Codec.Serialise.Tutorial

cborg is a library for the serialisation of Haskell values.

Introduction

As in modern serialisation libraries, cborg offers instance derivation via GHC's Generic mechanism,

import Codec.Serialise
import qualified Data.ByteString.Lazy as BSL

data Animal = HoppingAnimal { animalName :: String, hoppingHeight :: Int }
            | WalkingAnimal { animalName :: String, walkingSpeed :: Int }
            deriving (Generic)

instance Serialise Animal

fredTheFrog :: Animal
fredTheFrog = HoppingAnimal "Fred" 4

main = BSL.writeFile "hi" (serialise fredTheFrog)

We can then later read Fred,

main = do
    fred <- deserialise <$> BSL.readFile "hi"
    print fred

The CBOR format

cborg uses the Concise Binary Object Representation, CBOR (IETF RFC 7049, https://tools.ietf.org/html/rfc7049), as its serialised representation. This encoding is efficient in both encoding/decoding complexity as well as space, and is generally machine-independent.

The CBOR data model resembles that of JSON, having arrays, key/value maps, integers, floating point numbers, binary strings, and text. In addition, CBOR allows items to be tagged with a number which describes the type of data that follows. This can be used both to identify which data constructor of a type an encoding represents, as well as representing different versions of the same constructor.

A note on interoperability

cborg is intended primarily as a serialisation library for Haskell values. That is, a means of stably storing Haskell values for later reading by cborg. While it uses the CBOR encoding format, the library is not primarily aimed to facilitate serialisation and deserialisation across different CBOR implementations.

If you want to use cborg to serialise/deserialise values for/from another CBOR implementation (either in Haskell or another language), you should keep a few things in mind,

1. The Serialise instances for some "basic" Haskell types (e.g. Maybe, ByteString, tuples) don't carry a tag, in contrast to common convention. This is an intentional design decision to minimize encoding size for types which are primitive enough that their representation can be considered stable.
2. The library reserves the right to change encodings in non-backwards-compatible ways across super-major versions. For example the library may start producing a new representation for some type. The new version of the library will be able to decode the old and new representation, but your different CBOR decoder would not be expecting the new representation and would have to be updated to match.
3. While the library tries to use standard encodings in its instances wherever possible, these instances aren't guaranteed to implement all valid variants of the encodings in the specification. For instance, the UTCTime instance only implements a small subset of the encodings described by the Extended Date RFC.

The Serialise class

cborg provides a Serialise class for convenient access to serialisers and deserialisers. Writing a serialiser can be as simple as deriving Generic and Serialise,

-- with DerivingStrategies (GHC 8.2 and newer)
data Animal = ...
            deriving stock (Generic)
            deriving anyclass (Serialise)

-- older GHCs
data MyType = ...
            deriving (Generic)
instance Serialise MyType

Of course, you can also write the equivalent serialisers manually. A hand-rolled Serialise instance may be desireable for a variety of reasons,

• Deviating from the type-guided encoding that the Generic instance will provide
• Interfacing with other CBOR implementations
• Enabling migrations for future changes to the type or its encoding

A minimal hand-rolled instance will define the encode and decode methods,

instance Serialise Animal where
    encode = encodeAnimal
    decode = decodeAnimal

Below we will describe how to write these pieces.

Encoding terms

For the purposes of encoding, abstract CBOR representations are embodied by the Tokens type. Such a representation can be efficiently built using the Encoding Monoid.

For instance, to implement an encoder for the Animal type above we might write,

encodeAnimal :: Animal -> Encoding
encodeAnimal (HoppingAnimal name height) =
    encodeListLen 3 <> encodeWord 0 <> encode name <> encode height
encodeAnimal (WalkingAnimal name speed) =
    encodeListLen 3 <> encodeWord 1 <> encode name <> encode speed

Here we see that each encoding begins with a length, describing how many values belonging to our Animal will follow. We then encode a tag, which identifies which constructor. We then encode the fields using their respective Serialise instance.

It is recommended that you not deviate from this encoding scheme, including both the length and tag, to ensure that you have the option to migrate your types later on.

Also note that the recommended encoding represents Haskell constructor indexes as CBOR words, not CBOR tags.

Decoding terms

Decoding CBOR representations to Haskell values is done in the Decoder Monad. We can write a Decoder for the Animal type defined above as follows,

decodeAnimal :: Decoder s Animal
decodeAnimal = do
    len <- decodeListLen
    tag <- decodeWord
    case (len, tag) of
      (3, 0) -> HoppingAnimal <$> decode <*> decode
      (3, 1) -> WalkingAnimal <$> decode <*> decode
      _      -> fail "invalid Animal encoding"

Migrations

One eventuality that data serialisation schemes need to account for is the need for changes in the data's structure. There are two types of compatibility which we might want to strive for in our serialisers,

• Backward compatibility, such that newer versions of the serialiser can read older versions of an encoding
• Forward compatibility, such that older versions of the serialiser can read (or at least tolerate) newer versions of an encoding

Below we will look at a few of the types of changes which we may need to make and describe how these can be handled in a backwards-compatible manner with cborg.

