Here's a solution:
init :: [a] -> [a] init xs = tail (foldr keep [] xs) where keep :: a -> [a] -> [a] keep x [] = [x] keep x [y] = [x,x] keep x (y:z:zs) = x:x:y:zs
--Ham At 3:44 PM +0900 2005/4/10, Kaoru Hosokawa wrote:
I've been working through Thompson's exercises and got to one I could not solve. It's Exercise 9.13. This is where I need to define init using foldr.
init :: [a] -> [a] init "Greggery Peccary" ~> "Greggary Peccar"
This is as far as I got:
init xs = foldr left [] xs
left :: a -> [a] -> [a] left x [] = [] left x1 : (x2 : xs) = x1 : (left x2 xs)
But this returns [] and doesn't work. I can't get "left" to know that it is working with the rightmost element. It just throws away every element when its right hand side is empty.
I found a solution that works for Strings, but I am looking for a more general solution. This exercise may yet again be one of those that is difficult to solve with my current knowledge of Haskell, but I'm asking anyway.
-- Kaoru Hosokawa khosokawa@gmail.com
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-- ------------------------------------------------------------------ Hamilton Richards, PhD Department of Computer Sciences Senior Lecturer The University of Texas at Austin 512-471-9525 1 University Station C0500 Taylor Hall 5.138 Austin, Texas 78712-0233 ham@cs.utexas.edu hrichrds@swbell.net http://www.cs.utexas.edu/users/ham/richards ------------------------------------------------------------------