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{-# OPTIONS_GHC -O2 -fforce-recomp -dno-typeable-binds #-}
module T18401 where
-- | A safe version of `init`.
-- @safeInit [] = Nothing@
-- @safeInit xs = Just $ init xs@
safeInit :: [a] -> Maybe [a]
safeInit xs = case si xs of
(False, _) -> Nothing
(_, ys) -> Just ys
si :: [a] -> (Bool, [a])
si xs0 = foldr go stop xs0 Nothing
where
stop Nothing = (False, [])
stop _ = (True, [])
go x r Nothing = (True, snd (r (Just x)))
go x r (Just p) = (True, p : snd (r (Just x)))
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