Safe Haskell | None |
---|

*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.

- data SerializableDict a where
- SerializableDict :: Serializable a => SerializableDict a

- staticDecode :: Typeable a => Static (SerializableDict a) -> Static (ByteString -> a)
- sdictUnit :: Static (SerializableDict ())
- sdictProcessId :: Static (SerializableDict ProcessId)
- sdictSendPort :: Typeable a => Static (SerializableDict a) -> Static (SerializableDict (SendPort a))
- type CP a b = Closure (a -> Process b)
- idCP :: Typeable a => CP a a
- splitCP :: (Typeable a, Typeable b, Typeable c, Typeable d) => CP a c -> CP b d -> CP (a, b) (c, d)
- returnCP :: forall a. Serializable a => Static (SerializableDict a) -> a -> Closure (Process a)
- bindCP :: forall a b. (Typeable a, Typeable b) => Closure (Process a) -> CP a b -> Closure (Process b)
- seqCP :: (Typeable a, Typeable b) => Closure (Process a) -> Closure (Process b) -> Closure (Process b)
- cpLink :: ProcessId -> Closure (Process ())
- cpUnlink :: ProcessId -> Closure (Process ())
- cpSend :: forall a. Typeable a => Static (SerializableDict a) -> ProcessId -> CP a ()
- cpExpect :: Typeable a => Static (SerializableDict a) -> Closure (Process a)
- cpNewChan :: Typeable a => Static (SerializableDict a) -> Closure (Process (SendPort a, ReceivePort a))
- remotable :: [Name] -> Q [Dec]
- remotableDecl :: [Q [Dec]] -> Q [Dec]
- mkStatic :: Name -> Q Exp
- mkClosure :: Name -> Q Exp
- functionSDict :: Name -> Q Exp
- functionTDict :: Name -> Q Exp

# Serialization dictionaries (and their static versions)

data SerializableDict a whereSource

Reification of `Serializable`

(see Control.Distributed.Process.Closure)

SerializableDict :: Serializable a => SerializableDict a |

staticDecode :: Typeable a => Static (SerializableDict a) -> Static (ByteString -> a)Source

Static decoder, given a static serialization dictionary.

See module documentation of Control.Distributed.Process.Closure for an example.

sdictUnit :: Static (SerializableDict ())Source

Serialization dictionary for '()'

sdictProcessId :: Static (SerializableDict ProcessId)Source

Serialization dictionary for `ProcessId`

sdictSendPort :: Typeable a => Static (SerializableDict a) -> Static (SerializableDict (SendPort a))Source

Serialization dictionary for `SendPort`

# The CP type and associated combinators

type CP a b = Closure (a -> Process b)Source

`CP a b`

is a process with input of type `a`

and output of type `b`

splitCP :: (Typeable a, Typeable b, Typeable c, Typeable d) => CP a c -> CP b d -> CP (a, b) (c, d)Source

returnCP :: forall a. Serializable a => Static (SerializableDict a) -> a -> Closure (Process a)Source

bindCP :: forall a b. (Typeable a, Typeable b) => Closure (Process a) -> CP a b -> Closure (Process b)Source

seqCP :: (Typeable a, Typeable b) => Closure (Process a) -> Closure (Process b) -> Closure (Process b)Source

# CP versions of Cloud Haskell primitives

cpNewChan :: Typeable a => Static (SerializableDict a) -> Closure (Process (SendPort a, ReceivePort a))Source

# Template Haskell support for creating static values and closures

remotable :: [Name] -> Q [Dec]Source

Create the closure, decoder, and metadata definitions for the given list of functions

remotableDecl :: [Q [Dec]] -> Q [Dec]Source

Like `remotable`

, but parameterized by the declaration of a function
instead of the function name. So where for `remotable`

you'd do

f :: T1 -> T2 f = ... remotable ['f]

with `remotableDecl`

you would instead do

remotableDecl [ [d| f :: T1 -> T2 ; f = ... |] ]

`remotableDecl`

creates the function specified as well as the various
dictionaries and static versions that `remotable`

also creates.
`remotableDecl`

is sometimes necessary when you want to refer to, say,
`$(mkClosure 'f)`

within the definition of `f`

itself.

NOTE: `remotableDecl`

creates `__remoteTableDecl`

instead of `__remoteTable`

so that you can use both `remotable`

and `remotableDecl`

within the same
module.

mkStatic :: Name -> Q ExpSource

Construct a static value.

If `f : forall a1 .. an. T`

then `$(mkStatic 'f) :: forall a1 .. an. Static T`

.
Be sure to pass `f`

to `remotable`

.