monad-mock

monad-mock is a Haskell package that provides a monad transformer to help create “mocks” of mtl-style typeclasses, intended for use in unit tests. A mock can be executed by providing a sequence of expected monadic calls and their results, and the mock will verify that the computation conforms to the expectation.

For example, imagine a MonadFileSystem typeclass, which describes a class of monads that may perform filesystem operations:

class Monad m => MonadFileSystem m where
  readFile :: FilePath -> m String
  writeFile :: FilePath -> String -> m ()

Using MockT, it’s possible to test computations that use MonadFileSystem in a completely pure way:

copyFile :: MonadFileSystem m => FilePath -> FilePath -> m String
copyFile a b = do
  x <- readFile a
  writeFile b x
  return x

spec = describe "copyFile" $
  it "reads a file and writes its contents to another file" $
    evaluate $ copyFile "foo.txt" "bar.txt"
      & runMockT [ ReadFile "foo.txt" :-> "contents"
                 , WriteFile "bar.txt" "contents" :-> () ]

For more information, see the documentation on Hackage.