Re: [Haskell-cafe] Thompson's Exercise 9.13

On Mon, 11 Apr 2005, Christoph Bauer wrote:

Kaoru Hosokawa writes:

...

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))

left x = Just . maybe [] (x:)

init :: [a] -> [a] init xs = fromJust . foldr left Nothing xs

fromMaybe (error "init: empty list") instead of fromJust

Sure, there is a better solution...

Extended question: How to do it without foldr? :-) init xs = zipWith const xs (tail xs) But this does not check for empty lists. :-(