x, y, ...zs) -> if eq? (x, y)
		then loop (y, zs) with {
			(a, []) -> eq? (a, x)
			(a, [b, ...cs]) -> if eq? (a, x)
				then recur (b, cs)
				else false
			}
		else false
}

&&& true & false: boolean logic (part the first)
fn bool? {
	"Returns true if a value is of type :boolean."
	(false) -> true
	(true) -> true
	(_) -> false
}

fn true? {
	"Returns true if a value is boolean `true`. Useful to distinguish between `true` and anything else."
	(true) -> true
	(_) -> false
}

fn false? {
	"Returns `true` if a value is `false`, otherwise returns `false`. Useful to distinguish between `false` and `nil`."
	(false) -> true
	(_) -> false
}

fn bool {
	"Returns false if a value is nil or false, otherwise returns true."
	(nil) -> false
	(false) -> false
	(_) -> true
}

fn not {
	"Returns false if a value is truthy, true if a value is falsy."
	(nil) -> true
	(false) -> true
	(_) -> false
}

fn neq? {
	"Returns true if none of the arguments have the same value."
	(x) -> false
	(x, y) -> not (eq? (x, y))
	(x, y, ...zs) -> if eq? (x, y)
		then false
		else loop (y, zs) with {
			(a, []) -> neq? (a, x)
			(a, [b, ...cs]) -> if neq? (a, x)
				then recur (b, cs)
				else false
		}
}

& tuples: not a lot you can do with them functionally
fn tuple? {
	"Returns true if a value is a tuple."
	(tuple as :tuple) -> true
	(_) -> false
}

&&& functions: getting things done
fn fn? {
	"Returns true if an argument is a function."
	(f as :fn) -> true
	(_) -> false
}

& what we need for some very basic list manipulation 
fn rest {
	"Returns all but the first element of a list or tuple, as a list."
	([]) -> []
	(()) -> ()
	(xs as :list) -> base :rest (xs)
	(xs as :tuple) -> base :rest (xs)
	(xs as :string) -> base :str_slice (xs, 1)
}

fn inc {
	"Increments a number."
	(x as :number) -> base :inc (x)
}

fn dec {
	"Decrements a number."
	(x as :number) -> base :dec (x)
}

fn count {
	"Returns the number of elements in a collection (including string)."
	(xs as :list) -> base :count (xs)
	(xs as :tuple) -> base :count (xs)
	(xs as :dict) -> base :count (xs)
	(xs as :string) -> base :count (xs)
	(xs as :set) -> base :count (xs)
}

fn empty? {
	"Returns true if something is empty. Otherwise returns false (including for things that can't logically be empty, like numbers)."
	([]) -> true
	(#{}) -> true
	(s as :set) -> eq? (s, ${})
	(()) -> true
	("") -> true
	(_) -> false
}

fn any? {
	"Returns true if something is not empty, otherwise returns false (including for things that can't be logically full, like numbers)."
	([...]) -> true
	(#{...}) -> true
	(s as :set) -> not (empty? (s))
	((...)) -> true
	(s as :string) -> not (empty? (s))
	(_) -> false
}

fn list? {
	"Returns true if the value is a list."
	(l as :list) -> true
	(_) -> false
}

fn 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: sets and dicts don't have predictable or stable ordering in output. Dicts return lists of (key, value) tuples."
	(x) -> base :to_list (x)
}

fn fold {
	"Folds a list."
	(f as :fn, []) -> []
	(f as :fn, xs as :list) -> fold (f, xs, f ())
	(f as :fn, [], root) -> []
	(f as :fn, xs as :list, root) -> loop (root, first (xs), rest (xs)) with {
		(prev, curr, []) -> f (prev, curr)
		(prev, curr, remaining) -> recur (
			f (prev, curr)
			first (remaining)
			rest (remaining)
		)
	}
}

& TODO: optimize these with base :conj!
fn map {
	"Maps a function over a list: returns a new list with elements that are the result of applying the function to each element in the original list. E.g., `map ([1, 2, 3], inc) &=> [2, 3, 4]`."
	(f as :fn, xs) -> {
		fn mapper (prev, curr) -> append (prev, f (curr))
		fold (mapper, xs, [])
	}
	(kw as :keyword, xs) -> {
		fn mapper (prev, curr) -> append (prev, kw (curr))
		fold (mapper, xs, [])
	}
}

fn filter {
	"Takes a list and a predicate function, and returns a new list with only the items that produce truthy values when the function is called on them. E.g., `filter ([1, 2, 3, 4], odd?) &=> [1, 3]`."
	(p? as :fn, xs) -> {
		fn filterer (filtered, x) -> if p? (x)
			then append (filtered, x)
			else filtered
		fold (filterer, xs, [])
	}
}

fn keep {
	"Takes a list and returns a new list with any `nil` values omitted."
	(xs) -> filter (some?, xs)
}

fn append {
	"Adds an element to a list or set."
	() -> []
	(xs as :list) -> xs
	(xs as :list, x) -> base :conj (xs, x)
	(xs as :set) -> xs
	(xs as :set, x) -> base :conj (xs, x)
}

