19_a_1: Add solution

1 files changed, 31 insertions, 0 deletions

diff --git a/ch19/19_a_1.hs b/ch19/19_a_1.hs
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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)]