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(.module:
[lux #*
[control
functor
monad
comonad
[continuation (#+ pending Cont)]
["p" parser]]
[macro (#+ with-gensyms)
[code]
["s" syntax (#+ syntax: Syntax)]]
[data
bit
[collection [list ("list/" Monad<List>)]]]])
## [Types]
(type: #export (Sequence a)
{#.doc "An infinite sequence of values."}
(Cont [a (Sequence a)]))
## [Utils]
(def: (cycle' x xs init full)
(All [a]
(-> a (List a) a (List a) (Sequence a)))
(case xs
#.Nil (pending [x (cycle' init full init full)])
(#.Cons x' xs') (pending [x (cycle' x' xs' init full)])))
## [Functions]
(def: #export (iterate f x)
{#.doc "Create a sequence by applying a function to a value, and to its result, on and on..."}
(All [a]
(-> (-> a a) a (Sequence a)))
(pending [x (iterate f (f x))]))
(def: #export (repeat x)
{#.doc "Repeat a value forever."}
(All [a]
(-> a (Sequence a)))
(pending [x (repeat x)]))
(def: #export (cycle xs)
{#.doc "Go over the elements of a list forever.
The list should not be empty."}
(All [a]
(-> (List a) (Maybe (Sequence a))))
(case xs
#.Nil #.None
(#.Cons x xs') (#.Some (cycle' x xs' x xs'))))
(do-template [<name> <return> <part>]
[(def: #export (<name> s)
(All [a] (-> (Sequence a) <return>))
(let [[h t] (continuation.run s)]
<part>))]
[head a h]
[tail (Sequence a) t])
(def: #export (nth idx s)
(All [a] (-> Nat (Sequence a) a))
(let [[h t] (continuation.run s)]
(if (n/> +0 idx)
(nth (dec idx) t)
h)))
(do-template [<taker> <dropper> <splitter> <pred-type> <pred-test> <pred-step>]
[(def: #export (<taker> pred xs)
(All [a]
(-> <pred-type> (Sequence a) (List a)))
(let [[x xs'] (continuation.run xs)]
(if <pred-test>
(list& x (<taker> <pred-step> xs'))
(list))))
(def: #export (<dropper> pred xs)
(All [a]
(-> <pred-type> (Sequence a) (Sequence a)))
(let [[x xs'] (continuation.run xs)]
(if <pred-test>
(<dropper> <pred-step> xs')
xs)))
(def: #export (<splitter> pred xs)
(All [a]
(-> <pred-type> (Sequence a) [(List a) (Sequence a)]))
(let [[x xs'] (continuation.run xs)]
(if <pred-test>
(let [[tail next] (<splitter> <pred-step> xs')]
[(#.Cons [x tail]) next])
[(list) xs])))]
[take-while drop-while split-while (-> a Bit) (pred x) pred]
[take drop split Nat (n/> +0 pred) (dec pred)]
)
(def: #export (unfold step init)
{#.doc "A stateful way of infinitely calculating the values of a sequence."}
(All [a b]
(-> (-> a [a b]) a (Sequence b)))
(let [[next x] (step init)]
(pending [x (unfold step next)])))
(def: #export (filter p xs)
(All [a] (-> (-> a Bit) (Sequence a) (Sequence a)))
(let [[x xs'] (continuation.run xs)]
(if (p x)
(pending [x (filter p xs')])
(filter p xs'))))
(def: #export (partition p xs)
{#.doc "Split a sequence in two based on a predicate.
The left side contains all entries for which the predicate is #1.
The right side contains all entries for which the predicate is #0."}
(All [a] (-> (-> a Bit) (Sequence a) [(Sequence a) (Sequence a)]))
[(filter p xs) (filter (complement p) xs)])
## [Structures]
(structure: #export _ (Functor Sequence)
(def: (map f fa)
(let [[h t] (continuation.run fa)]
(pending [(f h) (map f t)]))))
(structure: #export _ (CoMonad Sequence)
(def: functor Functor<Sequence>)
(def: unwrap head)
(def: (split wa)
(let [[head tail] (continuation.run wa)]
(pending [wa (split tail)]))))
## [Pattern-matching]
(syntax: #export (^sequence& {patterns (s.form (p.many s.any))}
body
{branches (p.some s.any)})
{#.doc (doc "Allows destructuring of sequences in pattern-matching expressions."
"Caveat emptor: Only use it for destructuring, and not for testing values within the sequences."
(let [(^sequence& x y z _tail) (some-sequence-func 1 2 3)]
(func x y z)))}
(with-gensyms [g!sequence]
(let [body+ (` (let [(~+ (list/join (list/map (function (_ pattern)
(list (` [(~ pattern) (~ g!sequence)])
(` ((~! continuation.run) (~ g!sequence)))))
patterns)))]
(~ body)))]
(wrap (list& g!sequence body+ branches)))))
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