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{-# LANGUAGE NoImplicitPrelude #-}
module Test where
import GHC.Base (build, foldr, id, Maybe(..))
catMaybes :: [Maybe a] -> [a]
catMaybes = mapMaybe id
mapMaybe :: (a -> Maybe b) -> [a] -> [b]
mapMaybe _ [] = []
mapMaybe f (x:xs) =
let rs = mapMaybe f xs in
case f x of
Nothing -> rs
Just r -> r:rs
{-# NOINLINE [1] mapMaybe #-}
{-# RULES
"mapMaybe" [~1] forall f xs. mapMaybe f xs
= build (\c n -> foldr (mapMaybeFB c f) n xs)
"mapMaybeList" [1] forall f. foldr (mapMaybeFB (:) f) [] = mapMaybe f
#-}
{-# INLINE [0] mapMaybeFB #-} -- See Note [Inline FB functions] in GHC.List
mapMaybeFB :: (b -> r -> r) -> (a -> Maybe b) -> a -> r -> r
mapMaybeFB cons f x next = case f x of
Nothing -> next
Just r -> cons r next
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