fn append! {
	"Adds an element to a list, modifying it."
	() -> []
	(xs as :list) -> xs
	(xs as :list, x) -> base :conj! (xs, x)
}

fn concat {
	"Combines two lists, strings, or sets."
	(x as :string, y as :string) -> base :concat (x, y)
	(xs as :list, ys as :list) -> base :concat (xs, ys)
	(xs as :set, ys as :set) -> base :concat (xs, ys)
	(xs, ys, ...zs) -> fold (concat, zs, concat (xs, ys))
}

fn set {
	"Takes an ordered collection--list or tuple--and turns it into a set."
	(xs as :list) -> fold (append, xs, ${})
	(xs as :tuple) -> do xs > list > set
}

fn set? {
	"Returns true if a value is a set."
	(xs as :set) -> true
	(_) -> false
}

fn contains? {
	"Returns true if a set or list contains a value."
	(value, s as :set) -> bool (base :get	(s, value))
	(value, l as :list) -> contains? (value, set (list))
}

fn omit {
	"Returns a new set with the value omitted."
	(value, s as :set) -> base :disj (s, value)
}

fn print! {
	"Sends a text representation of Ludus values to the console."
	(...args) -> {
		base :print! (args)
		:ok
	}
}

fn show {
	"Returns a text representation of a Ludus value as a string."
	(x) -> base :show (x)
}

fn report! {
	"Prints a value, then returns it."
	(x) -> {
		print! (x)
		x
	}
	(msg as :string, x) -> {
		print! (concat ("{msg} ", show (x)))
		x
	}
}

fn doc! {
	"Prints the documentation of a function to the console."
	(f as :fn) -> do f > base :doc > print!
	(_) -> :none
}

&&& strings: harder than they look!
fn string? {
	"Returns true if a value is a string."
	(x as :string) -> true
	(_) -> false
}

fn 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 different kinds of values."
	(x as :string) -> x
	(x) -> show (x)
	(x, ...xs) -> loop (x, xs) with {
		(out, [y]) -> concat (out, show (y))
		(out, [y, ...ys]) -> recur (concat (out, show (y)), ys)
	}
}

fn join {
	"Takes a list of strings, and joins them into a single string, interposing an optional separator."
	([]) -> ""
	([str as :string]) -> str
	(strs as :list) -> join (strs, "")
	([], separator as :string) -> ""
	([str as :string], separator as :string) -> str
	([str, ...strs], separator as :string) -> fold (
		fn (joined, to_join) -> concat (joined, separator, to_join)
		strs
		str
	)
}

fn split {
	"Takes a string, and turns it into a list of strings, breaking on the separator."
	(str as :string, break as :string) -> base :split (break, str)
}

fn trim {
	"Trims whitespace from a string. Takes an optional argument, `:left` or `:right`, to trim only on the left or right."
	(str as :string) -> base :trim (str)
	(str as :string, :left) -> base :triml (str)
	(str as :string, :right) -> base :trimr (str)
}

fn upcase {
	"Takes a string and returns it in all uppercase. Works only for ascii characters."
	(str as :string) -> base :upcase (str)
}

fn downcase {
	"Takes a string and returns it in all lowercase. Works only for ascii characters."
	(str as :string) -> base :downcase (str)
}

fn chars {
	"Takes a string and returns its characters as a list. Works only for strings with only ascii characters. Panics on any non-ascii characters."
	(str as :string) -> match base :chars (str) with {
		(:ok, chrs) -> chrs
		(:err, msg) -> panic! msg
	}
}

fn chars/safe {
	"Takes a string and returns its characters as a list, wrapped in a result tuple. Works only for strings with only ascii characters. Returns an error tuple on any non-ascii characters."
	(str as :string) -> base :chars (str)
}

fn ws? {
	"Tells if a string is a whitespace character."
	(" ") -> true
	("\n") -> true
	("\t") -> true
	(_) -> false
}

fn strip {
	"Removes punctuation from a string, removing all instances of ,.;:?!"
	("{x},{y}") -> strip ("{x}{y}")
	("{x}.{y}") -> strip ("{x}{y}")
	("{x};{y}") -> strip ("{x}{y}")
	("{x}:{y}") -> strip ("{x}{y}")
	("{x}?{y}") -> strip ("{x}{y}")
	("{x}!{y}") -> strip ("{x}{y}")
	(x) -> x
}

fn words {
	"Takes a string and returns a list of the words in the string. Strips all whitespace."
	(str as :string) -> {
		let no_punct = strip (str)
		let strs = split (no_punct, " ")
		fn worder (l, s) -> if empty? (s) 
			then l
			else append (l, s)
		fold (worder, strs, [])
	}
}

fn sentence {
	"Takes a list of words and turns it into a sentence."
	(strs as :list) -> join (strs, " ")
}

