While there is an important sense in which even IO values in Haskell are still pure values, it is not helpful in the case of testing.
$ ghci
GHCi, version 8.8.4: https://www.haskell.org/ghc/ :? for help
Prelude> putStrLn "hello" == putStrLn "hello"
<interactive>:1:1: error:
• No instance for (Eq (IO ())) arising from a use of ‘==’
• In the expression: putStrLn "hello" == putStrLn "hello"
In an equation for ‘it’: it = putStrLn "hello" == putStrLn "hello"
If you want to test your IO code, and you should, you still have to test them from the perspective of them actually executing in the real world. There's no way to test them as pure values. The snippet above is only the beginning of your problems if you wanted to do that, but it a plenty sufficient problem on its own. In addition to the obvious problem that it points out that IO values are already not comparable to each other in the naive sense, there's also the more philosophical problem that the IO values are your programs, so even if you could run the test I show above, on more complicated expressions, your tests would be asserting that you wrote the program you thought you wrote, not that the program you wrote does what you think it should do.
