[Haskell-beginners] Non-exhaustive patterns

[Jeffrey Brown]

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 <borgauf at gmail.com> wrote:

> 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 at gmail.com
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