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|
(.module:
[library
[lux (#- list)
[abstract
[monoid (#+ Monoid)]
[equivalence (#+ Equivalence)]
[hash (#+ Hash)]
[apply (#+ Apply)]
["." functor (#+ Functor)]
["." monad (#+ Monad do)]]
[meta
["." location]]]])
... (type: (Maybe a)
... #.None
... (#.Some a))
(implementation: .public monoid
(All [a] (Monoid (Maybe a)))
(def: identity #.None)
(def: (compose mx my)
(case mx
#.None
my
(#.Some x)
(#.Some x))))
(implementation: .public functor
(Functor Maybe)
(def: (map f ma)
(case ma
#.None #.None
(#.Some a) (#.Some (f a)))))
(implementation: .public apply
(Apply Maybe)
(def: &functor ..functor)
(def: (apply ff fa)
(case [ff fa]
[(#.Some f) (#.Some a)]
(#.Some (f a))
_
#.None)))
(implementation: .public monad
(Monad Maybe)
(def: &functor ..functor)
(def: (in x)
(#.Some x))
(def: (join mma)
(case mma
#.None
#.None
(#.Some mx)
mx)))
(implementation: .public (equivalence super)
(All [a] (-> (Equivalence a) (Equivalence (Maybe a))))
(def: (= mx my)
(case [mx my]
[#.None #.None]
#1
[(#.Some x) (#.Some y)]
(\ super = x y)
_
#0)))
(implementation: .public (hash super)
(All [a] (-> (Hash a) (Hash (Maybe a))))
(def: &equivalence
(..equivalence (\ super &equivalence)))
(def: (hash value)
(case value
#.None
0
(#.Some value)
(\ super hash value))))
(implementation: .public (with monad)
(All [M] (-> (Monad M) (Monad (All [a] (M (Maybe a))))))
(def: &functor (functor.compose (get@ #monad.&functor monad) ..functor))
(def: in (|>> (\ ..monad in) (\ monad in)))
(def: (join MmMma)
(do monad
[mMma MmMma]
(case mMma
#.None
(in #.None)
(#.Some Mma)
Mma))))
(def: .public (lift monad)
{#.doc (doc "Wraps a monadic value with Maybe machinery.")}
(All [M a] (-> (Monad M) (-> (M a) (M (Maybe a)))))
(\ monad map (\ ..monad in)))
(macro: .public (else tokens state)
{#.doc (doc "Allows you to provide a default value that will be used"
"if a (Maybe x) value turns out to be #.None."
"Note: the expression for the default value will not be computed if the base computation succeeds."
(else +20 (#.Some +10))
"=>"
+10
--------------------------
(else +20 #.None)
"=>"
+20)}
(case tokens
(^ (.list else maybe))
(let [g!temp (: Code [location.dummy (#.Identifier ["" ""])])]
(#.Right [state (.list (` (case (~ maybe)
(#.Some (~ g!temp))
(~ g!temp)
#.None
(~ else))))]))
_
(#.Left "Wrong syntax for else")))
(def: .public assume
{#.doc (doc "Assumes that a Maybe value is a #.Some and yields its value."
"Raises/throws a runtime error otherwise."
"WARNING: Use with caution.")}
(All [a] (-> (Maybe a) a))
(|>> (..else (undefined))))
(def: .public (list value)
(All [a] (-> (Maybe a) (List a)))
(case value
#.None
#.End
(#.Some value)
(#.Item value #.End)))
(macro: .public (when tokens state)
{#.doc (doc "Can be used as a guard in (co)monadic be/do expressions."
(do monad
[value (do_something 1 2 3)
..when (passes_test? value)]
(do_something_else 4 5 6)))}
(case tokens
(^ (.list test then))
(#.Right [state (.list (` (.if (~ test)
(~ then)
#.None)))])
_
(#.Left "Wrong syntax for when")))
|
