[Haskell-cafe] Thompson's Exercise 9.13

[Kaoru Hosokawa]

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 at gmail.com