On 01/12/2015 02:25 PM, Alex Hammel wrote:
Hello list,
I've got a collection of types which come from generated code (it's protocol buffer stuff). I'd like to write a polymorphic function which maps the protocol buffer generated data types to native data types. I've tried something like this:
This should get you started. My ProtoBool and ProtoInt types are just dumb wrappers -- your conversions will be more complicated. But this will compile and run in ghci: ghci> toProto (3::Int) ProtoInt {mkProtoInt = 3} ghci> toProto True ProtoBool {mkProtoBool = True} If the toProto and fromProto definitions look goofy in the instances it's because they're in point-free style. You could just as well have e.g., toProto b = ProtoBool b or, fromProto i = mkProtoInt i ---- {-# LANGUAGE TypeFamilies #-} module Proto where -- | Just a wrapper around a 'Bool'. newtype ProtoBool = ProtoBool { mkProtoBool :: Bool } deriving (Show) -- | Just a wrapper around an 'Int'. newtype ProtoInt = ProtoInt { mkProtoInt :: Int } deriving (Show) class ToFromProto a where -- | The type 'a' has another type associated with it, the -- "protocol buffer type" that we can convert to/from. 'Proto' -- below is a type function which when applied to 'a' should -- return this associated type. type Proto a :: * toProto :: a -> (Proto a) fromProto :: (Proto a) -> a instance ToFromProto Bool where -- | The type associated with 'Bool' is 'ProtoBool' type Proto Bool = ProtoBool -- | How do we make a 'ProtoBool' from a 'Bool'? Just wrap it. toProto = ProtoBool -- | How do we get a 'Bool' From a 'ProtoBool'? Unwrap it. fromProto = mkProtoBool -- | The same for 'Int' and 'ProtoInt'. instance ToFromProto Int where type Proto Int = ProtoInt toProto = ProtoInt fromProto = mkProtoInt