Monad Transformers
In the stream tutorials we mostly used streams in the IO monad. In
general, the type SerialT is a monad transformer, @SerialT m a@
represents a stream of values of type ‘a’ in some underlying monad
‘m’. For example, @SerialT IO Int@ is a stream of ‘Int’ in ‘IO’
monad. Similarly, SerialT Identity Int would be a pure stream
equivalent to [a].
Similarly we have monad transformer types for other stream types as well viz. ‘WSerialT’, ‘AsyncT’, ‘WAsyncT’ and ‘ParallelT’.
To lift a value from an underlying monad in a monad transformer stack into a singleton stream use ‘lift’ and to lift from an IO action use ‘liftIO’.
>>> import Control.Monad.IO.Class (liftIO)
>>> Stream.drain $ liftIO $ putStrLn "Hello world!"
Hello world!
>>> import Control.Monad.Trans.Class (MonadTrans(lift))
>>> Stream.drain $ lift $ putStrLn "Hello world!"
Hello world!Using Monad Transformers
Common monad transformers can be used with streamly serial streams, without any
issues. ReaderT can be used with concurrent streams as well without any
issues.
The semantics of monads other than ReaderT with concurrent streams are
not yet finalized and will change in future, therefore, as of now they are not
recommended to be used with concurrent streams.
Ordering of Monad Transformers
In most cases it is a good idea to keep streamly as the top level monad. This example demonstrates how various control flow modifying monads can be combined with streamly stream monads.
State Sharing
Serial Applications
Read only global state can always be shared using the Reader monad.
Read-write global state can be shared either using an IORef in the Reader
monad or using the State monad.
See AcidRain.hs example for a usage of StateT in the serially executing
portion of the program.
Concurrent Applications
The current recommended method for sharing modifiable global state across
concurrent tasks is to put the shared state inside an IORef in a Reader
monad or just share the IORef by passing it to the required functions. The
IORef can be updated atomically using atomicModifyIORef.
The CirclingSquare.hs example shares an IORef across parallel tasks.