Question

Learning Haskell is killing me. If I am going to write a function that takes an array of arrays of order-able elements, and outputs the same thing, how do I do that?

I try:

main = testfn [[1],[2]]

testfn :: (Ord a) => [[a]] -> [[a]]
testfn x = x

But the message I get is:

Couldn’t match expected type ‘IO t0’ with actual type ‘[[a0]]’ In the
expression: main When checking the type of the function ‘main’

Answer 1

Your problem here is that main must be of a type of the form IO a (you can see this in the error – GHC is expecting main’s inferred type [[a0]] to match against IO t0). Here main has type [[Int]]. You can easily fix this by simply printing the result:

main = print (testfn [[1],[2]])