&&& boxes: mutable state and state changes

fn box? {
	"Returns true if a value is a box."
	(b as :box) -> true
	(_) -> false
}

fn unbox {
	"Returns the value that is stored in a box."
	(b as :box) -> base :unbox (b)
}

fn store! {
	"Stores a value in a box, replacing the value that was previously there. Returns the value."
	(b as :box, value) -> {
		base :store! (b, value)
		value
	}
}

fn update! {
	"Updates a box by applying a function to its value. Returns the new value."
	(b as :box, f as :fn) -> {
		let current = unbox (b)
		let new = f (current)
		store! (b, new)
	}
}

&&& numbers, basically: arithmetic and not much else, yet
& TODO: add nan?, 
fn number? {
	"Returns true if a value is a number."
	(x as :number) -> true
	(_) -> false
}

fn add {
	"Adds numbers or vectors."
	() -> 0
	(x as :number) -> x
	(x as :number, y as :number) -> base :add (x, y)
	(x, y, ...zs) -> fold (add, zs, base :add (x, y))
	& add vectors
	((x1, y1), (x2, y2)) -> (add (x1, x2), add (y1, y2))
}

fn sub {
	"Subtracts numbers or vectors."
	() -> 0
	(x as :number) -> x
	(x as :number, y as :number) -> base :sub (x, y)
	(x, y, ...zs) -> fold (sub, zs, base :sub (x, y))
	((x1, y1), (x2, y2)) -> (base :sub (x1, x2), base :sub (y1, y2))
}

fn mult {
	"Multiplies numbers or vectors."
	() -> 1
	(x as :number) -> x
	(x as :number, y as :number) -> base :mult (x, y)
	(x, y, ...zs) -> fold (mult, zs, mult (x, y))
	(scalar as :number, (x, y)) -> (mult (x, scalar), mult (y, scalar))
	((x, y), scalar as :number) -> mult (scalar, (x, y))
}

fn div {
	"Divides numbers. Panics on division by zero."
	(x as :number) -> x
	(_, 0) -> panic! "Division by zero."
	(x as :number, y as :number) -> base :div (x, y)
	(x, y, ...zs) -> {
		let divisor = fold (mult, zs, y)
		div (x, divisor)
	}
}

fn div/0 {
	"Divides numbers. Returns 0 on division by zero."
	(x as :number) -> x
	(_, 0) -> 0
	(x as :number, y as :number) -> base :div (x, y)
	(x, y, ...zs) -> {
		let divisor = fold (mult, zs, y)
		div/0 (x, divisor)
	}
}

fn div/safe {
	"Divides a number. Returns a result tuple."
	(x as :number) -> (:ok, x)
	(_, 0) -> (:err, "Division by zero")
	(x, y) -> (:ok, div (x, y))
	(x, y, ...zs) -> {
		let divisor = fold (mult, zs, y)
		div/safe (x, divisor)
	}
}

fn inv {
	"Returns the inverse of a number: 1/n or `div (1, n)`. Panics on division by zero."
	(x as :number) -> div (1, x)
}

fn inv/0 {
	"Returns the inverse of a number: 1/n or `div/0 (1, n)`. Returns 0 on division by zero."
	(x as :number) -> div/0 (1, x)
}

fn inv/safe {
	"Returns the inverse of a number: 1/n or `div/safe (1, n)`. Returns a result tuple."
	(x as :number) -> div/safe (1, x)
}

fn abs {
	"Returns the absolute value of a number."
	(0) -> 0
	(n as :number) -> if neg? (n) then mult (-1, n) else n
}

fn neg {
	"Multiplies a number by -1, negating it."
	(n as :number) -> mult (n, -1)
}

fn angle {
	"Calculates the angle between two vectors."
	(v1, v2) -> sub (atan/2 (v2), atan/2 (v1))
}

fn zero? {
	"Returns true if a number is 0."
	(0) -> true
	(_) -> false
}

fn gt? {
	"Returns true if numbers are in decreasing order."
	(x as :number) -> true
	(x as :number, y as :number) -> base :gt (x, y)
	(x, y, ...zs) -> loop (y, zs) with {
		(a, [b]) -> base :gt (a, b)
		(a, [b, ...cs]) -> if base :gt (a, b)
			then recur (b, cs)
			else false 
	}
}

fn gte? {
	"Returns true if numbers are in decreasing or flat order."
	(x as :number) -> true
	(x as :number, y as :number) -> base :gte (x, y)
	(x, y, ...zs) -> loop (y, zs) with {
		(a, [b]) -> base :gte (a, b)
		(a, [b, ...cs]) -> if base :gte (a, b)
			then recur (b, cs)
			else false
	}
}

fn lt? {
	"Returns true if numbers are in increasing order."
	(x as :number) -> true
	(x as :number, y as :number) -> base :lt (x, y)
	(x, y, ...zs) -> loop (y, zs) with {
		(a, [b]) -> base :lt (a, b)
		(a, [b, ...cs]) -> if base :lt (a, b)
			then recur (b, cs)
			else false
	}
}

