Testing functions in Haskell that do IO

Question

Working through Real World Haskell right now. Here\'s a solution to a very early exercise in the book:

-- | 4) Counts the number of characters in a file
numC         


        

Answer 1

Based on my layman's understanding of Haskell, I've come to the following conclusions:

1. If a function makes use of the IO monad, mock testing is going to be impossible. Avoid hard-coding the IO monad in your function.

2. Make a helper version of your function that takes in other functions that may do IO. The result will look like this:

numCharactersInFile' :: Monad m => (FilePath -> m String) -> FilePath -> m Int
numCharactersInFile' f filePath = do
    contents <- f filePath
    return (length contents)

numCharactersInFile' is now testable with mocks!

mockFileSystem :: FilePath -> Identity String
mockFileSystem "fileName" = return "mock file contents"

Now you can verify that numCharactersInFile' returns the the expected results w/o IO:

18 == (runIdentity .  numCharactersInFile' mockFileSystem $ "fileName")

Finally, export a version of your original function signature for use with IO

numCharactersInFile :: IO Int
numCharactersInFile = NumCharactersInFile' readFile

So, at the end of the day, numCharactersInFile' is testable with mocks. numCharactersInFile is just a variation of numCharactersInFile'.