Question

diag :: [[Double]] -> [Double]
    
Given a well-formed matrix, return its diagonal (that is, the values where
the row and column number are equal).

    Project1> diag [[1,2],[3,4],[5,6]]
    [1,4]
    Project1> diag (ident 5)
    [1.0,1.0,1.0,1.0,1.0]

This is done in Haskell, I need to make a function that returns a matrix's diagonal in the form of a list. The function should also return the diagonal of a matrix that is not square

Answer 1

I have implemented the function for getting diagonal of a matrix. I am pasting the code, as well as the screenshot with output.
Also as ident () is not a built-in function, I am guessing you already have such a function, in that case, that will work as well.

Code:

diag :: [[Double]] -> [Double]
diag [[]] = []
diag (xs:[]) = [head xs]
diag (x:xs) = head x : diag (map tail xs)

main = do
print(diag ([[1,0,0],[0,1,0],[0,0,1]]))
print(diag ([[1,2],[3,4],[5,6]]))

Screenshot: