To count the number of elements in an array matching a condition in Haskell, you can use the length and filter functions.
The filter function takes a conditional function and an array as arguments, and returns a new array that only includes the elements of the original array that satisfy the condition.
The length function simply takes an array as argument and returns its length.
Here's an example code snippet that demonstrates how to count the number of even numbers in an array:
countEvenNumbers :: [Int] -> Int
countEvenNumbers xs = length (filter even xs)
In this example, the filter even xs expression will return a new array that only includes the even numbers from the input array xs. The length function is then used to get the length of this new array, which gives us the number of even numbers in xs.
Here's another example where we count the number of strings in an array that start with the letter 'A':
countAStrings :: [String] -> Int
countAStrings xs = length (filter (\x -> head x == 'A') xs)
In this example, the filter (\x -> head x == 'A') xs expression will return a new array that only includes the strings from the input array xs that start with the letter 'A'. The length function is then used to get the length of this new array, which gives us the number of 'A'-starting strings in xs.