Control.Monad.TestFixture

Introduction and motivation

This package provides a set of helper functions and types that are designed to assist with writing tests for functions that encode side-effects into monads using effect-specific typeclasses. Consider a function that performs some sort of side effect, such as a function that looks up a user from a database:

lookupUser :: UserId -> IO (Maybe User)

Now consider a function that uses the lookupUser function:

lookupUserIsAdmin :: UserId -> IO Bool
lookupUserIsAdmin userId = do
  maybeUser <- lookupUser userId
  return $ maybe False isAdmin maybeUser

This function works fine, but it’s very difficult to test, even though it is extremely simple. Since lookupUser just runs in IO, it isn’t easy to test lookupUserIsAdmin in isolation. To fix this, it’s possible to create a layer of indirection between lookupUserIsAdmin and lookupUser by making lookupUser a method of a typeclass instead of a free function:

class Monad m => LookupUser m where
  lookupUser :: UserId -> m (Maybe User)

Implementing the original, IO-bound version of lookupUser is easy; we just create a LookupUser instance for IO:

instance LookupUser IO where
  lookupUser = lookupUserIO

However, we can also create other monads that implement the LookupUser typeclass. For example, we could create a very simple newtype wrapper around Identity with an implementation that always returns a user successfully:

newtype SuccessMonad a = SuccessMonad (Identity a)
  deriving (Functor, Applicative, Monad)

runSuccess :: SuccessMonad a -> a
runSuccess (SuccessMonad (Identity x)) = x

instance LookupUser SuccessMonad where
  lookupUser _ = return $ Just User { isAdmin = True }

Now we can test lookupUserIsAdmin completely deterministically without ever needing to touch a real database (using hspec syntax as an example):

lookupUserIsAdmin :: LookupUser m => UserId -> m Bool
lookupUserIsAdmin userId = do
  maybeUser <- lookupUser userId
  return $ maybe False isAdmin maybeUser

spec = describe "lookupUserIsAdmin" $ do
  it "returns True when the UserId corresponds to an admin user" $
    runSuccess (lookupUserIsAdmin (UserId 42)) `shouldBe` True

Similarly, we can also test the failure case by creating a monad that will always return Nothing:

newtype FailureMonad a = FailureMonad (Identity a)
  deriving (Functor, Applicative, Monad)

runFailure :: FailureMonad a -> a
runFailure (FailureMonad (Identity x)) = x

instance LookupUser FailureMonad where
  lookupUser _ = return Nothing

  it "returns False when the UserId does not have a corresponding User" $
    runFailure (lookupUserIsAdmin (UserId 42)) `shouldBe` False

This is great, but it comes at a pretty significant cost: lots and lots of boilerplate. It could get even worse when you have a typeclass with many methods, or even multiple typeclasses at a time! Clearly, there needs to be some way to abstract this pattern a little bit to make it easier to use.

Creating a customizable monad

To permit creating easily customizable implementations of monadic interfaces, we can reify a typeclass at the value level by creating a record type with a field that corresponds to each method:

data Fixture m = Fixture { _lookupUser :: UserId -> m (Maybe User) }

We have to prefix each method name with an underscore to avoid name clashes, but now we have the ability to create a first-class value that represents a particular implementation of the LookupUser typeclass. The next step is turning one of these values into something that can actually be supplied as a monad implementation. One way to do this is to use a reader monad to thread a particular Fixture value around. We can create a newtype that will do that for us:

newtype FixtureM a = FixtureM (Fixture Identity -> a)
  deriving (Functor, Applicative, Monad)

runFixture :: Fixture Identity -> FixtureM a -> a
runFixture fixture (FixtureM func) = func fixture

By making this new FixtureM type an instance of LookupUser, we can use the runFixture function that we defined to run a particular computation with any arbitrary fixture at runtime:

instance LookupUser FixtureM where
  lookupUser userId = FixtureM $ \fixture ->
    runIdentity $ _lookupUser fixture userId

