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furu/exercise-8-1-safe-pred.hs

Created June 12, 2017 18:20
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exercise-8-1-safe-pred.hs
import Control.Spoon (spoon)
import Control.DeepSeq (NFData)
predMay :: (Enum a, NFData a) => a -> Maybe a
predMay = spoon . pred
pred3 :: (Enum a, NFData a) => a -> Maybe a
pred3 x = return x >>= predMay >>= predMay >>= predMay
predN :: (Enum a, NFData a) => Int -> a -> Maybe a
predN 0 x = return x
predN n x = predN (n - 1) x >>= predMay
main :: IO ()
main = do
putStrLn "hello world"
print $ pred3 True
print $ predN 256 'a'
print $ predN 32 'z'
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