fn lte? {
	"Returns true if numbers are in increasing or flat order."
	(x as :number) -> true
	(x as :number, y as :number) -> base :lte (x, y)
	(x, y, ...zs) -> loop (y, zs) with {
		(a, [b]) -> base :lte (a, b)
		(a, [b, ...cs]) -> if base :lte (a, b)
			then recur (b, cs)
			else false
	}
}

fn between? {
	"Returns true if a number is in the range [lower, higher): greater than or equal to the lower number, less than the higher."
	(lower as :number, higher as :number, x as :number) -> and (
		gte? (x, lower)
		lt? (x, higher)
	)
}

fn neg? {
	"Returns true if a value is a negative number, otherwise returns false."
	(x as :number) if lt? (x, 0) -> true
	(_) -> false
}

fn pos? {
	"Returns true if a value is a positive number, otherwise returns false."
	(x as :number) if gt? (x, 0) -> true
	(_) -> false
}

fn even? {
	"Returns true if a value is an even number, otherwise returns false."
	(x as :number) if eq? (0, mod (x, 2)) -> true
	(_) -> false
}

fn odd? {
	"Returns true if a value is an odd number, otherwise returns false."
	(x as :number) if eq? (1, mod (x, 2)) -> true
	(_) -> false
}

fn min {
	"Returns the number in its arguments that is closest to negative infinity."
	(x as :number) -> x
	(x as :number, y as :number) -> if lt? (x, y) then x else y
	(x, y, ...zs) -> fold (min, zs, min (x, y)) 
}

fn max {
	"Returns the number in its arguments that is closest to positive infinity."
	(x as :number) -> x
	(x as :number, y as :number) -> if gt? (x, y) then x else y
	(x, y, ...zs) -> fold (max, zs, max (x, y))
}

& additional list operations now that we have comparitors
fn at {
	"Returns the element at index n of a list or tuple, or the byte at index n of a string. Zero-indexed: the first element is at index 0. Returns nil if nothing is found in a list or tuple; returns an empty string if nothing is found in a string."
	(xs as :list, n as :number) -> base :nth (n, xs)
	(xs as :tuple, n as :number) -> base :nth (n, xs)
	(str as :string, n as :number) -> {
		let raw = base :nth (n, str)
		when {
			nil? (raw) -> nil
			gte? (raw, 128) -> panic! "not an ASCII char"
			true -> base :str_slice (str, n, inc (n))
		}
	}
	(_) -> nil
}

fn first {
	"Returns the first element of a list or tuple."
	(xs) if ordered? (xs) -> at (xs, 0)
}

fn second {
	"Returns the second element of a list or tuple."
	(xs) if ordered? (xs) -> at (xs, 1)
}

fn last {
	"Returns the last element of a list or tuple."
	(xs) if ordered? (xs) -> at (xs, dec (count (xs)))
}

fn butlast {
	"Returns a list, omitting the last element."
	(xs as :list) -> base :slice (xs, 0, dec (count (xs)))
}

fn slice {
	"Returns a slice of a list or a string, representing a sub-list or sub-string."
	(xs as :list, end as :number) -> slice (xs, 0, end) 
	(xs as :list, start as :number, end as :number) -> when {
		gte? (start, end) -> []
		gt? (end, count (xs)) -> slice (xs, start, count (xs))
		neg? (start) -> slice (xs, 0, end)
		true -> base :slice (xs, start, end)
	}
	(str as :string, end as :number) -> base :str_slice (str, 0, end)
	(str as :string, start as :number, end as :number) -> base :str_slice (str, start, end)
}

&&& keywords: funny names
fn keyword? {
	"Returns true if a value is a keyword, otherwise returns false."
	(kw as :keyword) -> true
	(_) -> false
}

& TODO: determine if Ludus should have a `keyword` function that takes a string and returns a keyword. Too many panics, it has weird memory consequences, etc.

& TODO: make `and` and `or` special forms which lazily evaluate arguments
fn and {
	"Returns true if all values passed in are truthy. Note that this does not short-circuit: all arguments are evaulated before they are passed in."
	() -> true
	(x) -> bool (x)
	(x, y) -> base :and (x, y)
	(x, y, ...zs) -> fold (and, zs, base :and (x, y))
}

fn or {
	"Returns true if any value passed in is truthy. Note that this does not short-circuit: all arguments are evaluated before they are passed in."
	() -> true
	(x) -> bool (x)
	(x, y) -> base :or (x, y)
	(x, y, ...zs) -> fold (or, zs, base :or (x, y))
}

fn assoc {
	"Takes a dict, key, and value, and returns a new dict with the key set to value."
	() -> #{}
	(d as :dict) -> d
	(d as :dict, k as :keyword, val) -> base :assoc (d, k, val)
	(d as :dict, (k as :keyword, val)) -> base :assoc (d, k, val)
}

fn dissoc {
	"Takes a dict and a key, and returns a new dict with the key and associated value omitted."
	(d as :dict) -> d
	(d as :dict, k as :keyword) -> base :dissoc (d, k)
}

fn 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."
	(d as :dict) -> d
	(d as :dict, k as :keyword, updater as :fn) -> base :assoc (d, k, updater (get (k, d)))
}

