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.
Ok, my second haskell program ;-): module Init where import Maybe left :: a -> Maybe [a] -> Maybe [a] left x None = (Just []) left x (Just l) = (Just (x:l)) init :: [a] -> [a] init xs = fromJust . foldr left Nothing xs Sure, there is a better solution... Best Regards, Christoph Bauer -- let () = let rec f a w i j = Printf.printf "%.20f\r" a; let a1 = a *. i /. j in if w then f a1 false (i +. 2.0) j else f a1 true i (j +. 2.0) in f 2.0 false 2.0 1.0