Am Sonntag, 10. April 2005 08:44 schrieb 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)
Try left :: (Bool, [a]) -> (Bool, [a]), res :: (Bool, [a]) -> [a], init' = res . foldr left (False, []) for example. It's not too difficult then.
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.
I cannot imagine how you could do it for Strings, but not for general lists. Mind sending me the code? Hope this helps, Daniel