I'm trying to create minimal working example of how to use GHC.Generics, by following the wiki article . Here's what I have:
{-# LANGUAGE DefaultSignatures, DeriveGeneric, TypeOperators, FlexibleContexts #-}
import GHC.Generics
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)
class GSerialize f where
gput :: f a -> [Bit]
instance GSerialize U1 where
gput U1 = []
instance (GSerialize a, GSerialize b) => GSerialize (a :*: b) where
gput (a :*: b) = gput a ++ gput b
instance (GSerialize a, GSerialize b) => GSerialize (a :+: b) where
gput (L1 x) = O : gput x
gput (R1 x) = I : gput x
instance (GSerialize a) => GSerialize (M1 i c a) where
gput (M1 x) = gput x
instance (Serialize a) => GSerialize (K1 i a) where
gput (K1 x) = put x
--
-- Try it out...
--
data UserTree a = Node a (UserTree a) (UserTree a) | Leaf
deriving Generic
instance (Serialize a) => Serialize (UserTree a)
instance Serialize Int
main = do
print . put $ (Leaf :: UserTree Int)
print . put $ (Node 7 Leaf Leaf :: UserTree Int)
print . put $ (3 :: Int)
However, when I try to run this, the program hangs:
λ> main
[I]
[O -- the program hangs here
What am I doing wrong?
You need a proper instance for Int
. That's a built-in type, and you cannot expect magic here. Giving an empty instance for Int
will cause the looping (that's arguably a bad design decision, but that's how it currently is).
Here's one that works (but is in no way efficient):
import Data.Bits
boolToBit :: Bool -> Bit
boolToBit False = O
boolToBit True = I
instance Serialize Int where
put x = map (boolToBit . testBit x) [0 .. bitSize x - 1]
If you really want a minimal example, then do not use Int
, use Tree ()
or Tree Bool
instead.
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