Index of element in Haskell

To get the index of an element in an array in Haskell, you can use the elemIndex function from the Data.List module. Here's the syntax:

import Data.List
elemIndex :: (Eq a) => a -> [a] -> Maybe Int

The elemIndex function takes two parameters: the element you want to find the index of, and the array in which you want to search for the element. It returns a Maybe Int value, which means that it will return Just i if the element is found at index i in the array, and Nothing otherwise.

Here's an example of using elemIndex to find the index of the element 3 in the array [1, 2, 3, 4, 3, 5]:

import Data.List (elemIndex)
array = [1, 2, 3, 4, 3, 5]

index = elemIndex 3 array -- Returns: Just 2

In this example, elemIndex 3 array will return Just 2, because the element 3 is found at index 2 in the array.

If the element you're searching for isn't in the array, elemIndex will return Nothing. Here's an example:

import Data.List (elemIndex)
array = [1, 2, 3, 4, 3, 5]

index = elemIndex 6 array -- Returns: Nothing

In this example, elemIndex 6 array will return Nothing, because 6 isn't in the array.