mono.hs:7:4: error:
    • Couldn't match type ‘Bool’ with ‘Int’
      Expected: [Bool]
        Actual: [Int]
    • In the pattern: Single x
      In an equation for ‘f’: f (Single x) = x

mono.hs:7:16: error:
    • Couldn't match expected type ‘Bool’ with actual type ‘Int’
    • In the expression: x
      In an equation for ‘f’: f (Single x) = x