[Haskell-cafe] Thompson's Exercise 9.13

[Kaoru Hosokawa]

My solution based on Bernie's solution:

init :: [a] -> [a]
init xs = foldr (left (length xs)) xs xs

left :: Int -> a -> [a] -> [a]
left n x xs
      | n == length xs = []
      | otherwise      = x : xs

I use the foldr return value for the empty list as the input itself. 
Use this value as a marker to signify that I am at the  end of the list 
by checking its length.

On 2005/04/10, at 17:13, Bernard Pope wrote:

> On Sun, 2005-04-10 at 15:44 +0900, 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"
>
> Hi,
>
> Here's a tentative solution:
>
> myinit xs
>    = foldr (f (length xs)) [] xs
>    where
>    f len next []
>       | len == 1 = []
>       | otherwise = [next]
>    f len next list@(x:xs)
>       | length list + 1 == len = next : xs
>       | otherwise = x : next : xs
>

--
Kaoru Hosokawa
khosokawa at gmail.com