winery

winery is a serialisation library for Haskell. It tries to achieve two goals: compact representation and perpetual inspectability.

The binary library provides a compact representation, but there is no way to inspect the serialised value without the original instance.

There's serialise, which is an alternative library based on CBOR. Every value has to be accompanied with tags, so it tends to be redundant for arrays of small values. Encoding records with field names is also redudant.

Interface

The interface is simple; serialise encodes a value with its schema, and deserialise decodes a ByteString using the schema in it.

class Serialise a

serialise :: Serialise a => a -> B.ByteString
deserialise :: Serialise a => B.ByteString -> Either String a

It's also possible to serialise schemata and data separately.

-- Note that 'Schema' is an instance of 'Serialise'
schema :: Serialise a => proxy a -> Schema
serialiseOnly :: Serialise a => a -> B.ByteString

getDecoder gives you a deserialiser.

getDecoder :: Serialise a => Schema -> Either StrategyError (ByteString -> a)

For user-defined datatypes, you can either define instances

instance Serialise Foo where
  schemaVia = gschemaViaRecord
  toEncoding = gtoEncodingRecord
  deserialiser = gdeserialiserRecord Nothing

for single-constructor records, or just

instance Serialise Foo

for any ADT. The former explicitly describes field names in the schema, and the latter does constructor names.

The schema

The definition of Schema is as follows:

data Schema = SSchema !Word8
  | SUnit
  | SBool
  | SChar
  | SWord8
  | SWord16
  | SWord32
  | SWord64
  | SInt8
  | SInt16
  | SInt32
  | SInt64
  | SInteger
  | SFloat
  | SDouble
  | SBytes
  | SText
  | SList !Schema
  | SArray !(VarInt Int) !Schema -- fixed size
  | SProduct [Schema]
  | SProductFixed [(VarInt Int, Schema)] -- fixed size
  | SRecord [(T.Text, Schema)]
  | SVariant [(T.Text, [Schema])]
  | SFix Schema -- ^ binds a fixpoint
  | SSelf !Word8 -- ^ @SSelf n@ refers to the n-th innermost fixpoint
  deriving (Show, Read, Eq, Generic)

The Serialise instance is derived by generics.

There are some special schemata:

• SSchema n is a schema of schema. The winery library stores the concrete schema of Schema for each version, so it can deserialise data even if the schema changes.
• SFix binds a fixpoint.
• SSelf n refers to the n-th innermost fixpoint bound by SFix. This allows it to provide schemata for inductive datatypes.

Backward compatibility

If having default values for missing fields is sufficient, you can pass a default value to gdeserialiserRecord:

  deserialiser = gdeserialiserRecord $ Just $ Foo "" 42 0

You can also build a custom deserialiser using the Alternative instance and combinators such as extractField, extractConstructor, etc.

Pretty-printing

Term can be deserialised from any winery data. It can be pretty-printed using the Pretty instance:

{ bar: "hello"
, baz: 3.141592653589793
, foo: Just 42
}