-- Hoogle documentation, generated by Haddock -- See Hoogle, http://www.haskell.org/hoogle/ -- | Compositional, type-safe, polymorphic static values and closures -- -- Towards Haskell in the Cloud (Epstein et al, Haskell Symposium -- 2011) introduces the concept of static values: values that are -- known at compile time. In a distributed setting where all nodes are -- running the same executable, static values can be serialized simply by -- transmitting a code pointer to the value. This however requires -- special compiler support, which is not yet available in ghc. We can -- mimick the behaviour by keeping an explicit mapping -- (RemoteTable) from labels to values (and making sure that all -- distributed nodes are using the same RemoteTable). In this -- module we implement this mimickry and various extensions: type safety -- (including for polymorphic static values) and compositionality. @package distributed-static @version 0.2.0.0 -- | Towards Haskell in the Cloud (Epstein et al, Haskell Symposium -- 2011) introduces the concept of static values: values that are -- known at compile time. In a distributed setting where all nodes are -- running the same executable, static values can be serialized simply by -- transmitting a code pointer to the value. This however requires -- special compiler support, which is not yet available in ghc. We can -- mimick the behaviour by keeping an explicit mapping -- (RemoteTable) from labels to values (and making sure that all -- distributed nodes are using the same RemoteTable). In this -- module we implement this mimickry and various extensions. -- -- -- -- The paper stipulates that Static values should have a free -- Binary instance: -- -- 
--   instance Binary (Static a) 
--
-- -- This however is not (runtime) type safe: for instance, what would be -- the behaviour of -- -- 
--   f :: Static Int -> Static Bool 
--   f = decode . encode 
--
-- -- For this reason we work only with Typeable terms in this -- module, and implement runtime checks -- -- 
--   instance Typeable a => Binary (Static a)
--
-- -- The above function f typechecks but throws an exception if -- executed. The type representation we use, however, is not the standard -- TypeRep from Data.Typeable but TypeRep from -- Data.Rank1Typeable. This means that we can represent -- polymorphic static values (see below for an example). -- -- Since the runtime mapping (RemoteTable) contains values of -- different types, it maps labels (Strings) to Dynamic -- values. Again, we use the implementation from Data.Rank1Dynamic -- so that we can store polymorphic dynamic values. -- -- -- -- Static values as described in the paper are not compositional: there -- is no way to combine two static values and get a static value out of -- it. This makes sense when interpreting static strictly as known at -- compile time, but it severely limits expressiveness. However, the -- main motivation for static is not that they are known at -- compile time but rather that they provide a free Binary -- instance. We therefore provide two basic constructors for -- Static values: -- -- 
--   staticLabel :: String -> Static a
--   staticApply :: Static (a -> b) -> Static a -> Static b
--
-- -- The first constructor refers to a label in a RemoteTable. The -- second allows to apply a static function to a static argument, and -- makes Static compositional: once we have staticApply we -- can implement numerous derived combinators on Static values (we -- define a few in this module; see staticCompose, -- staticSplit, and staticConst). -- -- -- -- Closures in functional programming arise when we partially apply a -- function. A closure is a code pointer together with a runtime data -- structure that represents the value of the free variables of the -- function. A Closure represents these closures explicitly so -- that they can be serialized: -- -- 
--   data Closure a = Closure (Static (ByteString -> a)) ByteString
--
-- -- See Towards Haskell in the Cloud for the rationale behind -- representing the function closure environment in serialized -- (ByteString) form. Any static value can trivially be turned -- into a Closure (staticClosure). Moreover, since -- Static is now compositional, we can also define derived -- operators on Closure values (closureApplyStatic, -- closureApply, closureCompose, closureSplit). -- -- -- -- Suppose we are working in the context of some distributed environment, -- with a monadic type Process representing processes, -- NodeId representing node addresses and ProcessId -- representing process addresses. Suppose further that we have a -- primitive -- -- 
--   sendInt :: ProcessId -> Int -> Process ()
--
-- -- We might want to define -- -- 
--   sendIntClosure :: ProcessId -> Closure (Int -> Process ())
--
-- -- In order to do that, we need a static version of send, and a -- static decoder for ProcessId: -- -- 
--   sendIntStatic :: Static (ProcessId -> Int -> Process ())
--   sendIntStatic = staticLabel "$send"
--
-- -- 
--   decodeProcessIdStatic :: Static (ByteString -> Int)
--   decodeProcessIdStatic = staticLabel "$decodeProcessId"
--
-- -- where of course we have to make sure to use an appropriate -- RemoteTable: -- -- 
--   rtable :: RemoteTable
--   rtable = registerStatic "$send" (toDynamic sendInt)
--          . registerStatic "$decodeProcessId" (toDynamic (decode :: ByteString -> Int))
--          $ initRemoteTable
--
-- -- We can now define sendIntClosure: -- -- 
--   sendIntClosure :: ProcessId -> Closure (Int -> Process ())
--   sendIntClosure pid = Closure decoder (encode pid)
--     where
--       decoder :: Static (ByteString -> Int -> Process ()) 
--       decoder = sendIntStatic `staticCompose` decodeProcessIdStatic
--
-- -- -- -- Suppose we wanted to define a primitive -- -- 
--   sendIntResult :: ProcessId -> Closure (Process Int) -> Closure (Process ())
--
-- -- which turns a process that computes an integer into a process that -- computes the integer and then sends it someplace else. -- -- We can define -- -- 
--   bindStatic :: (Typeable a, Typeable b) => Static (Process a -> (a -> Process b) -> Process b)
--   bindStatic = staticLabel "$bind"
--
-- -- provided that we register this label: -- -- 
--   rtable :: RemoteTable
--   rtable = ...