fn keys {
	"Takes a dict and returns a list of keys in that dict."
	(d as :dict) -> do d > list > map (first, _)
}

fn values {
	"Takes a dict and returns a list of values in that dict."
	(d as :dict) -> do d > list > map (second, _)
}

fn diff {
	"Takes two dicts 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) -> {
		let key1 = keys (d1)
		let key2 = keys (d2)
		let all = do concat (d1, d2) > set > list
		let diffs = loop (all, []) with {
			& TODO: reduce this redundancy?
			([k, ...ks], diffs) -> {
				let v1 = get (k, d1)
				let v2 = get (k, d2)
				if eq? (v1, v2)
					then recur (ks, diffs)
					else recur (ks, append (diffs, (k, (v1, v2))))
			}
			([k], diffs) -> {
				let v1 = get (k, d1)
				let v2 = get (k, d2)
				if eq? (v1, v2)
					then diffs
					else append (diffs, (k, (v1, v2)))
			}
		}
		dict (diffs)
	}
}

& TODO: consider merging `get` and `at`
fn get {
	"Takes a key, dict, and optional default value; returns the value at key. If the value is not found, returns nil or the default value."
	(k as :keyword) -> get (k, _)
	(k as :keyword, d as :dict) -> get (k, d, nil)
	(k as :keyword, d as :dict, default) -> base :get (k, d, default) 
}

& TODO: add sets to this?
fn has? {
	"Takes a key and a dict, and returns true if there is a non-`nil` value stored at the key."
	(k as :keyword) -> has? (k, _)
	(k as :keyword, d as :dict) -> do d> k > nil?
}

fn dict {
	"Takes a list or tuple of (key, value) tuples and returns it as a dict. Returns dicts unharmed."
	(d as :dict) -> d
	(l as :list) -> fold (assoc, l)
	(t as :tuple) -> do t > list > dict
}

fn dict? {
	"Returns true if a value is a dict."
	(d as :dict) -> true
	(_) -> false
}

fn each! {
	"Takes a list and applies a function, presumably with side effects, to each element in the list. Returns nil."
	(f! as :fn, []) -> nil
	(f! as :fn, [x]) -> { f! (x); nil }
	(f! as :fn, [x, ...xs]) -> { f! (x); each! (f!, xs) } 
}

&&& Trigonometry functions

& Ludus uses turns as its default unit to measure angles
& However, anything that takes an angle can also take a
& units argument, that's a keyword of :turns, :degrees, or :radians

let pi = base :pi

let tau = mult (2, pi)

fn 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) -> do a > turn/rad > base :sin
	(a as :number, :turns) -> do a > turn/rad > base :sin
	(a as :number, :degrees) -> do a > deg/rad > base :sin
	(a as :number, :radians) -> base :sin (a)
}

fn 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) -> do a > turn/rad > base :cos
	(a as :number, :turns) -> do a > turn/rad > base :cos
	(a as :number, :degrees) -> do a > deg/rad > base :cos
	(a as :number, :radians) -> base :cos (a)
}

fn 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) -> do a > turn/rad > base :tan
	(a as :number, :turns) -> do a > turn/rad > base :tan
	(a as :number, :degrees) -> do a > deg/rad > base :tan
	(a as :number, :radians) -> base :tan (a)
}

fn rotate {
	"Rotates a vector by an angle. Default angle measure is turns. An optional keyword argument specifies the units of the angle passed in."
	((x, y), a) -> rotate ((x, y), a, :turns)
	((x, y), a, units as :keyword) -> (
		sub (mult (x, cos (a, units)), mult (y, sin (a, units)))
		add (mult (x, sin (a, units)), mult (y, cos (a, units)))
	)
}

fn turn/deg {
	"Converts an angle in turns to an angle in degrees."
	(a as :number) -> mult (a, 360)
}

fn deg/turn {
	"Converts an angle in degrees to an angle in turns."
	(a as :number) -> div (a, 360)
}

fn turn/rad {
	"Converts an angle in turns to an angle in radians."
	(a as :number) -> mult (a, tau)
}

fn rad/turn {
	"Converts an angle in radians to an angle in turns."
	(a as :number) -> div (a, tau)
}

fn deg/rad {
	"Converts an angle in degrees to an angle in radians."
	(a as :number) -> mult (tau, div (a, 360))
}

fn rad/deg {
	"Converts an angle in radians to an angle in degrees."
	(a as :number) -> mult (360, div (a, tau))
}

fn 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) -> do base :atan_2 (x, y) > rad/turn
	(x, y, :turns) -> atan/2 (x, y)
	(x, y, :radians) -> base :atan_2 (x, y)
	(x, y, :degrees) -> do base :atan_2 (x, y) > rad/deg
	((x, y)) -> atan/2 (x, y)
	((x, y), units as :keyword) -> atan/2 (x, y, units)
}

fn mod {
	"Returns the modulus of x and y. Truncates towards negative infinity. Panics if y is 0."
	(x as :number, 0) -> panic! "Division by zero."
	(x as :number, y as :number) -> base :mod (x, y)
}

