Haskell Flatbuffers

An implementation of the flatbuffers protocol in Haskell.

• Getting started
  • Codegen
  • Enums
  • Bit flags / Bitmasks
  • Structs
  • Unions
  • File Identifiers
• TODO

Getting started

1. Start off by writing a flatbuffers schema with the data structures you want to serialize/deserialize.

   namespace Data.Game;
   
   table Monster {
     name: string;
     hp: int;
     locations: [string] (required);
   }
   

2. Create a Haskell module named after the namespace in the schema.

   module Data.Game where
   

3. Use mkFlatBuffers to generate constructors and accessors for the data types in your schema.

   {-# LANGUAGE TemplateHaskell #-}
   
   module Data.Game where
   import FlatBuffers
   
   $(mkFlatBuffers "schemas/game.fbs" defaultOptions)
   

4. The following declarations will be generated for you.

   data Monster
   
   -- Constructor
   monster :: Maybe Text -> Maybe Int32 -> WriteVector Text -> WriteTable Monster
   
   -- Accessors
   monsterName      :: Table Monster -> Either ReadError (Maybe Text)
   monsterHp        :: Table Monster -> Either ReadError Int32
   monsterLocations :: Table Monster -> Either ReadError (Vector Text)
   

We can now construct a flatbuffer using encode and read it using decode:

{-# LANGUAGE OverloadedStrings #-}

import           Data.ByteString.Lazy (ByteString)
import           FlatBuffers
import qualified FlatBuffers.Vector as Vector

-- Writing
byteString = encode $
      monster
        (Just "Poring")
        (Just 50)
        (Vector.fromList 2 ["Prontera Field", "Payon Forest"])

-- Reading
readMonster :: ByteString -> Either ReadError String
readMonster byteString = do
  someMonster <- decode byteString
  name        <- monsterName someMonster
  hp          <- monsterHp someMonster
  locations   <- monsterLocations someMonster >>= Vector.toList
  Right ("Monster: " <> show name <> " (" <> show hp <> " HP) can be found in " <> show locations)

For the rest of this document, we'll assume these imports/extensions are enabled:

{-# LANGUAGE OverloadedStrings #-}

import           Data.ByteString.Lazy (ByteString)
import           Data.Text (Text)
import qualified Data.Text as Text
import           FlatBuffers
import qualified FlatBuffers.Vector as Vector

Codegen

You can check exactly which declarations were generated by browsing your module in ghci:

λ> :m Data.Game FlatBuffers FlatBuffers.Vector
λ> :browse Data.Game
data Monster
monster :: Maybe Int32 -> WriteTable Monster
monsterHp :: Table Monster -> Either ReadError Int32

Or by launching a local hoogle server with Stack:

> stack hoogle --rebuild --server

There are lots of examples in the test/Examples folder and the THSpec module.

In particular, test/Examples/schema.fbs contains a variety of data structures and Examples.HandWritten demonstrates what the code generated by mkFlatBuffers would look like.

Enums

enum Color: short {
  Red, Green, Blue
}

Given the enum declaration above, the following code will be generated:

data Color
  = ColorRed
  | ColorGreen
  | ColorBlue
  deriving (Eq, Show, Read, Ord, Bounded)

toColor   :: Int16 -> Maybe Color
fromColor :: Color -> Int16

colorName :: Color -> Text

Usage:

table Monster {
  color: Color;
}

data Monster

monster      :: Maybe Int16 -> WriteTable Monster
monsterColor :: Table Monster -> Either ReadError Int16

-- Writing
byteString = encode $
      monster (Just (fromColor ColorBlue))

-- Reading
readMonster :: ByteString -> Either ReadError Text
readMonster byteString = do
  someMonster <- decode byteString
  i           <- monsterColor someMonster
  case toColor i of
    Just color -> Right ("This monster is " <> colorName color)
    Nothing    -> Left ("Unknown color: " <> show i) -- Forwards compatibility

Bit flags / Bitmasks

enum Colors: uint16 (bit_flags) {
  Red, Green, Blue
}

Given the enum declaration above, the following code will be generated:

colorsRed, colorsGreen, colorsBlue :: Word16
colorsRed = 1
colorsGreen = 2
colorsBlue = 4

allColors :: [Word16]

colorsNames :: Word16 -> [Text]

Usage:

table Monster {
  colors: Colors = "Red Blue";
}

data Monster

monster       :: Maybe Word16 -> WriteTable Monster
monsterColors :: Table Monster -> Either ReadError Word16

import Control.Monad.Except (MonadError, MonadIO, liftEither, liftIO)
import Data.Bits ((.|.), (.&.))
import qualified Data.Text.IO as Text

-- Writing
byteString = encode $
      monster (Just (colorsBlue .|. colorsGreen))

-- Reading
readMonster :: (MonadIO m, MonadError ReadError m) => ByteString -> m ()
readMonster byteString = do
  someMonster <- liftEither $ decode byteString
  colors      <- liftEither $ monsterColors someMonster

  let isRed = colors .&. colorsRed /= 0
  liftIO $ putStrLn $ "Is this monster red? " <> if isRed then "Yes" else "No"

  liftIO $ Text.putStrLn $ "Monster colors: " <> Text.intercalate ", " (colorsNames colors)

