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

For that you will need to modify the function such that it becomes:

numCharactersInFile :: (FilePath -> IO String) -> FilePath -> IO Int
numCharactersInFile reader fileName = do
                         contents <- reader fileName
                         return (length contents)

Now you can pass any mock function that takes a file path and return IO string such as:

fakeFile :: FilePath -> IO String
fakeFile fileName = return "Fake content"

and pass this function to numCharactersInFile.