fn mod/0 {
	"Returns the modulus of x and y. Truncates towards negative infinity. Returns 0 if y is 0."
	(x as :number, 0) -> 0
	(x as :number, y as :number) -> base :mod (x, y)
}

fn mod/safe {
	"Returns the modulus of x and y in a result tuple, or an error if y is 0. Truncates towards negative infinity."
	(x as :number, 0) -> (:err, "Division by zero.")
	(x as :number, y as :number) -> (:ok, base :mod (x, y))
}

fn square {
	"Squares a number."
	(x as :number) -> mult (x, x)
}

fn sqrt {
	"Returns the square root of a number. Panics if the number is negative."
	(x as :number) if not (neg? (x)) -> base :sqrt (x) 
}

fn sqrt/safe {
	"Returns a result containing the square root of a number, or an error if the number is negative."
	(x as :number) -> if not (neg? (x))
		then (:ok, base :sqrt (x))
		else (:err, "sqrt of negative number")
}

fn sum_of_squares {
	"Returns the sum of squares of numbers."
	() -> 0
	(x as :number) -> square (x)
	(x as :number, y as :number) -> add (square (x), square (y))
	(x, y, ...zs) -> fold (
		fn (sum, z) -> add (sum, square (z))
		zs
		sum_of_squares (x, y))
}

fn dist {
	"Returns the distance from the origin to a point described by x and y, or by the vector (x, y)."
	(x as :number, y as :number) -> sqrt (sum_of_squares (x, y))
	((x, y)) -> dist (x, y)
}

fn heading/vector {
	"Takes a turtle heading, and returns a unit vector of that heading."
	(heading) -> {
		& 0 is 90º/0.25T, 0.25 is 180º/0.5T, 0.5 is 270º/0.75T, 0.75 is 0º/0T
		let a = add (neg (heading), 0.25)
		(cos (a), sin (a))
	}
}

&&& more number functions
fn random {
	"Returns a random something. 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. Alternately, given a collection (list, dict, set), it returns a random member of that collection."
	() -> base :random ()
	(n as :number) -> mult (n, random ())
	(m as :number, n as :number) -> add (m, random (sub (n, m)))
	(l as :list) -> {
		let i = do l > count > random > floor
		at (l, i)
	}
	(d as :dict) -> {
		let key = do d > keys > random 
		get (key, d)
	}
	(s as :set) -> do s > list > random
}

fn random_int {
	"Returns a random integer. With one argument, returns a random integer between 0 and that number. With two arguments, returns a random integer between them."
	(n as :number) -> do n > random > floor
	(m as :number, n as :number) -> floor (random (m, n))
}

fn 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) -> base :floor (n)
}

fn 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) -> base :ceil (n)
}

fn round {
	"Rounds a number to the nearest integer."
	(n as :number) -> base :round (n)
}

fn range {
	"Returns the set of integers between start (inclusive) and end (exclusive) as a list: [start, end). With one argument, starts at 0. If end is less than start, returns an empty list."
	(end as :number) -> base :range (0, end)
	(start as :number, end as :number) -> base :range (start, end)
}

&&& Results, errors and other unhappy values

fn ok {
	"Takes a value and wraps it in an :ok result tuple."
	(value) -> (:ok, value)	
}

fn ok? {
	"Takes a value and returns true if it is an :ok result tuple."
	((:ok, _)) -> true
	(_) -> false
}

fn err {
	"Takes a value and wraps it in an :err result tuple, presumably as an error message."
	(msg) -> (:err, msg)
}

fn err? {
	"Takes a value and returns true if it is an :err result tuple."
	((:err, _)) -> true
	(_) -> false
}

fn 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)) -> value
	((:err, msg)) -> panic! string ("Unwrapped :err! ", msg)
	(_) -> panic! "Cannot unwrap something that's not an error tuple."
}

fn 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), _) -> value
	((:err, _), default) -> default
}

fn assert! {
	"Asserts a condition: returns the value if the value is truthy, panics if the value is falsy. Takes an optional message."
	(value) -> if value 
		then value 
		else panic! "Assert failed: {value}"
	(msg, value) -> if value 
		then value 
		else panic! "Assert failed: {msg} with {value}"
}

&&& Turtle & other graphics

& some basic colors
& these are the "basic" css colors
& https://developer.mozilla.org/en-US/docs/Web/CSS/named-color
let colors = #{
	:black (0, 0, 0, 255)
	:silver (192, 192, 192, 255)
	:gray (128, 128, 128, 255)
	:white (255, 255, 255, 255)
	:maroon (128, 0, 0, 255)
	:red (255, 0, 0, 255)
	:purple (128, 0, 128, 255)
	:fuchsia (255, 0, 255, 255)
	:green (0, 128, 0, 255)
	:lime (0, 255, 0, 255)
	:olive (128, 128, 0, 255)
	:yellow (255, 255, 0, 255)
	:navy (0, 0, 128, 255)
	:blue (0, 0, 255, 255)
	:teal (0, 128, 128, 255)
	:aqua (0, 255, 25, 255)
}

