Deserialising with GHC.Generics

Question

I'm trying to add a get function to the Generic serialization described in the wiki . Some parts seem straightforward, but there are a few places where I'm very unsure what to write, and unsurprisingly, I'm getting compilation errors. I've looked at the original paper, and at the implementation in cereal , but those resources are a bit over my head. If I can get this simple example working, I will have a better understanding of how to use Generics.

Please see LINE 33, LINE 41, LINE 43 and LINE 54 below.

{-# LANGUAGE DefaultSignatures, DeriveGeneric, TypeOperators, FlexibleContexts #-}

import GHC.Generics
import Data.Bits


data Bit = O | I deriving Show

class Serialize a where
  put :: a -> [Bit]

  default put :: (Generic a, GSerialize (Rep a)) => a -> [Bit]
  put a = gput (from a)

  get :: [Bit] -> (a, [Bit])

  default get :: (Generic a, GSerialize (Rep a)) => [Bit] -> (a, [Bit])
  get xs = (to x, xs')
    where (x, xs') = gget xs

class GSerialize f where
  gput :: f a -> [Bit]
  gget :: [Bit] -> f a

-- | Unit: used for constructors without arguments
instance GSerialize U1 where
  gput U1 = []
  gget xs = U1

-- | Constants, additional parameters and recursion of kind *
instance (GSerialize a, GSerialize b) => GSerialize (a :*: b) where
  gput (a :*: b) = gput a ++ gput b
  gget xs = (a :*: b, xs'') -- LINE 33
    where (a, xs') = gget xs
          (b, xs'') = gget xs'

-- | Meta-information (constructor names, etc.)
instance (GSerialize a, GSerialize b) => GSerialize (a :+: b) where
  gput (L1 x) = O : gput x
  gput (R1 x) = I : gput x
  gget (O:xs) = (L1 x, xs') -- LINE 41
    where (x, xs') = gget xs
  gget (I:xs) = (R1 x, xs') -- LINE 43
    where (x, xs') = gget xs

-- | Sums: encode choice between constructors
instance (GSerialize a) => GSerialize (M1 i c a) where
  gput (M1 x) = gput x
  gget = M1 . gget

-- | Products: encode multiple arguments to constructors
instance (Serialize a) => GSerialize (K1 i a) where
  gput (K1 x) = put x
  gget xs = K1 . get -- LINE 54

instance Serialize Bool where
  put True = [I]
  put False = [O]
  get (I:xs) = (True, xs)
  get (O:xs) = (False, xs)

--
-- Try it out...
--

data UserTree a = Node a (UserTree a) (UserTree a) | Leaf
  deriving (Generic, Show)

instance (Serialize a) => Serialize (UserTree a)


main = do
  let xs = put True
  print (fst . get $ xs :: Bool)
  let ys = put (Leaf :: UserTree Bool)
  print (fst . get $ ys :: UserTree Bool)
  let zs = put (Node False Leaf Leaf :: UserTree Bool)
  print (fst . get $ zs :: UserTree Bool)

Here are the errors:

amy11.hs:33:13:
    Couldn't match expected type `(:*:) a b a1'
                with actual type `((:*:) f2 g2 p3, t2)'
    In the expression: (a :*: b, xs'')
    In an equation for `gget':
        gget xs
          = (a :*: b, xs'')
          where
              (a, xs') = gget xs
              (b, xs'') = gget xs'
    In the instance declaration for `GSerialize (a :*: b)'

amy11.hs:41:17:
    Couldn't match expected type `(:+:) a b a1'
                with actual type `((:+:) f0 g0 p1, t0)'
    In the expression: (L1 x, xs')
    In an equation for `gget':
        gget (O : xs)
          = (L1 x, xs')
          where
              (x, xs') = gget xs
    In the instance declaration for `GSerialize (a :+: b)'

amy11.hs:43:17:
    Couldn't match expected type `(:+:) a b a1'
                with actual type `((:+:) f1 g1 p2, t1)'
    In the expression: (R1 x, xs')
    In an equation for `gget':
        gget (I : xs)
          = (R1 x, xs')
          where
              (x, xs') = gget xs
    In the instance declaration for `GSerialize (a :+: b)'

amy11.hs:54:13:
    Couldn't match expected type `K1 i a a1'
                with actual type `[Bit] -> K1 i0 (a0, [Bit]) p0'
    In the expression: K1 . get
    In an equation for `gget': gget xs = K1 . get
    In the instance declaration for `GSerialize (K1 i a)'
Failed, modules loaded: none.

Answer 1

The main problem is that the type of your gget is not compatible with the type of your get . You're saying that

get :: [Bit] -> (a, [Bit])

Then, correspondingly, you should have

gget :: [Bit] -> (f a, [Bit])

You're then actually doing the right thing in the sum and product instances, but corrections are needed in the cases of U1 and K1 .

For completeness, a version of your code that typechecks:

{-# LANGUAGE DefaultSignatures, DeriveGeneric, TypeOperators, FlexibleContexts #-}

import GHC.Generics
import Data.Bits


data Bit = O | I deriving Show

class Serialize a where
  put :: a -> [Bit]

  default put :: (Generic a, GSerialize (Rep a)) => a -> [Bit]
  put a = gput (from a)

  get :: [Bit] -> (a, [Bit])

  default get :: (Generic a, GSerialize (Rep a)) => [Bit] -> (a, [Bit])
  get xs = (to x, xs')
    where (x, xs') = gget xs

class GSerialize f where
  gput :: f a -> [Bit]
  gget :: [Bit] -> (f a, [Bit])

-- | Unit: used for constructors without arguments
instance GSerialize U1 where
  gput U1 = []
  gget xs = (U1, xs)

-- | Constants, additional parameters and recursion of kind *
instance (GSerialize a, GSerialize b) => GSerialize (a :*: b) where
  gput (a :*: b) = gput a ++ gput b
  gget xs = (a :*: b, xs'') -- LINE 33
    where (a, xs') = gget xs
          (b, xs'') = gget xs'

-- | Meta-information (constructor names, etc.)
instance (GSerialize a, GSerialize b) => GSerialize (a :+: b) where
  gput (L1 x) = O : gput x
  gput (R1 x) = I : gput x
  gget (O:xs) = (L1 x, xs') -- LINE 41
    where (x, xs') = gget xs
  gget (I:xs) = (R1 x, xs') -- LINE 43
    where (x, xs') = gget xs

-- | Sums: encode choice between constructors
instance (GSerialize a) => GSerialize (M1 i c a) where
  gput (M1 x) = gput x
  gget xs = (M1 x, xs')
    where (x, xs') = gget xs

-- | Products: encode multiple arguments to constructors
instance (Serialize a) => GSerialize (K1 i a) where
  gput (K1 x) = put x
  gget xs = (K1 x, xs') -- LINE 54
    where (x, xs') = get xs

instance Serialize Bool where
  put True = [I]
  put False = [O]
  get (I:xs) = (True, xs)
  get (O:xs) = (False, xs)

--
-- Try it out...
--

data UserTree a = Node a (UserTree a) (UserTree a) | Leaf
  deriving (Generic, Show)

instance (Serialize a) => Serialize (UserTree a)


main = do
  let xs = put True
  print (fst . get $ xs :: Bool)
  let ys = put (Leaf :: UserTree Bool)
  print (fst . get $ ys :: UserTree Bool)
  let zs = put (Node False Leaf Leaf :: UserTree Bool)
  print (fst . get $ zs :: UserTree Bool)