This code myTakePM :: Int -> [a] -> [a] myTakePM 0 _ = [] myTakePM n (x:xs) = x : myTakePM (n-1) xs is bad because it allows myTakePM 4 [1,2,3] [1,2,3*** Exception: <interactive>:(395,1)-(396,41): Non-exhaustive patterns in function myTakePM I knew it would not work, but why is it calling this essentially a partial function? How does it know this? Again, I expected an error, but what is this Non-exhaustive patterns in function myTakePM saying? Or, said another way, what exactly is non-exhaustive about this? -- ⨽ Lawrence Bottorff Grand Marais, MN, USA borgauf@gmail.com
Hello - I think the pattern for the empty list (as second argument) is missing.
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From: Beginners
The function as written assumes that if the first argument is not zero,
then the second argument is nonempty. But if you try to take5 from a list
of length 2 that way, you'll eventually be asking for `myTake 3 []`, which
there's no way to evaluate.
On Sun, Dec 12, 2021 at 12:14 AM Galaxy Being
This code
myTakePM :: Int -> [a] -> [a] myTakePM 0 _ = [] myTakePM n (x:xs) = x : myTakePM (n-1) xs
is bad because it allows
myTakePM 4 [1,2,3] [1,2,3*** Exception: <interactive>:(395,1)-(396,41): Non-exhaustive patterns in function myTakePM
I knew it would not work, but why is it calling this essentially a partial function? How does it know this? Again, I expected an error, but what is this Non-exhaustive patterns in function myTakePM saying? Or, said another way, what exactly is non-exhaustive about this? -- ⨽ Lawrence Bottorff Grand Marais, MN, USA borgauf@gmail.com _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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participants (3)
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Galaxy Being -
Jeffrey Brown -
Pietro Grandinetti