& the initial turtle state
let turtle_init = #{
	:position (0, 0) & let's call this the origin for now
	:heading 0 & this is straight up
	:pendown? true
	:pencolor :white
	:penwidth 1
	:visible? true
}

& turtle states: refs that get modified by calls
& turtle_commands is a list of commands, expressed as tuples
box turtle_commands = []
box turtle_state = turtle_init

fn add_command! (command) -> {
	update! (turtle_commands, append! (_, command))
	let prev = unbox (turtle_state)
	let curr = apply_command (prev, command)
	store! (turtle_state, curr)
	:ok
}

fn forward! {
	"Moves the turtle forward by a number of steps. Alias: fd!"
	(steps as :number) -> add_command! ((:forward, steps))
}

let fd! = forward!

fn back! {
	"Moves the turtle backward by a number of steps. Alias: bk!"
	(steps as :number) -> add_command! ((:back, steps))
}

let bk! = back!

fn left! {
	"Rotates the turtle left, measured in turns. Alias: lt!"
	(turns as :number) -> add_command! ((:left, turns))
}

let lt! = left!

fn right! {
	"Rotates the turtle right, measured in turns. Alias: rt!"
	(turns as :number) -> add_command! ((:right, turns))
}

let rt! = right!

fn penup! {
	"Lifts the turtle's pen, stopping it from drawing. Alias: pu!"
	() -> add_command! ((:penup))
}

let pu! = penup!

fn pendown! {
	"Lowers the turtle's pen, causing it to draw. Alias: pd!"
	() -> add_command! ((:pendown))
}

let pd! = pendown!

fn pencolor! {
	"Changes the turtle's pen color. Takes a single grayscale value, an rgb tuple, or an rgba tuple. Alias: pc!"
	(color as :keyword) -> add_command! ((:pencolor, color))
	(gray as :number) -> add_command! ((:pencolor, gray, gray, gray, 255))
	((r as :number, g as :number, b as :number)) -> add_command! ((:pencolor, r, g, b, 255))
	((r as :number, g as :number, b as :number, a as :number)) -> add_command! ((:pencolor, r, g, b, a))
}

let pc! = pencolor!

fn penwidth! {
	"Sets the width of the turtle's pen, measured in pixels. Alias: pw!"
	(width as :number) -> add_command! ((:penwidth, width))
}

let pw! = penwidth!

fn background! {
	"Sets the background color behind the turtle and path. Alias: bg!"
	(color as :keyword) -> add_command! ((:background, color))
	(gray as :number) -> add_command! ((:background, gray, gray, gray, 255))
	((r as :number, g as :number, b as :number)) -> add_command! ((:background, r, g, b, 255))
	((r as :number, g as :number, b as :number, a as :number)) -> add_command! ((:background, r, g, b, a))
}

let bg! = background!

fn 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."
	() -> add_command! ((:home))
}

fn clear! {
	"Clears the canvas and sends the turtle home."
	() -> add_command! ((:clear))
}

fn 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) -> add_command! ((:goto, (x, y)))
	((x, y)) -> goto! (x, y)
}

fn setheading! {
	"Sets the turtle's heading. The angle is specified in turns, with 0 pointing up. Increasing values rotate the turtle counter-clockwise."
	(heading as :number) -> add_command! ((:setheading, heading))
}

fn showturtle! {
	"If the turtle is hidden, shows the turtle. If the turtle is already visible, does nothing."
	() -> add_command! ((:show))
}

fn hideturtle! {
	"If the turtle is visible, hides it. If the turtle is already hidden, does nothing."
	() -> add_command! ((:hide))
}

fn loadstate! {
	"Sets the turtle state to a previously saved state. Returns the state."
	(state) -> {
		let #{:position (x, y), heading, pendown?, pencolor, penwidth, visible?} = state
		let command = if tuple? (pencolor)
			then {
				let (r, g, b, a) = pencolor
				(:loadstate, x, y, heading, visible?, pendown?, penwidth, r, g, b, a)
			}
			else (:loadstate, x, y, heading, visible?, pendown?, penwidth, pencolor)
		add_command! (command)
		state
	}
}

