Bluefin

In brief

Bluefin is an effect system which allows you to freely mix a variety of effects, including

• Bluefin.EarlyReturn, for early return
• Bluefin.Exception, for exceptions
• Bluefin.IO, for I/O
• Bluefin.State, for mutable state
• Bluefin.Stream, for streams

and to create your own effects in terms of existing ones (Bluefin.Compound). Bluefin effects are accessed explicitly through value-level handles.

Introduction

Bluefin is a Haskell effect system with a new style of API. It is distinct from prior effect systems because effects are accessed explicitly through value-level handles which occur as arguments to effectful operations. Handles (such as State handles, which allow access to mutable state) are introduced by handlers (such as evalState, which sets the initial state). Here's an example where a mutable state effect handle, sn, is introduced by its handler, evalState.

-- If n < 10 then add 10 to it, otherwise
-- return it unchanged
example1 :: Int -> Int
example1 n = runPureEff $
  -- Create a new state handle, sn, and
  -- initialize the value of the state to n
  evalState n $ \sn -> do
    n' <- get sn
    when (n' < 10) $
      modify sn (+ 10)
    get sn

>>> example1 5
15
>>> example1 12
12

The handle sn is used in much the same way as an STRef or IORef.

Multiple effects of the same type

A benefit of value-level effect handles is that it's simple to have multiple effects of the same type in scope at the same time. It is simple to disambiguate them, because they are distinct values! By contrast, existing effect systems require the disambiguation to occur at the type level, which imposes challenges.

Here is a Bluefin example with two mutable Int state effects in scope.

-- Compare two values and add 10
-- to the smaller
example2 :: (Int, Int) -> (Int, Int)
example2 (m, n) = runPureEff $
  evalState m $ \sm -> do
    evalState n $ \sn -> do
      do
        n' <- get sn
        m' <- get sm

        if n' < m'
          then modify sn (+ 10)
          else modify sm (+ 10)

      n' <- get sn
      m' <- get sm

      pure (n', m')

>>> example2 (5, 10)
(15, 10)
>>> example2 (30, 3)
(30, 13)

Exception handles

Bluefin exceptions are accessed through Exception handles. An Exception handle is introduced by a handler, such as try, and that handler is where the exception, if thrown, will be handled. This arrangement differs from normal Haskell exceptions in two ways. Firstly, every Bluefin exception will be handled – it is not possible to have an unhandled Bluefin exception. Secondly, a Bluefin exception can be handled in only one place – normal Haskell exceptions can be handled in a variety of places, and the closest handler of matching type on the stack will be the one that will be chosen upon throw.

example3 shows how to use Bluefin to calculate the sum of numbers from 1 to n, but stop if the sum becomes bigger than 20. The exception handle, ex, which has type Exception String e, cannot escape the scope of its handler, try. If thrown it will be handled at that try, and nowhere else.

example3 :: Int -> Either String Int
example3 n = runPureEff $
  try $ \ex -> do
    evalState 0 $ \total -> do
      for_ [1..n] $ \i -> do
         soFar <- get total
         when (soFar > 20) $ do
           throw ex ("Became too big: " ++ show soFar)
         put total (soFar + i)

      get total

>>> example3 4
Right 10
>>> example3 10
Left "Became too big: 21"

Effect scoping

Bluefin's use of the type system is very similar to Control.Monad.ST: it ensures that a handle can never escape the scope of its handler. That is, once the handler has finished running there is no way you can use the handle anymore.

Type signatures

The type signatures of Bluefin functions follow a common pattern which looks like

(e1 :> es, ...) -> <Handle> e1 -> ... -> Eff es r

Here <Handle> could be, for example, State Int, Exception String or IOE. Consider the function below, incrementReadLine. It reads integers from standard input, accumulates them into a state; it returns when it reads the input integer 0 and it throws an exception if it encounters an input line it cannot parse.

Firstly, let's look at the arguments, which are all handles to Bluefin effects. There is a state handle, an exception handle, and an IO handle, which allow modification of an Int state, throwing a String exception, and performing IO operations respectively. They are each tagged with a different effect type, e1, e2 and e3 respectively, which are always kept polymorphic.

Secondly, let's look at the return value, Eff es (). This means the computation is performed in the Eff monad and the resulting value produced is of type (). Eff is tagged with the effect type es, which is also always kept polymorphic.

