Safe Haskell | Safe-Infered |
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*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 (`String`

s) 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

- data Static a
- staticLabel :: String -> Static a
- staticApply :: Static (a -> b) -> Static a -> Static b
- staticCompose :: (Typeable a, Typeable b, Typeable c) => Static (b -> c) -> Static (a -> b) -> Static (a -> c)
- staticSplit :: (Typeable a, Typeable a', Typeable b, Typeable b') => Static (a -> b) -> Static (a' -> b') -> Static ((a, a') -> (b, b'))
- staticConst :: (Typeable a, Typeable b) => Static a -> Static (b -> a)
- data Closure a
- closure :: Static (ByteString -> a) -> ByteString -> Closure a
- staticClosure :: Typeable a => Static a -> Closure a
- closureApplyStatic :: (Typeable a, Typeable b) => Static (a -> b) -> Closure a -> Closure b
- closureApply :: forall a b. (Typeable a, Typeable b) => Closure (a -> b) -> Closure a -> Closure b
- closureCompose :: (Typeable a, Typeable b, Typeable c) => Closure (b -> c) -> Closure (a -> b) -> Closure (a -> c)
- closureSplit :: (Typeable a, Typeable a', Typeable b, Typeable b') => Closure (a -> b) -> Closure (a' -> b') -> Closure ((a, a') -> (b, b'))
- data RemoteTable
- initRemoteTable :: RemoteTable
- registerStatic :: String -> Dynamic -> RemoteTable -> RemoteTable
- unstatic :: Typeable a => RemoteTable -> Static a -> Either String a
- unclosure :: Typeable a => RemoteTable -> Closure a -> Either String a

# Static values

A static value. Static is opaque; see `staticLabel`

and `staticApply`

.

staticLabel :: String -> Static aSource

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.

staticApply :: Static (a -> b) -> Static a -> Static bSource

Apply two static values

# Derived static combinators

staticCompose :: (Typeable a, Typeable b, Typeable c) => Static (b -> c) -> Static (a -> b) -> Static (a -> c)Source

Static version of (`.`

)

staticSplit :: (Typeable a, Typeable a', Typeable b, Typeable b') => Static (a -> b) -> Static (a' -> b') -> Static ((a, a') -> (b, b'))Source

Static version of (`***`

)

# Closures

A closure is a static value and an encoded environment

:: Static (ByteString -> a) | Decoder |

-> ByteString | Encoded closure environment |

-> Closure a |

# Derived closure combinators

staticClosure :: Typeable a => Static a -> Closure aSource

Convert a static value into a closure.

closureApplyStatic :: (Typeable a, Typeable b) => Static (a -> b) -> Closure a -> Closure bSource

Apply a static function to a closure

closureApply :: forall a b. (Typeable a, Typeable b) => Closure (a -> b) -> Closure a -> Closure bSource

Closure application

closureCompose :: (Typeable a, Typeable b, Typeable c) => Closure (b -> c) -> Closure (a -> b) -> Closure (a -> c)Source

Closure composition

closureSplit :: (Typeable a, Typeable a', Typeable b, Typeable b') => Closure (a -> b) -> Closure (a' -> b') -> Closure ((a, a') -> (b, b'))Source

Closure version of (`***`

)

# Resolution

data RemoteTable Source

Runtime dictionary for `unstatic`

lookups

initRemoteTable :: RemoteTableSource

Initial remote table

registerStatic :: String -> Dynamic -> RemoteTable -> RemoteTableSource

Register a static label