CODEWITHSUNDEEP
haskelltypesmonadstype-families
valderman
valderman

Reputation: 8755

Writing a function (a -> b -> ... -> t) -> (Monad m => m a -> m b -> ... -> m t)

Is there some way to write a function f :: (a -> b -> ... -> t) -> (Monad m => m a -> m b -> ... -> m t), basically liftMn for any n?

(EDIT: fixed nonsensical example.)

I'm writing an FRP library, and thought it'd be neat if I could have code vaguely like:

main = do
  input1 <- signalFromTextBoxTheUserMayTypeANumberInto
  input2 <- signalFromAnotherTextBox
  divided <- signal div input1 input2
  signal (\x -> showTextAlert ("input1 `div` input2 is " ++ show x)) divided

I've been fiddling with type families to get it working, but I'm starting to think that it's actually not doable. I'm currently doing something like this:

type Action a = IORef a -> IO ()
type Listener = IO ()
newtype Signal a = Signal (IORef (SigData a))

data SigData a = SigData {
    trigger   :: Action a,
    output    :: IORef a,
    listeners :: [Listener]
  }

class Sig a where
  type S a
  mkSig :: [AnySignal] -> a -> S a

instance Sig b => Sig (a -> b) where
  type S (a -> b) = Signal a -> S b
  mkSig dependencies f =
    \s@(Signal sig) ->
      let x  = unsafePerformIO $ readIORef sig >>= readIORef . output
      in mkSig (AnySignal s : dependencies) (f x)

instance Sig Int where
  type S Int = IO (Signal Int)
  out <- newIORef x
  self <- Signal <$> (newIORef $ SigData {
      trigger   = \ref -> writeIORef ref $! x,
      output    = out,
      listeners = []
    })
  mapM_ (self `listensTo`) deps
  return self

This obviously doesn't work, as the unsafePerformIO gets evaluated once and then keeps that value, and if did work it'd still be ugly, hacky and generally evil. Is there a way to do this, or will I just have to let go of the idea?

Upvotes: 2

Views: 237

Answers (2)

ehird
ehird

Reputation: 40797

How about the Strathclyde Haskell Environment preprocessor, which lets you use idiom brackets, the original notation for applicative functors? This lets you use (| f a b c |) for f <$> a <*> b <*> c. Your example would be (| input1 `div` input2 |).

By the way, it's probably a bad idea for your Signal type to have a Monad instance; this causes the well-known (in the FRP community) problem of time leaks; see this blog post for more information. An Applicative interface is OK, but a Monad interface is not. There are several solutions that prevent time leaks while still allowing the same dynamic event switching behaviour, involving things like an additional type parameter or another monad (as seen in, e.g. the sodium library).

Upvotes: 6

Geoff Huston
Geoff Huston

Reputation: 363

I'm kind of new to all of this, so forgive me if this is a silly answer, but isn't this exactly what applicative functors are for?

Applicatives let you do something like:

f :: a -> b -> ... -> c

f2 :: Applicative p => p a -> p b ... -> p c
f2 x ... y = f <$> x <*> ... <*> y

if I'm not mistaken. (The ellipses are any number of types/arguments)

Upvotes: 10

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