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
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. _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.
-- brandon s allbery kf8nh allbery.b@gmail.com