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-- Take the Either example and made it work with laziness in the style of the
-- Maybe example.
-- I chose the non-Monadic version of the Either example because the monadic one
-- required changes to make it work with recent GHC versions.
{-- From examples/examples/ch19/divby7.hs and modified --}
data DivByError a = DivBy0
| ForbiddenDenominator a
deriving (Eq, Read, Show)
divBy :: Integral a => a -> [a] -> [Either (DivByError a) a]
divBy numerator denominators =
map worker denominators
where worker 0 = Left DivBy0
worker 10 = Left (ForbiddenDenominator 10)
worker 20 = Left (ForbiddenDenominator 20)
worker x = Right (numerator `div` x)
{-- End of code from examples --}
-- ghci> :l 19_a_1.hs
-- [1 of 2] Compiling Main ( 19_a_1.hs, interpreted )
-- Ok, one module loaded.
-- ghci> take 8 (divBy 50 [1..])
-- [Right 50,Right 25,Right 16,Right 12,Right 10,Right 8,Right 7,Right 6]
-- ghci> divBy 50 [1,0,5,10,30,20]
-- [Right 50,Left DivBy0,Right 10,Left (ForbiddenDenominator 10),Right 1,Left (ForbiddenDenominator 20)]
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