Now we can write all our tests using one-off fixture implementations without creating entirely new types:

spec = describe "lookupUserIsAdmin" $ do
  it "returns True when the UserId corresponds to an admin user" $ do
    let fixture = Fixture { _lookupUser = return $ Just User { isAdmin = True } }
    runFixture fixture (lookupUserIsAdmin (UserId 42)) `shouldBe` True

  it "returns False when the UserId corresponds to a non-admin user" $ do
    let fixture = Fixture { _lookupUser = return $ Just User { isAdmin = False } }
    runFixture fixture (lookupUserIsAdmin (UserId 42)) `shouldBe` False

  it "returns False when the UserId does not have a corresponding User" $ do
    let fixture = Fixture { _lookupUser = return Nothing }
    runFixture fixture (lookupUserIsAdmin (UserId 42)) `shouldBe` False

Moving beyond a reader

The above example is relatively contrived, but it may be possible to see how this technique could be applied to a larger set of monadic typeclasses by creating more instances on a fixture with more methods.

However, it is sometimes useful to do even more with a fixture, such as verifying that a given function was called with a particular argument. For example, consider a function with the following signature:

insertUser :: User -> m ()

In this case, testing the result is likely not particulary interesting, but testing that the function itself is called with the right argument might be helpful. Even more subtly, a function might be called multiple times, and it might need to return different values each time! This requires some degree of state tracking that a reader monad simply cannot provide.

To solve this, the provided TestFixture monad combines a reader, writer, and state monad into a single system. This allows “logging” results from a fixture by using tell within the fixture definition and logTestFixture, and it also permits having fixture invocations depend on previous uses of the fixture by using get and put from MonadState.

Continuing from the above example but using TestFixture instead, we eschew the simpler FixtureM type and create instances over TestFixture instead:

instance Monoid log => LookupUser (TestFixture Fixture log state) where
  lookupUser userId = do
    fn <- asks _lookupUser
    fn userId

Now we can write our tests using the unTestFixture function, along with the similar logTestFixture functions and friends:

spec = describe "lookupUserIsAdmin" $ do
  it "returns True when the UserId corresponds to an admin user" $ do
    let fixture = Fixture { _lookupUser = return $ Just User { isAdmin = True } }
    unTestFixture (lookupUserIsAdmin (UserId 42)) fixture `shouldBe` True

  it "returns False when the UserId corresponds to a non-admin user" $ do
    let fixture = Fixture { _lookupUser = return $ Just User { isAdmin = False } }
    unTestFixture (lookupUserIsAdmin (UserId 42)) fixture `shouldBe` False

  it "returns False when the UserId does not have a corresponding User" $ do
    let fixture = Fixture { _lookupUser = return Nothing }
    unTestFixture (lookupUserIsAdmin (UserId 42)) fixture `shouldBe` False

As a final note, writing out all of these fixture record definitions and instance declarations can be extremely tedious with large numbers of typeclasses and tests. To mitigate this, the Control.Monad.TestFixture.TH module provides a mkFixture function, which uses Template Haskell to generate the necessary code instead.

The TestFixture monad

type TestFixture fixture log state = TestFixtureT fixture log state Identity

The TestFixture monad. A combination of a reader, writer, and state monad, where the reader portion contains a reified typeclass dictionary used as a fixture. For more information, see the module documentation for Control.Monad.TestFixture.

unTestFixture

Arguments

:: TestFixture fixture () () a

the monadic computation to run

-> fixture (TestFixture fixture () ())

the fixture dictionary to use

-> a

the computation’s result

The simplest way to run a test given a fixture, unTestFixture simply runs a monadic computation with a particular fixture and returns the computation’s result. Useful for testing impure functions that return useful values.

logTestFixture :: TestFixture fixture log () a -> fixture (TestFixture fixture log ()) -> [log]

Like unTestFixture, but instead of returning the result of the computation, logTestFixture returns the value written from the writer monad. Useful for testing impure functions called exclusively for side-effects that do not depend on complex prior state.

evalTestFixture :: TestFixture fixture log () a -> fixture (TestFixture fixture log ()) -> (a, [log])

