Towards Haskell in the Cloud (Epstein et al., Haskell Symposium 2011) proposes a new type construct called static that characterizes values that are known statically. Cloud Haskell uses the Static implementation from Control.Distributed.Static. That module comes with its own extensive documentation, which you should read if you want to know the details. Here we explain the Template Haskell support only.

Static values

Given a top-level (possibly polymorphic, but unqualified) definition

f :: forall a1 .. an. T
f = ...

you can use a Template Haskell splice to create a static version of f:

$(mkStatic 'f) :: forall a1 .. an. (Typeable a1, .., Typeable an) => Static T

Every module that you write that contains calls to mkStatic needs to have a call to remotable:

remotable [ 'f, 'g, ... ]

where you must pass every function (or other value) that you pass as an argument to mkStatic. The call to remotable will create a definition

__remoteTable :: RemoteTable -> RemoteTable

which can be used to construct the RemoteTable used to initialize Cloud Haskell. You should have (at most) one call to remotable per module, and compose all created functions when initializing Cloud Haskell:

let rtable :: RemoteTable
    rtable = M1.__remoteTable
           . M2.__remoteTable
           . ...
           . Mn.__remoteTable
           $ initRemoteTable

NOTE: If you get a type error from ghc along these lines

 The exact Name `a_a30k' is not in scope
      Probable cause: you used a unique name (NameU) in Template Haskell but did not bind it

then you need to enable the ScopedTypeVariables language extension.

Static serialization dictionaries

Some Cloud Haskell primitives require static serialization dictionaries (**):

call :: Serializable a => Static (SerializableDict a) -> NodeId -> Closure (Process a) -> Process a

Given some serializable type T you can define

sdictT :: SerializableDict T
sdictT = SerializableDict

and then have

$(mkStatic 'sdictT) :: Static (SerializableDict T)

However, since these dictionaries are so frequently required Cloud Haskell provides special support for them. When you call remotable on a monomorphic function f :: T1 -> T2

remotable ['f]

then a serialization dictionary is automatically created for you, which you can access with

$(functionSDict 'f) :: Static (SerializableDict T1)

In addition, if f :: T1 -> Process T2, then a second dictionary is created

$(functionTDict 'f) :: Static (SerializableDict T2)

Closures

Suppose you have a process

isPrime :: Integer -> Process Bool

Then

$(mkClosure 'isPrime) :: Integer -> Closure (Process Bool)

which you can then call, for example, to have a remote node check if a number is prime.

In general, if you have a monomorphic function

f :: T1 -> T2

then

$(mkClosure 'f) :: T1 -> Closure T2

provided that T1 is serializable (*) (remember to pass f to remotable).

(You can also create closures manually--see the documentation of Control.Distributed.Static for examples.)

Example

Here is a small self-contained example that uses closures and serialization dictionaries. It makes use of the Control.Distributed.Process.SimpleLocalnet Cloud Haskell backend.

{-# LANGUAGE TemplateHaskell #-}
import System.Environment (getArgs)
import Control.Distributed.Process
import Control.Distributed.Process.Closure
import Control.Distributed.Process.Backend.SimpleLocalnet
import Control.Distributed.Process.Node (initRemoteTable)

isPrime :: Integer -> Process Bool
isPrime n = return . (n `elem`) . takeWhile (<= n) . sieve $ [2..]
  where
    sieve :: [Integer] -> [Integer]
    sieve (p : xs) = p : sieve [x | x <- xs, x `mod` p > 0]

remotable ['isPrime]

master :: [NodeId] -> Process ()
master [] = liftIO $ putStrLn "no slaves"
master (slave:_) = do
  isPrime79 <- call $(functionTDict 'isPrime) slave ($(mkClosure 'isPrime) (79 :: Integer))
  liftIO $ print isPrime79

main :: IO ()
main = do
  args <- getArgs
  case args of
    ["master", host, port] -> do
      backend <- initializeBackend host port rtable
      startMaster backend master
    ["slave", host, port] -> do
      backend <- initializeBackend host port rtable
      startSlave backend
  where
    rtable :: RemoteTable
    rtable = __remoteTable initRemoteTable

Notes

(*) If T1 is not serializable you will get a type error in the generated code. Unfortunately, the Template Haskell infrastructure cannot check a priori if T1 is serializable or not due to a bug in the Template Haskell libraries (http://hackage.haskell.org/trac/ghc/ticket/7066)

(**) Even though call is passed an explicit serialization dictionary, we still need the Serializable constraint because Static is not the true static. If it was, we could unstatic the dictionary and pattern match on it to bring the Typeable instance into scope, but unless proper static support is added to ghc we need both the type class argument and the explicit dictionary.