Codec.Serialise.Tutorial

serialise-0.2.6.1: A binary serialisation library for Haskell values.

Copyright | (c) Duncan Coutts 2015-2017
License | BSD3-style (see LICENSE.txt)
Maintainer | duncan@community.haskell.org
Stability | experimental
Portability | non-portable (GHC extensions)
Safe Haskell | None
Language | Haskell2010

Codec.Serialise.Tutorial

Contents

- Basic use example
- The CBOR format
  - Interoperability with other CBOR implementations
- The `Serialise` class
  - How to write encoding terms
  - How to write decoding terms
- Migrations
- Working with foreign encodings
  - Working with arbitrary terms
  - Examining encodings

Description

`serialise` library is built on `cborg`, they implement CBOR (Concise Binary Object Representation, specified by IETF RFC 7049) and serialisers/deserializers for it.

Synopsis

# Basic use example

`serialise` offers ability to derive instances via `Generic` mechanism:

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

    fileName :: FilePath
    fileName = "out.cbor"

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

    instance Serialise Animal

    fredTheFrog :: Animal
    fredTheFrog = HoppingAnimal "Fred" 4

    -- | To output value into a file
    write :: Serialise a => FilePath -> a -> IO ()
    write file val = BSL.writeFile file (serialise val)

    -- | Outputs @Fred@ value into file
    writeIO :: IO ()
    writeIO = write fileName fredTheFrog

    -- | Reads the value from file
    readIO :: IO Animal
    readIO = deserialise <$> BSL.readFile fileName

    printIO :: IO ()
    printIO = do
        val <- readIO
        print val

# The CBOR format

CBOR encoding is efficient in encoding/decoding complexity and space, and is generally machine-independent.

CBOR data model has: * integers * floating point numbers * binary strings * text * arrays * key/value maps and resembles JSON.

CBOR allows items to be tagged with a number which identifies the type of data. This can be used both to identify which data constructor of a type is represented, as well as representing different versions of the same constructor.

## Interoperability with other CBOR implementations

Library provides means of stably storing Haskell values for later reading by the library.

The library is not aimed to facilitate serialisation and deserialisation across different CBOR implementations. But that is possible to setup practically.

A few things on compatibility with other CBOR implementations:

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 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 RFCs/standards mentioned 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

`Serialise` class provides convenient access to serialisers and deserialisers.

Creating & using a serialiser can be as simple as deriving `Generic` and `Serialise`,

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

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

Of course, equivalent implementations can be handwritten. A custom `Serialise` instance may be desireable for a variety of reasons:

- deviating from the type-guided encoding that the `Generic` instance provides
- interfacing with other CBOR implementations
- managing migration changes to the type and its encoding

`encode` and `decode` methods form a minimal `Serialise` instance definition:

    instance Serialise Animal where
        encode = encodeAnimal
        decode = decodeAnimal

## How to write 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 `Monoid` `Encoding`.

For instance, to implement an encoder for the `Animal` type above:

    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

Each encoding begins with a length, declaring how many values belonging to `Animal` constructor going to follow. Then a tag which identifies constructor. Fields are encoded using their respective `Serialise` instances.

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

Note: the recommended encoding represents Haskell constructor indexes as CBOR words, not CBOR tags.

## How to write decoding terms

Decoding CBOR representations to Haskell values is done in the `Decoder` `Monad`. A `decode` for the `Animal` type would be:

    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 future changes in the data's structure.

There are two types of compatibility to strive for in serialisers:

- backward compatibility: newer versions of the serialiser can read older versions of an encoding
- forward compatibility: older versions of the serialiser can read (or at least tolerate) newer versions of an encoding

Below are a few examples of how to provide backward-compatible serialisation.

### Adding a constructor

Example: adding a new constructor to `Animal` type, `SwimmingAnimal`,

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

To account for this in handwritten serialiser - add a new tag to 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

Example: adding a new field to `WalkingAnimal` constructor,

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

To account for this - represent `WalkingAnimal` with a new encoding with a new tag, while also providing default value for backward compatibility:

    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"

The same approach can be used to handle field removal and type changes.

# Working with foreign encodings

While `serialise` & `cborg` are primarily designed to be a Haskell-only values serialisation library, the fact that it implements the standard CBOR encoding means that it also can find uses in interacting with foreign CBOR producers & 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, to later re-serialise it in some later result). The `Term` type provides such representation, losslessly capturing a CBOR AST. It can be serialised and deserialised with its `Serialise` instance.

## Examining encodings

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 handwritten 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)