[Haskell-cafe] Thompson's Exercise 9.13

[Kaoru Hosokawa]

On 2005/04/16, at 12:57, Hamilton Richards wrote:

> At 10:50 AM +0900 2005/4/16, Kaoru Hosokawa wrote:
>> 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.
>
> Using length causes this definition of init to fail on infinite lists.
>

Oh. I haven't learned how to handle infinite data structures yet, but 
is init defined for infinite lists in the first place?

--
Kaoru Hosokawa
khosokawa at gmail.com