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(.module:
lux
(lux/control monoid
["F" functor]
["A" applicative]
[monad #+ do Monad]))
(type: #export (Writer l a)
{#.doc "Represents a value with an associated 'log' value to record arbitrary information."}
{#log l
#value a})
(struct: #export Functor<Writer> (All [l]
(F.Functor (Writer l)))
(def: (map f fa)
(let [[log datum] fa]
[log (f datum)])))
(struct: #export (Applicative<Writer> mon) (All [l]
(-> (Monoid l) (A.Applicative (Writer l))))
(def: functor Functor<Writer>)
(def: (wrap x)
[(:: mon identity) x])
(def: (apply ff fa)
(let [[log1 f] ff
[log2 a] fa]
[(:: mon compose log1 log2) (f a)])))
(struct: #export (Monad<Writer> mon) (All [l]
(-> (Monoid l) (Monad (Writer l))))
(def: applicative (Applicative<Writer> mon))
(def: (join mma)
(let [[log1 [log2 a]] mma]
[(:: mon compose log1 log2) a])))
(def: #export (log l)
{#.doc "Set the log to a particular value."}
(All [l] (-> l (Writer l Unit)))
[l []])
(struct: #export (WriterT Monoid<l> Monad<M>)
(All [l M] (-> (Monoid l) (Monad M) (Monad (All [a] (M (Writer l a))))))
(def: applicative (A.compose (get@ #monad.applicative Monad<M>) (Applicative<Writer> Monoid<l>)))
(def: (join MlMla)
(do Monad<M>
[## TODO: Remove once new-luxc is the standard compiler.
[l1 Mla] (: (($ +1) (Writer ($ +0) (($ +1) (Writer ($ +0) ($ +2)))))
MlMla)
## [l1 Mla] MlMla
[l2 a] Mla]
(wrap [(:: Monoid<l> compose l1 l2) a]))))
(def: #export (lift Monoid<l> Monad<M>)
(All [l M a] (-> (Monoid l) (Monad M) (-> (M a) (M (Writer l a)))))
(function (_ ma)
(do Monad<M>
[a ma]
(wrap [(:: Monoid<l> identity) a]))))
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