Data.Binary.Tagged

Provides tools for serializing and decoding data into ByteString tagged with information about its type. Really, most of this should be used by libraries and frameworks and abstracted over. Typical use cases are the polymorphic communication channels in distributed computing used by Cloud Haskell and distributed-process --- data of any type can come through the channel, and the framework can chose to ignore, queue, or accept data depending on the type the data is tagged with. Designed to work with cross-platform GHC backends like ghcjs.

When decoding data, the result is polymorphic, and you should either allow GHC to infer what you want somehow somewhere, or specify it explicitly.

Quick example:

> let x = encodeTagged (1 :: Int)
> decodeTagged x :: Maybe Bool
Nothing
> decodeTagged x :: Maybe Int
Just 1

The interface is very similar to that of Dynamic.

Also provided here is the internal TagFingerprint data type, so that you can categorize, sort, and queue Tagged or ByteString based on the types they represent.

It might be significant to note that the current TagFingerprint implementation is a little shaky; it's a bit tricky getting all GHC platforms to agree on a meaningful TypeRep serialization, and we will have a better implementation eventually. For now, it just uses an MD5 hash of the string name of the type. So for now, don't encode/decode things with the same type name but exist in different modules (Text or Text, for example) through the same polymorphic channel! This is a bit limiting, admittedly, but until I or the backend maintainers find out a way to ensure that type fingerprints match up per backend, be aware of this limitation.

Encoding and decoding tagged data

encodeTagged :: (Binary a, Typeable a) => a -> ByteString

Encode data into a ByteString with its type data tagged.

Remember that for now, types are distinguished by their string names, so two types of the same name in different modules will not have unique tags.

decodeTagged :: (Binary a, Typeable a) => ByteString -> Maybe a

Decode tagged data from a ByteString. The return type is polymorphic, so it'll attempt to decode it by inferred or specified type.

• If the data is not decoded, Nothing is returned.
• If successfully decoded data is tagged with a Fingerprint not matching the desired type, Nothing is also returned.
• If the data is successfully decoded *and* the tagged Fingerprint matches the desired type, Just x is returned, where x is the originally encoded data (with its tag stripped).

bsFingerprint :: ByteString -> Maybe TagFingerprint

With a ByteString, expecting tagged data, returns the Fingerprint that the data is tagged with. Returns Nothing if the data is not decodable as tagged data. Might accidentally decode untagged data though!

Manipulating

data Tagged a

A data type tupling together data with a TagFingerprint, representing data tagged with its type.

It's best to interface directly with data using encodeTagged, decodeTagged, etc, using tag to tag data and extractTagged to extract data from valid tagged data. This type is exported mostly when you want to specifically decode a ByteString into tagged data, and manually extract it yourself. If you are writing a framework, it is preferred to handle this for the end user.

Instances

Eq a => Eq (Tagged a)

Show a => Show (Tagged a)

Generic (Tagged a)

Binary a => Binary (Tagged a)

Typeable (* -> *) Tagged

type Rep (Tagged a)

data TagFingerprint

A data type representing a fingerprint for a Typeable type. Ideally, this would be Internal's own Fingerprint types; however, for some reason, the fingerprints for the same data type from the same modules differ between different GHC backends. So for now, it is just a ByteString representation of the name of the type. This is literally a bad idea, and so two types with the same name but from different modules will share a non-unique TagFingerprint. Hopefully in the future when I find out a way to fix this or the GHC backend maintainers find a way to provide consistent type fingerprints, this will be fixed.

This type is mostly used for the ability to categorized Tagged items by their type.

There is a Default instance, because the constructor is hidden. For now, it is just an empty ByteString, but when fingerprinting works for real, think of it as a way to generate a fingerprint that will most likely not be matched by any type, in case the need ever comes up.

Instances

Eq TagFingerprint

Ord TagFingerprint

Show TagFingerprint

Generic TagFingerprint

Binary TagFingerprint

Default TagFingerprint

MD5Digest of an empty string: d41d8cd98f00b204e9800998ecf8427e

Typeable * TagFingerprint

type Rep TagFingerprint

typeFingerprint :: Typeable a => a -> TagFingerprint

Compute the Fingerprint representing a type. It is non-strict on its parameter, so passing in undefined should work if you want to just get the Fingerprint of a specific type without having data of that type on hand:

typeFingerprint (undefined :: Int)