--          . registerStatic "$bind" ((>>=) :: Process ANY1 -> (ANY1 -> Process ANY2) -> Process ANY2)
--          $ initRemoteTable
--
-- -- (Note that we are using the special ANY1 and ANY2 types -- from Data.Rank1Typeable to represent this polymorphic value.) -- Once we have a static bind we can define -- -- 
--   sendIntResult :: ProcessId -> Closure (Process Int) -> Closure (Process ())
--   sendIntResult pid cl = bindStatic `closureApplyStatic` cl `closureApply` sendIntClosure pid
--
-- -- -- -- In the above we were careful to avoid qualified types. Suppose that we -- have instead -- -- 
--   send :: Binary a => ProcessId -> a -> Process ()
--
-- -- If we now want to define sendClosure, analogous to -- sendIntClosure above, we somehow need to include the -- Binary instance in the closure -- after all, we can ship this -- closure someplace else, where it needs to accept an a, -- then encode it, and send it off. In order to do this, we need -- to turn the Binary instance into an explicit dictionary: -- -- 
--   data BinaryDict a where
--     BinaryDict :: Binary a => BinaryDict a
--   
--   sendDict :: BinaryDict a -> ProcessId -> a -> Process ()
--   sendDict BinaryDict = send
--
-- -- Now sendDict is a normal polymorphic value: -- -- 
--   sendDictStatic :: Static (BinaryDict a -> ProcessId -> a -> Process ())
--   sendDictStatic = staticLabel "$sendDict"
--   
--   rtable :: RemoteTable
--   rtable = ...
--          . registerStatic "$sendDict" (sendDict :: BinaryDict ANY -> ProcessId -> ANY -> Process ())
--          $ initRemoteTable
--
-- -- so that we can define -- -- 
--   sendClosure :: Static (BinaryDict a) -> Process a -> Closure (a -> Process ())
--   sendClosure dict pid = Closure decoder (encode pid)
--     where
--       decoder :: Static (ByteString -> a -> Process ())
--       decoder = (sendDictStatic `staticApply` dict) `staticCompose` decodeProcessIdStatic 
--
-- -- -- -- You should not define functions on ANY and co. For -- example, the following definition of rtable is incorrect: -- -- 
--   rtable :: RemoteTable
--   rtable = registerStatic "$sdictSendPort" sdictSendPort
--          $ initRemoteTable
--     where
--       sdictSendPort :: SerializableDict ANY -> SerializableDict (SendPort ANY)
--       sdictSendPort SerializableDict = SerializableDict
--
-- -- This definition of sdictSendPort ignores its argument -- completely, and constructs a SerializableDict for the -- monomorphic type SendPort ANY, which isn't what you -- want. Instead, you should do -- -- 
--   rtable :: RemoteTable
--   rtable = registerStatic "$sdictSendPort" (sdictSendPort :: SerializableDict ANY -> SerializableDict (SendPort ANY))
--          $ initRemoteTable
--     where
--       sdictSendPort :: forall a. SerializableDict a -> SerializableDict (SendPort a)
--       sdictSendPort SerializableDict = SerializableDict
--
module Control.Distributed.Static -- | A static value. Static is opaque; see staticLabel and -- staticApply. data Static a -- | Create a primitive static value. -- -- It is the responsibility of the client code to make sure the -- corresponding entry in the RemoteTable has the appropriate -- type. staticLabel :: String -> Static a -- | Apply two static values staticApply :: Static (a -> b) -> Static a -> Static b -- | Static version of (.) staticCompose :: (Typeable a, Typeable b, Typeable c) => Static (b -> c) -> Static (a -> b) -> Static (a -> c) -- | Static version of (***) staticSplit :: (Typeable a, Typeable a', Typeable b, Typeable b') => Static (a -> b) -> Static (a' -> b') -> Static ((a, a') -> (b, b')) -- | Static version of const staticConst :: (Typeable a, Typeable b) => Static a -> Static (b -> a) -- | A closure is a static value and an encoded environment data Closure a closure :: Static (ByteString -> a) -> ByteString -> Closure a -- | Convert a static value into a closure. staticClosure :: Typeable a => Static a -> Closure a -- | Apply a static function to a closure closureApplyStatic :: (Typeable a, Typeable b) => Static (a -> b) -> Closure a -> Closure b -- | Closure application closureApply :: (Typeable a, Typeable b) => Closure (a -> b) -> Closure a -> Closure b -- | Closure composition closureCompose :: (Typeable a, Typeable b, Typeable c) => Closure (b -> c) -> Closure (a -> b) -> Closure (a -> c) -- | Closure version of (***) closureSplit :: (Typeable a, Typeable a', Typeable b, Typeable b') => Closure (a -> b) -> Closure (a' -> b') -> Closure ((a, a') -> (b, b')) -- | Runtime dictionary for unstatic lookups data RemoteTable -- | Initial remote table initRemoteTable :: RemoteTable -- | Register a static label registerStatic :: String -> Dynamic -> RemoteTable -> RemoteTable -- | Resolve a static value unstatic :: Typeable a => RemoteTable -> Static a -> Either String a -- | Resolve a closure unclosure :: Typeable a => RemoteTable -> Closure a -> Either String a instance Typeable StaticLabel instance Typeable1 Static instance Typeable1 Closure instance Show StaticLabel instance Show (Static a) instance Show (Closure a) instance Typeable a => Binary (Closure a) instance Typeable a => Binary (Static a)