CODEWITHSUNDEEP
haskell
DiVeRsi0n
DiVeRsi0n

Reputation: 51

Code doesnt work with infinite lists

I'm currently working on function which takes 2 integers and a list as arguments and returns a list with the elements of the first list which are at the positions between these 2 integers (function :: Int->Int->[u]->[u]). For example:

>function 2 5 [1..10]

returns 

[2,3,4,5]

The above example is working with my current code...but if I insert an infinite list like [0..] it returns an "ERROR - Garbage collection fails to reclaim sufficient space". Any suggestions? Here is the code

function :: Int->Int->[u]->[u]                                                       
function i j s
    |i<0 = function 0 j s
    |j< length s = function i j (init s) 
    |j-i< (length s)-1 = function i j (tail s)
    |otherwise = s

Upvotes: 0

Views: 164

Answers (3)

elm
elm

Reputation: 20415

An approach based in zipping with indexes and filtering out those not within the given range,

f :: Int -> Int -> [a] -> [a]
f i j = map (snd) . filter (\v -> fst v `elem` [i..j]) . zip [1..]

Upvotes: 0

ThreeFx
ThreeFx

Reputation: 7360

The problem with your implementation is the use of length. Using length with an infinite list as parameter is not a good idea because, well, the list in infinitely long.

I would suggest altering the implementation to use other functions which can work on infinite lists, such as drop and take:

function :: Int -> Int -> [a] -> [a]
function i j list = take (i - j + 1) (drop (i - 1) list)

Or, pointfree:

function i j = take (j - i + 1) . drop (i - 1)

Upvotes: 8

Jack
Jack

Reputation: 31

How about: the length of an infinite list? I think that is not possible.

Upvotes: 1

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