fn apply_command {
	"Takes a turtle state and a command and calculates a new state."
	(state, command) -> match command with {
		(:goto, (x, y)) -> assoc (state, :position, (x, y))
		(:home) -> do state > 
			assoc (_, :position, (0, 0)) >
			assoc (_, :heading, 0)
		(:clear) -> do state >
			assoc (state, :position, (0, 0)) >
			assoc (_, :heading, 0)
		(:right, turns) -> update (state, :heading, add (_, turns))
		(:left, turns) -> update (state, :heading, sub (_, turns))
		(:forward, steps) -> {
			let #{heading, position, ...} = state
			let unit = heading/vector (heading)
			let vect = mult (steps, unit)
			update (state, :position, add (vect, _))
		}
		(:back, steps) -> {
			let #{heading, position, ...} = state
			let unit = heading/vector (heading)
			let vect = mult (steps, unit)
			update (state, :position, sub (_, vect))
		}
		(:penup) -> assoc (state, :pendown?, false)
		(:pendown) -> assoc (state, :pendown?, true)
		(:penwidth, pixels) -> assoc (state, :penwidth, pixels)
		(:pencolor, color) -> assoc (state, :pencolor, color)
		(:setheading, heading) -> assoc (state, :heading, heading)
		(:loadstate, x, y, heading, visible?, pendown?, penwidth, pencolor) -> #{:position (x, y), heading, visible?, pendown?, penwidth, pencolor}
		(:loadstate, x, y, heading, visible?, pendown?, penwidth, r, g, b, a) -> #{:position (x, y), heading, visible?, pendown?, penwidth, :pencolor (r, g, b, a)}
		(:show) -> assoc (state, :visible?, true)
		(:hide) -> assoc (state, :visible?, false)
		(:background, _) -> state
	}
}

& position () -> (x, y)
fn position {
	"Returns the turtle's current position."
	() -> do turtle_state > unbox > :position
}

fn heading {
	"Returns the turtle's current heading."
	() -> do turtle_state > unbox > :heading
}

fn pendown? {
	"Returns the turtle's pen state: true if the pen is down."
	() -> do turtle_state > unbox > :pendown?
}

fn pencolor {
	"Returns the turtle's pen color as an (r, g, b, a) tuple or keyword."
	() -> do turtle_state > unbox > :pencolor
}

fn penwidth {
	"Returns the turtle's pen width in pixels."
	() -> do turtle_state > unbox > :penwidth
}

box state = nil

pkg Prelude {
	abs & math
	add & math
	and & bool
	angle & math
	any? & dicts lists strings sets tuples
	append & lists sets
 	assert! & errors
	assoc & dicts
	assoc? & dicts 
	at & lists strings
	atan/2 & math
	back! & turtles
	background! & turtles
	between? & math
	bg! & turtles
	bk! & turtles
	bool & bool
	bool? & bool
	box? & boxes
	butlast & lists strings tuples
	ceil & math
	chars & strings
	clear! & turtles
	coll? & dicts lists sets tuples
	colors & turtles
	concat & string list set 
	contains? & list set
	cos & math
	count & string list set tuple dict
	dec & math
	deg/rad & math
	deg/turn & math
	dict & dict
	dict? & dict
	diff & dict
	dissoc & dict
	dist & math
	div & math
	div/0 & math
	div/safe & math
	doc! & env
	downcase & string
	each! & list 
	empty? & list dict set string tuple
	eq? & values
	err & result
	err? & result
	even? & math
	false? & bool
	fd! & turtles
	filter & list
	first & list tuple
	floor & math
	fn? & functions
	fold & lists
	forward! & turtles
	get & dicts
	goto! & turtles
	gt? & math
	gte? & math
	heading & turtles
	heading/vector & math
	hideturtle! & turtles
	home! & turtles
	inc & math
	inv & math
	inv/0 & math
	inv/safe & math
	join & lists strings
	keep & lists
	keys & dicts
	keyword? & keywords
	last & lists tuples
	left! & turtles
	list & lists
	list? & lists
	loadstate! & turtles
	lt! & turtles
	lt? & math
	lte? & math
	map & lists
	max & math
	min & math
	mod & math
	mod/0
	mod/safe
	mult & math
	neg & math
	neg? & math
	neq? & values
	nil? & nil
	not & bool
	odd? & math
	ok & results
	ok? & results
	omit & set
	or & bool
	ordered? & lists tuples strings
	pc! & turtles
	pd! & turtles
	pencolor & turtles
	pencolor! & turtles
	pendown! & turtles
	pendown? & turtles
	penup! & turtles
	penwidth & turtles
	penwidth! & turtles
	pi & math
	pos? & math
	position & turtles
	print! & environment
	pu! & turtles
	pw! & turtles
	rad/deg & math
	rad/turn & math
	random & math dicts lists tuples sets
	random_int & math
	range & math lists
	report! & environment
	rest & lists tuples
	right! & turtles
	round & math
	rt! & turtles
	second & lists tuples
	sentence & lists strings
	set & sets
	set? & sets
	setheading! & turtles
	show & strings
	showturtle! & turtles
	sin & math
	slice & lists tuples strings
	some & values
	some? & values
	split & strings
	sqrt & math
	sqrt/safe & math
	square & math
	state & environment
	store! & boxes
	string & strings
	string? & strings
	strip & strings
	sub & math
	sum_of_squares & math 
	tan & math
	tau & math
	trim & strings
	tuple? & tuples
	turn/deg & math
	turn/rad & math
	turtle_commands & turtles
	turtle_init & turtles
	turtle_state & turtles
	type & values
	unbox & boxes
	unwrap! & results
	unwrap_or & results
	upcase & strings
	update & dicts
	update! & boxes
	values & dicts
	words & strings lists
	ws? & strings
	zero? & math
}