Bluefin is an effect system which allows you, though value-level handles, 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

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.

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's easy to disambiguate them because they are distinct values! It is not simple with existing effect systems because they require the disambiguation to occur at the type level. Here is an 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)

Bluefin's use of the type system is very similar to 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.

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.

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 functions-that-return-IO 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 there are two Bluefin features that I don't know to implement in terms of effectful: Coroutines and Compound effects.

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, which doesn't exist in effectful, 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.

• Use NoMonoLocalBinds and NoMonomorphismRestriction for better type inference.
• Writing a handler often requires an explicit type signature.

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"