[Haskell-cafe] Newbie question

[Brandon Allbery]

You've covered 0 elements, 1 element, 3 or more elements, but not 2 elements.

On Sat, Jan 15, 2022 at 9:33 AM Terry Phelps <tgphelps50 at gmail.com> wrote:
>
> I'm trying yet again to "get" Haskell. I'm reading Graham Hutton's book "Programming in Haskell", and am working on one of the exercises. Sounds easy enough:
>
> Make a function 'last' that returns the last element of a non-empty list.
>
> After struggling a while, I looked at the answer: last = head . reverse.
> I wish I had thought of that. But I kept trying to get my recursive version done, but I can't seem to make ghci happy. (Version 8.10.5, if you care). I wrote:
>
> last [] = []
> (Yes, I know that's not a non-empty list, but I don't want ghci whining about non-exhaustive patterns). Then I added:
>
> last [x] = x
> And I checked the type:
> :t last
> last:: [a] -> [a]
>
> Yes, that looks right. There seems to be only one other case: 2 or more elements. So I wrote:
>
> last (x:y:xs) = last (y:xs)
>
> I ran it:
>
> Prelude> last [1,2]
> *** Exception: <interactive>:2:1-27: Non-exhaustive patterns in function last
>
> What the heck is the problem? I've covered every possible list, haven't I?
>
> Stranger yet, I check the type again:
>
> Prelude> :t last
> last :: [a] -> t
>
> The type has changed. And I don't understand what it means. What's the 't'?
>
> I'm sure this is a simple beginner error, but I'm really confused. I don't see how it isn't exhaustive, and I don't see why the 3rd clause (or whatever it's called) caused the type to change.
>
> Someone help, please.
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-- 
brandon s allbery kf8nh
allbery.b at gmail.com