Finally, let's look at the constraints. They are what tie together the effect tags of the arguments to the effect tag of the result. For every argument effect tag en we have a constraint en :> es. That tells us the that effect handle with tag en is allowed to be used within the effectful computation. If we didn't have the e1 :> es constraint, for example, that would tell us that the State Int e1 isn't actually used anywhere in the computation.

GHC and editor tools like HLS do a good job of inferring these type signatures.

incrementReadLine ::
  (e1 :> es, e2 :> es, e3 :> es) =>
  State Int e1  ->
  Exception String e2  ->
  IOE e3 ->
  Eff es ()
incrementReadLine state exception io = do
  withJump $ \break -> forever $ do
    line <- effIO io getLine
    i <- case readMaybe line of
      Nothing ->
        throw exception ("Couldn't read: " ++ line)
      Just i ->
        pure i

    when (i == 0) $
      jumpTo break

    modify state (+ i)

Now let's look at how we can run such a function. Each effect must be handled by a corresponding handler, for example runState for the state effect, try for the exception effect and runEff for the IO effect. The type signatures of handlers also follow a common pattern, which looks like

(forall e. <Handle> e -> Eff (e :& es) a) -> Eff es r

This means that the effect e, corresponding to the handle <Handle> e, has been handled and removed from the set of remaining effects, es. (The signatures for runEff and runPureEff are slightly different because they remove Eff itself.) Here, then, is how we can run incrementReadLine:

runIncrementReadLine :: IO (Either String Int)
runIncrementReadLine = runEff $ \io -> do
  try $ \exception -> do
    ((), r) <- runState 0 $ \state -> do
      incrementReadLine state exception io
    pure r

>>> runIncrementReadLine
1
2
3
0
Right 6
>>>> runIncrementReadLine
1
2
3
Hello
Left "Couldn't read: Hello"

Comparison to other effect systems

Everything except effectful

The design of Bluefin is strongly inspired by and based on effectful. All the points in effectful's comparison of itself to other effect systems apply to Bluefin too.

effectful

The major difference between Bluefin and effectful is that in Bluefin effects are represented as value-level handles whereas in effectful they are represented only at the type level. effectful could be described as "a well-typed implementation of the ReaderT IO pattern", and Bluefin could be described as a well-typed implementation of something even simpler: the Handle pattern. The aim of the Bluefin style of value-level effect tracking is to make it even easier to mix effects, especially effects of the same type. Only time will tell which approach is preferable in practice.

"Why not just implement Bluefin as an alternative API on top of effectful?"

It would be great to share code between the two projects! But I don't know to implement Bluefin's Bluefin.Compound effects in effectful.

Implementation

Bluefin has a similar implementation style to effectful. Eff is an opaque wrapper around IO, State is an opaque wrapper around IORef, and throw throws an actual IO exception. Coroutine is implemented simply as a function.

newtype Eff (es :: Effects) a = UnsafeMkEff (IO a)
newtype State s (st :: Effects) = UnsafeMkState (IORef s)
newtype Coroutine a b (s :: Effects) = UnsafeMkCoroutine (a -> IO b)

The type parameters of kind Effects are phantom type parameters which track which effects can be used in an operation. Bluefin uses them to ensure that effects cannot escape the scope of their handler, in the same way that the type parameter to the ST monad ensures that mutable state references cannot escape runST. When the type system indicates that there are no unhandled effects it is safe to run the underlying IO action using unsafePerformIO, which is the approach taken to implement runPureEff. Consequently, it is impossible for a pure value retured from runPureEff to access any Bluefin internal state or throw a Bluefin internal exception.

Tips

• Use NoMonoLocalBinds and NoMonomorphismRestriction for better type inference. (You can always change back to the default after adding inferred type signatures.)
• Writing a handler often requires an explicit type signature.

Creating your own effects

See Bluefin.Compound.

Example

countPositivesNegatives :: [Int] -> String
countPositivesNegatives is = runPureEff $
  evalState (0 :: Int) $ \positives -> do
      r <- try $ \ex ->
          evalState (0 :: Int) $ \negatives -> do
              for_ is $ \i -> do
                  case compare i 0 of
                      GT -> modify positives (+ 1)
                      EQ -> throw ex ()
                      LT -> modify negatives (+ 1)

              p <- get positives
              n <- get negatives

              pure $
                "Positives: "
                  ++ show p
                  ++ ", negatives "
                  ++ show n

      case r of
          Right r' -> pure r'
          Left () -> do
              p <- get positives
              pure $
                "We saw a zero, but before that there were "
                  ++ show p
                  ++ " positives"