Adding a constructor

Say we want to add a new constructor to our Animal type, SwimmingAnimal,

data Animal = HoppingAnimal { animalName :: String, hoppingHeight :: Int }
            | WalkingAnimal { animalName :: String, walkingSpeed :: Int }
            | SwimmingAnimal { numberOfFins :: Int }
            deriving (Generic)

We can account for this in our hand-rolled serialiser by simply adding a new tag to our encoder and decoder,

encodeAnimal :: Animal -> Encoding
-- HoppingAnimal, SwimmingAnimal cases are unchanged...
encodeAnimal (HoppingAnimal name height) =
    encodeListLen 3 <> encodeWord 0 <> encode name <> encode height
encodeAnimal (WalkingAnimal name speed) =
    encodeListLen 3 <> encodeWord 1 <> encode name <> encode speed
-- Here is out new case...
encodeAnimal (SwimmingAnimal numberOfFins) =
    encodeListLen 2 <> encodeWord 2 <> encode numberOfFins

decodeAnimal :: Decoder s Animal
decodeAnimal = do
    len <- decodeListLen
    tag <- decodeWord
    case (len, tag) of
      -- these cases are unchanged...
      (3, 0) -> HoppingAnimal <$> decode <*> decode
      (3, 1) -> WalkingAnimal <$> decode <*> decode
      -- this is new...
      (2, 2) -> SwimmingAnimal <$> decode
      _      -> fail "invalid Animal encoding"

Adding/removing/modifying fields

Say then we want to add a new field to our WalkingAnimal constructor,

data Animal = HoppingAnimal { animalName :: String, hoppingHeight :: Int }
            | WalkingAnimal { animalName :: String, walkingSpeed :: Int, numberOfFeet :: Int }
            | SwimmingAnimal { numberOfFins :: Int }
            deriving (Generic)

We can account for this by representing WalkingAnimal with a new encoding with a new tag,

encodeAnimal :: Animal -> Encoding
-- HoppingAnimal, SwimmingAnimal cases are unchanged...
encodeAnimal (HoppingAnimal name height) =
    encodeListLen 3 <> encodeWord 0 <> encode name <> encode height
encodeAnimal (SwimmingAnimal numberOfFins) =
    encodeListLen 2 <> encodeWord 2 <> encode numberOfFins
-- This is new...
encodeAnimal (WalkingAnimal animalName walkingSpeed numberOfFeet) =
    encodeListLen 4 <> encodeWord 3 <> encode animalName <> encode walkingSpeed <> encode numberOfFeet

decodeAnimal :: Decoder s Animal
decodeAnimal = do
    len <- decodeListLen
    tag <- decodeWord
    case (len, tag) of
      -- these cases are unchanged...
      (3, 0) -> HoppingAnimal <$> decode <*> decode
      (2, 2) -> SwimmingAnimal <$> decode
      -- this is new...
      (3, 1) -> WalkingAnimal <$> decode <*> decode <*> pure 4
                                                     -- ^ note the default for backwards compat
      (4, 3) -> WalkingAnimal <$> decode <*> decode <*> decode
      _      -> fail "invalid Animal encoding"

We can use this same approach to handle field removal and type changes.

Working with foreign encodings

While cborg is primarily designed to be a Haskell serialisation library, the fact that it uses the standard CBOR encoding means that it can also find uses in interacting with foreign non-cborg producers and consumers. In this section we will describe a few features of the library which may be useful in such applications.

Working with arbitrary terms

When working with foreign encodings, it can sometimes be useful to capture a serialised CBOR term verbatim (for instance, so you can later re-serialise it in some later result). The Term type provides such a representation, losslessly capturing a CBOR AST. It can be serialised and deserialised with its Serialise instance.

Examining encodings

We can also look In addition to serialisation and deserialisation, cborg provides a variety of tools for representing arbitrary CBOR encodings in the Codec.CBOR.FlatTerm and Codec.CBOR.Pretty modules.

The FlatTerm type represents a single CBOR term, as would be found in the ultimate CBOR representation. For instance, we can easily look at the structure of our Animal encoding above,

>>> toFlatTerm $ encode $ HoppingAnimal "Fred" 42
[TkListLen 3,TkInt 0,TkString "Fred",TkInt 42]
>>> fromFlatTerm (decode @Animal) $ toFlatTerm $ encode (HoppingAnimal "Fred" 42)
Right (HoppingAnimal {animalName = "Fred", hoppingHeight = 42})

This can be useful both for understanding external CBOR formats, as well as understanding and testing your own hand-rolled encodings.

The package also includes a pretty-printer in Codec.CBOR.Pretty, for visualising the CBOR wire protocol alongside its semantic structure. For instance,

>>> putStrLn $ Codec.CBOR.Pretty.prettyHexEnc $ encode $ HoppingAnimal "Fred" 42
83  # list(3)
   00  # word(0)
   64 46 72 65 64  # text("Fred")
   18 2a  # int(42)