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| -rw-r--r-- | README.md | 2 | ||||
| -rw-r--r-- | ch19/19_a_1.hs | 31 |
2 files changed, 32 insertions, 1 deletions
@@ -180,7 +180,7 @@ are prefixed with 'Module_'. | 18_a_2 | yes | | | | 18_a_3 | yes, in 18_a_2 | | | | **_18_b_1_** | yes | 441 | | -| **_19_a_1_** | | 462 | 19. Error handling | +| **_19_a_1_** | yes | 462 | 19. Error handling | | **_19_b_1_** | | 465 | | | 19_b_2 | | | | | 19_b_3 | | | | diff --git a/ch19/19_a_1.hs b/ch19/19_a_1.hs new file mode 100644 index 0000000..877ca57 --- /dev/null +++ b/ch19/19_a_1.hs @@ -0,0 +1,31 @@ +-- 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)] |