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

My question is: How would you test this function? Is there a way to make a "mock" input instead of actually needing to interact with the file system to test it out? Haskell places such an emphasis on pure functions that I have to imagine that this is easy to do.

Answer 1

You can make your code testable by using a type-class-constrained type variable instead of IO.

First, let's get the imports out of the way.

{-# LANGUAGE FlexibleInstances #-} import qualified Prelude import Prelude hiding(readFile) import Control.Monad.State 

The code we want to test:

class Monad m => FSMonad m where     readFile :: FilePath -> m String  -- | 4) Counts the number of characters in a file numCharactersInFile :: FSMonad m => FilePath -> m Int numCharactersInFile fileName = do     contents <- readFile fileName     return (length contents) 

Later, we can run it:

instance FSMonad IO where     readFile = Prelude.readFile 

And test it too:

data MockFS = SingleFile FilePath String  instance FSMonad (State MockFS) where                 -- ^ Reader would be enough in this particular case though     readFile pathRequested = do         (SingleFile pathExisting contents) <- get         if pathExisting == pathRequested             then return contents             else fail "file not found"   testNumCharactersInFile :: Bool testNumCharactersInFile =     evalState         (numCharactersInFile "test.txt")          (SingleFile "test.txt" "hello world")       == 11 

This way your code under test needs very little modification.