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 ()
copyFile a b = do
x <- readFile a
writeFile b x
makeMock "FileSystemAction" [ts| MonadFileSystem |]
spec = describe "copyFile" $
it "reads a file and writes its contents to another file" $
evaluate $ copyFile "foo.txt" "bar.txt"
& runMock [ ReadFile "foo.txt" :-> "contents"
, WriteFile "bar.txt" "contents" :-> () ]For more information, see the documentation on Hackage.