Combines unTestFixture and logTestFixture to return both the computation’s result and the written value as a tuple.

execTestFixture :: TestFixture fixture log state a -> fixture (TestFixture fixture log state) -> state -> (state, [log])

Like logTestFixture but accepts an initial state and returns the final monadic state tupled with the value written from the writer monad. Useful for testing stateful side-effectful computations.

runTestFixture :: TestFixture fixture log state a -> fixture (TestFixture fixture log state) -> state -> (a, state, [log])

Runs a test fixture given an initial state and returns all three pieces of resulting information: the computation’s result, the final monadic state, and the value written from the writer.

The TestFixtureT monad transformer

data TestFixtureT fixture log state m a

TestFixture as a monad transformer instead of as a monad.

Instances

MonadError e m => MonadError e (TestFixtureT fixture log state m)

Methods

throwError :: e -> TestFixtureT fixture log state m a

catchError :: TestFixtureT fixture log state m a -> (e -> TestFixtureT fixture log state m a) -> TestFixtureT fixture log state m a

Monad m => MonadState state (TestFixtureT fixture log state m)

Methods

get :: TestFixtureT fixture log state m state

put :: state -> TestFixtureT fixture log state m ()

state :: (state -> (a, state)) -> TestFixtureT fixture log state m a

Monad m => MonadReader (fixture (TestFixtureT fixture log state m)) (TestFixtureT fixture log state m)

Methods

ask :: TestFixtureT fixture log state m (fixture (TestFixtureT fixture log state m))

local :: (fixture (TestFixtureT fixture log state m) -> fixture (TestFixtureT fixture log state m)) -> TestFixtureT fixture log state m a -> TestFixtureT fixture log state m a

reader :: (fixture (TestFixtureT fixture log state m) -> a) -> TestFixtureT fixture log state m a

Monad m => MonadWriter [log] (TestFixtureT fixture log state m)

Methods

writer :: (a, [log]) -> TestFixtureT fixture log state m a

tell :: [log] -> TestFixtureT fixture log state m ()

listen :: TestFixtureT fixture log state m a -> TestFixtureT fixture log state m (a, [log])

pass :: TestFixtureT fixture log state m (a, [log] -> [log]) -> TestFixtureT fixture log state m a

MonadTrans (TestFixtureT fixture log state)

Methods

lift :: Monad m => m a -> TestFixtureT fixture log state m a

Monad m => Monad (TestFixtureT fixture log state m)

Methods

(>>=) :: TestFixtureT fixture log state m a -> (a -> TestFixtureT fixture log state m b) -> TestFixtureT fixture log state m b

(>>) :: TestFixtureT fixture log state m a -> TestFixtureT fixture log state m b -> TestFixtureT fixture log state m b

return :: a -> TestFixtureT fixture log state m a

fail :: String -> TestFixtureT fixture log state m a

Functor m => Functor (TestFixtureT fixture log state m)

Methods

fmap :: (a -> b) -> TestFixtureT fixture log state m a -> TestFixtureT fixture log state m b

(<$) :: a -> TestFixtureT fixture log state m b -> TestFixtureT fixture log state m a

Monad m => Applicative (TestFixtureT fixture log state m)

Methods

pure :: a -> TestFixtureT fixture log state m a

(<*>) :: TestFixtureT fixture log state m (a -> b) -> TestFixtureT fixture log state m a -> TestFixtureT fixture log state m b

(*>) :: TestFixtureT fixture log state m a -> TestFixtureT fixture log state m b -> TestFixtureT fixture log state m b

(<*) :: TestFixtureT fixture log state m a -> TestFixtureT fixture log state m b -> TestFixtureT fixture log state m a

unTestFixtureT :: Monad m => TestFixtureT fixture () () m a -> fixture (TestFixtureT fixture () () m) -> m a

The transformer equivalent of unTestFixture.

logTestFixtureT :: Monad m => TestFixtureT fixture log () m a -> fixture (TestFixtureT fixture log () m) -> m [log]

The transformer equivalent of logTestFixture.

evalTestFixtureT :: Monad m => TestFixtureT fixture log () m a -> fixture (TestFixtureT fixture log () m) -> m (a, [log])

The transformer equivalent of evalTestFixture.

execTestFixtureT :: Monad m => TestFixtureT fixture log state m a -> fixture (TestFixtureT fixture log state m) -> state -> m (state, [log])

The transformer equivalent of execTestFixture.

runTestFixtureT :: Monad m => TestFixtureT fixture log state m a -> fixture (TestFixtureT fixture log state m) -> state -> m (a, state, [log])

The transformer equivalent of runTestFixture.

