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module ErrorState where
infixl 1 >+, >+=
infixl 4 <$>, <*>
type ES e s a = s > Either e (a, s)
evalES :: ES e s a > s > Either e a
evalES m s = case m s of
Left e > Left e
Right (x, _) > Right x
returnES :: a > ES e s a
returnES x s = Right (x, s)
failES :: e > ES e s a
failES e _ = Left e
(>+=) :: ES e s a > (a > ES e s b) > ES e s b
m >+= f = \s > case m s of
Left e > Left e
Right (x, s') > f x s'
(>+) :: ES e s a > ES e s b > ES e s b
m >+ n = m >+= \_ > n
(<$>) :: (a > b) > ES e s a > ES e s b
f <$> act = act >+= \x > returnES (f x)
(<*>) :: ES e s (a > b) > ES e s a > ES e s b
sf <*> sx = sf >+= \f > sx >+= \x > returnES (f x)
gets :: ES e s s
gets s = Right (s, s)
puts :: s > ES e s ()
puts s _ = Right ((), s)
modify :: (s > s) > ES e s ()
modify f s = Right ((), f s)
mapES :: (a > ES e s b) > [a] > ES e s [b]
mapES _ [] = returnES []
mapES f (x : xs) = f x >+= \y >
mapES f xs >+= \ys >
returnES (y:ys)
concatMapES :: (a > ES e s [b]) > [a] > ES e s [b]
concatMapES f xs = concat <$> mapES f xs
mapAccumES :: (a > b > ES e s (a, c)) > a > [b] > ES e s (a, [c])
mapAccumES _ s [] = returnES (s, [])
mapAccumES f s (x : xs) = f s x >+= \(s', y) >
mapAccumES f s' xs >+= \(s'', ys) >
returnES (s'', y:ys)
