Timeout.purs

foreign import data Timeout :: !

type Milliseconds = Number

type WithTimeout eff = Eff (t :: Timeout | eff)
type ContTimeout eff = ContT Unit (WithTimeout eff) 

setTimeoutCont :: forall eff. Milliseconds -> ContTimeout eff Unit
setTimeoutCont = ContT <<< setTimeout

type WithRef eff = Eff (ref :: Ref | eff)
type ContRef eff = ContT Unit (WithRef eff)

race :: forall a eff. ContRef eff a -> ContRef eff a -> ContRef eff a
race ca cb = ContT $ \k -> do
    rflag <- newRef false
    let run c = runContT c $ \a -> do
                  flag <- readRef rflag
                  unless flag $ writeRef rflag true *> k a
    run ca *> run cb
    
timeout :: forall a eff. Milliseconds -> ContRef eff a -> ContRef eff (Maybe a)
timeout t c = race (Just <$> c) 
                   (setTimeoutCont t *> return Nothing)
                  
{-
Error in declaration timeout
Cannot unify (t :: Network.HTTP.Client.Timeout | u13977) with (ref :: Control.Monad.Eff.Ref.Ref | eff13964) 
Use --force to continue.

The type inferred by compiler for timeout is:
> :t Network.HTTP.Client.timeout
Compiling Network.HTTP.Client
forall t184 t198. Prim.Number -> Control.Monad.Cont.Trans.ContT Prelude.Unit 
                                  (Control.Monad.Eff.Eff (t :: Network.HTTP.Client.Timeout, 
                                                          ref :: Control.Monad.Eff.Ref.Ref | t198)) t184 -> 
                                  Network.HTTP.Client.ContRef (t :: Network.HTTP.Client.Timeout | t198) (Data.Maybe.Maybe t184)