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(.module:
[lux #*
[abstract
[functor (#+ Functor)]
[comonad (#+ CoMonad)]]
[control
["//" continuation (#+ Cont)]
["<>" parser
["<.>" code (#+ Parser)]]]
[meta (#+ with-gensyms)]
[macro
[syntax (#+ syntax:)]
["." code]]
[data
["." bit]
[number
["n" nat]]
[collection
["." list ("#@." monad)]]]])
(type: #export (Sequence a)
{#.doc "An infinite sequence of values."}
(Cont [a (Sequence a)]))
(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 [start next])
{#.doc (doc "Go over the elements of a list forever."
"The list should not be empty.")}
(All [a]
(-> [a (List a)] (Sequence a)))
(loop [head start
tail next]
(//.pending [head (case tail
#.Nil
(recur start next)
(#.Cons head' tail')
(recur head' tail'))])))
(template [<name> <return>]
[(def: #export (<name> sequence)
(All [a] (-> (Sequence a) <return>))
(let [[head tail] (//.run sequence)]
<name>))]
[head a]
[tail (Sequence a)]
)
(def: #export (nth idx sequence)
(All [a] (-> Nat (Sequence a) a))
(let [[head tail] (//.run sequence)]
(case idx
0 head
_ (nth (dec idx) tail))))
(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'] (//.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'] (//.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'] (//.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 predicate sequence)
(All [a] (-> (-> a Bit) (Sequence a) (Sequence a)))
(let [[head tail] (//.run sequence)]
(if (predicate head)
(//.pending [head (filter predicate tail)])
(filter predicate tail))))
(def: #export (partition left? xs)
{#.doc (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 left? xs) (filter (bit.complement left?) xs)])
(structure: #export functor
(Functor Sequence)
(def: (map f fa)
(let [[head tail] (//.run fa)]
(//.pending [(f head) (map f tail)]))))
(structure: #export comonad
(CoMonad Sequence)
(def: &functor ..functor)
(def: unwrap head)
(def: (split wa)
(let [[head tail] (//.run wa)]
(//.pending [wa (split tail)]))))
(syntax: #export (^sequence& {patterns (<code>.form (<>.many <code>.any))}
body
{branches (<>.some <code>.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)])
(` ((~! //.run) (~ g!sequence)))))
patterns)))]
(~ body)))]
(wrap (list& g!sequence body+ branches)))))
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