# Time complexity of a haskell function

Consider the following code for a function isPrime.

``````f :: Integer -> Integer
f n = head (filter isPrime [0..n])
``````

Assuming that `isPrime k` computes a boolean in O(k) time, what is the time complexity of `f n`?

My thought process was that filter is linear time, and we are given `isPrime` is also linear time, so the time complexity of `f n` should be O(n).

Is this correct and is there an easy way to eyeball the time complexity of code?

### >Solution :

If we can assume that `isPrime` will return `True` for `2`, then it takes constant time. Indeed, it will first call `isPrime 0`, then `isPrime 1`, and finally `isPrime 2`. All these function calls take constant time, since `isPrime k` runs in O(k).

If we make no assumptions what `isPrime` will return, then thefact that `isPrime` is a linear function, means that it will take O(n2) time. Indeed, we each time call `isPrime k`, which runs in O(k), and this thus means that it runs in O(Σnkk) and thus (n2).

Is this correct and is there an easy way to eyeball the time complexity of code?

Due to Haskell’s laziness, analyzing the timecomplexity is more complicated. Especially since for example the time complexity of `filter isPrime [0..n]` depends on the function that uses that list (here `head`). `head` will stop at the first item of the list. Since `filter` is lazy, it will thus not determine the rest of the items. So the notion of time complexity of a function for lazy functions, is probably less straight forward.