Helper functions

module Control.Monad.Writer.Class

module Control.Monad.State.Class

arg0 :: (fixture (TestFixture fixture log state) -> TestFixture fixture log state a) -> TestFixture fixture log state a

A helper function for implementing typeclass instances over TestFixture that pull a value out of a monadic dictionary. For example, given the following instance:

instance Monoid log => MonadSomething (TestFixture Fixture log state) where
  getSomething = do
    something <- asks _getSomething
    something

Using arg0, it can be rewritten like this:

instance Monoid log => MonadSomething (TestFixture Fixture log state) where
  getSomething = arg0 _getSomething

For functions of various arities instead of plain values, use arg1 through arg7, instead.

arg1 :: Monad m => (fixture (TestFixtureT fixture log state m) -> a -> TestFixtureT fixture log state m b) -> a -> TestFixtureT fixture log state m b

Like arg0, but for lifting record accessors containing functions of arity one. For example, given the following instance:

instance Monoid log => MonadSomething (TestFixture Fixture log state) where
  doSomething x = do
    fn <- asks _doSomething
    fn x

Using arg1, it can be rewritten like this:

instance Monoid log => MonadSomething (TestFixture Fixture log state) where
  doSomething = arg1 _doSomething

For functions of higher arities, use arg2 through arg7.

arg2 :: Monad m => (fixture (TestFixtureT fixture log state m) -> a -> b -> TestFixtureT fixture log state m c) -> a -> b -> TestFixtureT fixture log state m c

Like arg1, but for functions of arity 2.

arg3 :: Monad m => (fixture (TestFixtureT fixture log state m) -> a -> b -> c -> TestFixtureT fixture log state m d) -> a -> b -> c -> TestFixtureT fixture log state m d

Like arg1, but for functions of arity 3.

arg4 :: Monad m => (fixture (TestFixtureT fixture log state m) -> a -> b -> c -> d -> TestFixtureT fixture log state m e) -> a -> b -> c -> d -> TestFixtureT fixture log state m e

Like arg1, but for functions of arity 4.

arg5 :: Monad m => (fixture (TestFixtureT fixture log state m) -> a -> b -> c -> d -> e -> TestFixtureT fixture log state m f) -> a -> b -> c -> d -> e -> TestFixtureT fixture log state m f

Like arg1, but for functions of arity 5.

arg6 :: Monad m => (fixture (TestFixtureT fixture log state m) -> a -> b -> c -> d -> e -> f -> TestFixtureT fixture log state m g) -> a -> b -> c -> d -> e -> f -> TestFixtureT fixture log state m g

Like arg1, but for functions of arity 6.

arg7 :: Monad m => (fixture (TestFixtureT fixture log state m) -> a -> b -> c -> d -> e -> f -> g -> TestFixtureT fixture log state m h) -> a -> b -> c -> d -> e -> f -> g -> TestFixtureT fixture log state m h

Like arg1, but for functions of arity 7.

unimplemented :: String -> a

An extremely simple helper function for creating “base” fixture dictionaries with implementations that will simply throw as soon as they are called using a helpful error message. The provided argument should be the name of a method being implemented.

>>> unimplemented "_getSomething"
*** Exception: unimplemented fixture method `_getSomething`

log :: MonadWriter [log] m => log -> m ()

Logs a single value using MonadWriter when the writer state is a list. Equivalent to tell . pure. Useful with TestFixture implementations to record values when using functions like logTestFixture.