Structs

struct Coord {
  x: long;
  y: long;
}

Given the struct declaration above, the following code will be generated:

data Coord
instance IsStruct Coord

--  Constructor
coord :: Int64 -> Int64 -> WriteStruct Coord

-- Accessors
coordX :: Struct Coord -> Either ReadError Int64
coordY :: Struct Coord -> Either ReadError Int64

Usage:

table Monster {
  position: Coord (required);
}

data Monster

monster         :: WriteStruct Coord -> WriteTable Monster
monsterPosition :: Table Monster -> Either ReadError (Struct Coord)

-- Writing
byteString = encode $
      monster (coord 123 456)

-- Reading
readMonster :: ByteString -> Either ReadError String
readMonster byteString = do
  someMonster <- decode byteString
  pos         <- monsterPosition someMonster
  x           <- coordX pos
  y           <- coordY pos
  Right ("Monster is located at " <> show x <> ", " <> show y)

Unions

table Sword { power: int; }
table Axe { power: int; }
union Weapon { Sword, Axe }

Given the union declaration above, the following code will be generated:

-- Accessors
data Weapon
  = WeaponSword !(Table Sword)
  | WeaponAxe   !(Table Axe)

-- Constructors
weaponSword :: WriteTable Sword -> WriteUnion Weapon
weaponAxe   :: WriteTable Axe   -> WriteUnion Weapon

Usage:

table Character {
  weapon: Weapon;
}

data Character

character       :: WriteUnion Weapon -> WriteTable Character
characterWeapon :: Table Character -> Either ReadError (Union Weapon)

-- Writing
byteString = encode $
      character
        (weaponSword (sword (Just 1000)))

-- Reading
readCharacter :: ByteString -> Either ReadError String
readCharacter byteString = do
  someCharacter <- decode byteString
  weapon        <- characterWeapon someCharacter
  case weapon of
    Union (WeaponSword sword) -> do
      power <- swordPower sword
      Right ("Weilding a sword with " <> show power <> " Power.")
    Union (WeaponAxe axe) -> do
      power <- axePower axe
      Right ("Weilding an axe with " <> show power <> " Power.")
    UnionNone         -> Right "Character has no weapon"
    UnionUnknown byte -> Left "Unknown weapon" -- Forwards compatibility

Note that, like in the official FlatBuffers implementation, unions are always optional. Adding the required attribute to a union field has no effect.

To create a character with no weapon, use none :: WriteUnion a

byteString = encode $
      character none

File Identifiers

From "File identification and extension":

Typically, a FlatBuffer binary buffer is not self-describing, i.e. it needs you to know its schema to parse it correctly. But if you want to use a FlatBuffer as a file format, it would be convenient to be able to have a "magic number" in there, like most file formats have, to be able to do a sanity check to see if you're reading the kind of file you're expecting.

Now, you can always prefix a FlatBuffer with your own file header, but FlatBuffers has a built-in way to add an identifier to a FlatBuffer that takes up minimal space, and keeps the buffer compatible with buffers that don't have such an identifier.

table Monster { name: string; }

root_type Monster;
file_identifier "MONS";

data Monster
instance HasFileIdentifier Monster

-- Usual constructor and accessors...

We can now construct a flatbuffer using encodeWithFileIdentifier and use checkFileIdentifier to check if it's safe to decode it to a specific type:

{-# LANGUAGE TypeApplications #-}

-- Writing
byteString = encodeWithFileIdentifier $
      monster (Just "Poring")

-- Reading
readName :: ByteString -> Either ReadError (Maybe Text)
readName byteString = do
  if checkFileIdentifier @Monster byteString then do
    someMonster <- decode byteString
    monsterName someMonster
  else if checkFileIdentifier @Character byteString then do
    someCharacter <- decode byteString
    characterName someCharacter
  else
    Left "Unexpected flatbuffer identifier"

TODO

Features

• gRPC support
• Size-prefixed buffers (needed for streaming multiple messages)
  • flatbuffers/3898
  • FlatCC
• Fixed length arrays in structs
  • flatbuffers/63
  • flatbuffers/3987
  • flatbuffers/5313
  • FlatCC
• unions of strings / structs
  • FlatCC
• key attribute (See "Storing dictionaries in a FlatBuffer" section)
• nested_flatbuffer attribute
• hash attribute
  • Docs
  • Docs
• DSL that allows sharing of data (e.g. reuse an offset to a string/table)
• shared attribute
  • Docs
• Attach Haddock documentation to the generated code.

Other

• TH: sort table fields by size + support original_order attribute
• Enrich Vector API: drop, take, null, folds, sum, elem, for_, traverse_, ideally support most of operations in Data.Foldable
• Improve error messages during SemanticAnalysis stage, provide source code location
• Try alternative bytestring builders: fast-builder, blaze-builder
• Try alternative bytestring parsers: cereal