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.

Dynamic type checking

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.

Compositionality

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

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).

Monomorphic example

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

Polymorphic example

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

Dealing with qualified types

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 

Word of Caution

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