Can Haskell functions be serialized?

Question

The best way to do it would be to get the representation of the function (if it can be recovered somehow). Binary serialization is preferred for efficiency reasons.

I think there is a way to do it in Clean, because it would be impossible to implement iTask, which relies on that tasks (and so functions) can be saved and continued when the server is running again.

This must be important for distributed haskell computations.

I'm not looking for parsing haskell code at runtime as described here: Serialization of functions in Haskell. I also need to serialize not just deserialize.

Answer 1

Unfortunately, it's not possible with the current ghc runtime system. Serialization of functions, and other arbitrary data, requires some low level runtime support that the ghc implementors have been reluctant to add.

Serializing functions requires that you can serialize anything, since arbitrary data (evaluated and unevaluated) can be part of a function (e.g., a partial application).

Answer 2

No. However, the CloudHaskell project is driving home the need for explicit closure serialization support in GHC. The closest thing CloudHaskell has to explicit closures is the distributed-static package. Another attempt is the HdpH closure representation. However, both use Template Haskell in the way Thomas describes below.

The limitation is a lack of static support in GHC, for which there is a currently unactioned GHC ticket. (Any takers?). There has been a discussion on the CloudHaskell mailing list about what static support should actually look like, but nothing has yet progressed as far as I know.

The closest anyone has come to a design and implementation is Jost Berthold, who has implemented function serialisation in Eden. See his IFL 2010 paper "Orthogonal Serialisation for Haskell". The serialisation support is baked in to the Eden runtime system. (Now available as separate library: packman. Not sure whether it can be used with GHC or needs a patched GHC as in the Eden fork...) Something similar would be needed for GHC. This is the serialisation support Eden, in the version forked from GHC 7.4:

data Serialized a = Serialized { packetSize :: Int , packetData :: ByteArray# }
serialize   :: a -> IO (Serialized a)
deserialize :: Serialized a -> IO a

So: one can serialize functions and data structures. There is a Binary instance for Serialized a, allowing you to checkpoint a long-running computation to file! (See Secion 4.1).

Support for such a simple serialization API in the GHC base libraries would surely be the Holy Grail for distributed Haskell programming. It would likely simplify the composability between the distributed Haskell flavours (CloudHaskell, MetaPar, HdpH, Eden and so on...)