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(.module:
[library
[lux #*]]
[//
[monad (#+ Monad)]
["." functor (#+ Functor)]])
(interface: #export (Apply f)
{#.doc "Applicative functors."}
(: (Functor f)
&functor)
(: (All [a b]
(-> (f (-> a b)) (f a) (f b)))
apply))
(implementation: #export (compose f_monad f_apply g_apply)
{#.doc "Applicative functor composition."}
(All [F G]
(-> (Monad F) (Apply F) (Apply G)
## TODO: Replace (All [a] (F (G a))) with (functor.Then F G)
(Apply (All [a] (F (G a))))))
(def: &functor (functor.compose (get@ #&functor f_apply) (get@ #&functor g_apply)))
(def: (apply fgf fgx)
## TODO: Switch from this version to the one below (in comments) ASAP.
(let [fgf' (\ f_apply apply
(\ f_monad wrap (\ g_apply apply))
fgf)]
(\ f_apply apply fgf' fgx))
## (let [applyF (\ f_apply apply)
## applyG (\ g_apply apply)]
## ($_ applyF
## (\ f_monad wrap applyG)
## fgf
## fgx))
))
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