Associated data type confusion
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: class FromProto a where type InputDataType fromProto :: InputDataType -> a instance ToProto SomeNativeType where type OutputDataType = SomeGeneratedType toProto = {- etc -} instance FromProto SomeOtherNativeType where type InputDataType = SomeOtherGeneratedType fromProto = {- etc -} Which works fine for mapping one native data type to *one* generated data type, but breaks when I want to define different *InputDataType*s for different instances of *FromProto*. I feel like I'm making this more complicated than I need to. Is there an easy way to get the data type to data type mapping that I'm looking for? Cheers, Alex
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
Awesome, that did the trick! I think I need a bit more practice with 'type
functions' before I'm really comfortable with them.
Thanks a lot!
Cheers,
Alex
On Mon, Jan 12, 2015 at 12:39 PM, Michael Orlitzky
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
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participants (2)
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Alex Hammel -
Michael Orlitzky