type FieldEntry = (Text, FieldValue)

toFieldEntry ∷ ToFieldValue a ⇒ T.Text → a → FieldEntry
toFieldEntry k v = (k, toFieldValue v)

class FromFieldValue a where
    fromFieldValue ∷ FieldValue → Maybe a

instance FromFieldValue () where
    fromFieldValue FVVoid = Just ()
    fromFieldValue _      = Nothing

instance FromFieldValue Bool where
    fromFieldValue (FVBool b) = Just b
    fromFieldValue _          = Nothing

instance FromFieldValue Int where
    fromFieldValue (FVInt8  i) = Just (fromIntegral i)
    fromFieldValue (FVInt16 i) = Just (fromIntegral i)
    fromFieldValue (FVInt32 i) = Just (fromIntegral i)
    fromFieldValue (FVInt64 i) = Just (fromIntegral i)
    fromFieldValue _      = Nothing

instance FromFieldValue ByteString where
    fromFieldValue (FVByteArray b) = Just b
    fromFieldValue _               = Nothing

instance FromFieldValue T.Text where
    fromFieldValue (FVString t) = Just t
    fromFieldValue _            = Nothing

instance FromFieldValue Float where
    fromFieldValue (FVFloat f)  = Just f
    fromFieldValue (FVDouble f) = Just (double2Float f)
    fromFieldValue _            = Nothing

instance FromFieldValue Double where
    fromFieldValue (FVDouble f) = Just f
    fromFieldValue (FVFloat f)  = Just (float2Double f)
    fromFieldValue _            = Nothing

class ToFieldValue a where
    toFieldValue ∷ a → FieldValue

instance ToFieldValue () where
    toFieldValue = const FVVoid

instance ToFieldValue Bool where
    toFieldValue = FVBool

instance ToFieldValue Int where
    toFieldValue = FVInt64 ∘ fromIntegral

instance ToFieldValue ByteString where
    toFieldValue = FVByteArray

instance ToFieldValue T.Text where
    toFieldValue = FVString

instance ToFieldValue Float where
    toFieldValue = FVFloat

instance ToFieldValue Double where
    toFieldValue = FVDouble