ToSchema, proxy, and GeneralizedNewtypeDeriving

[michalrus]

This won’t work:

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

newtype SomeId =
  SomeId UUID
  deriving (Show, Eq, Ord, ToJSON, FromJSON, ToJSONKey, FromJSONKey, ToSchema, ToField, FromField)

with the following error:

    • Couldn't match representation of type ‘proxy UUID’
                               with that of ‘proxy SomeId’
        arising from the coercion of the method ‘Data.Swagger.Internal.Schema.declareNamedSchema’
          from type ‘forall (proxy :: * -> *).
                     proxy UUID
                     -> Data.Swagger.Declare.Declare
                          (Data.Swagger.Internal.Definitions Data.Swagger.Internal.Schema)
                          Data.Swagger.Internal.NamedSchema’
            to type ‘forall (proxy :: * -> *).
                     proxy SomeId
                     -> Data.Swagger.Declare.Declare
                          (Data.Swagger.Internal.Definitions Data.Swagger.Internal.Schema)
                          Data.Swagger.Internal.NamedSchema’
      NB: We cannot know what roles the parameters to ‘proxy’ have;
        we must assume that the role is nominal
    • When deriving the instance for (ToSchema SomeId)
   |
44 |   deriving (Show, Eq, Ord, ToJSON, FromJSON, ToJSONKey, FromJSONKey, ToSchema, ToField, FromField)
   |                                                                      ^^^^^^^^

Perhaps when using a concrete Data.Proxy.Proxy instead of forall proxy. proxy, this would’ve worked?

[phadej]

Alternatively we could have TaggedNamedSchema a. which might be even better..

[phadej]

@michalrus note that GND is questionable for ToSchema though, as the name of the schema won't change, it will be still UUID. It might be what you want, or not.

[michalrus]

Ohhhhhh, right, absolutely! I don’t want that. Sorry for the noise!

[fieldstrength]

Hi! I recently found myself affected by this design decision as well. I would be quite interested if there is some way to have a workaround. If so I'd be willing to help with the implementation.

My use case involves DerivingVia. I have a newtype that keeps track of how to transform the JSON encodings. This is quite valuable to us because it prevents the various type classes that know about the encoding from getting out of sync.

data MyRecord = MyRecord 
  { ...
  } deriving stock Generic
    deriving (FromJSON, ToJSON, Flow, ToSchema) via APIEncoding SnakeCase MyRecord

This is what I want to write (see here for a gist). Everything works beautifully until we get to ToSchema for the reason mentioned in this issue: the type parameter role is assumed to be nominal.

Now about the solution:

@phadej It sounds like you had an idea, but it is not immediately clear to me what it is. Maybe you could illuminate? Something involved the Tagged type?

I suspect there would be compatibility concerns around simply changing the function signature to take a concrete Proxy (is anyone using the fact that its polymorphic?) so I suppose we could add a new method with this signature, and require that either one be specified. Admittedly it does seem a bit unfortunate to pollute the class in this way though.

Is there a better way?

[phadej]

Adding new method won't help GND/DerivingVia. All methods have to be coercible

…

Sent from my iPhone

On 22 Feb 2019, at 19.34, Cliff Harvey ***@***.***> wrote: Hi! I recently found myself affected by this design decision as well. I would be quite interested if there is some way to have a workaround. If so I'd be willing to help with the implementation. My use case involves DerivingVia. I have a newtype that keeps track of how to transform the JSON encodings. This is quite valuable to us because it prevents the various type classes that know about the encoding from getting out of sync. data MyRecord = MyRecord { ... } deriving stock Generic deriving (FromJSON, ToJSON, Flow, ToSchema) via APIEncoding SnakeCase MyRecord This is what I want to write (see here for a gist). Everything works beautifully until we get to ToSchema for the reason mentioned in this issue: the type parameter role is assumed to be nominal. Now about the solution: @phadej It sounds like you had an idea, but it is not immediately clear to me what it is. Maybe you could illuminate? Something involved the Tagged type? I suspect there would be compatibility concerns around simply changing the function signature to take a concrete Proxy (is anyone using the fact that its polymorphic?) so I suppose we could add a new method with this signature, and require that either one be specified. Admittedly it does seem a bit unfortunate to pollute the class in this way though. Is there a better way? — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub, or mute the thread.

[fizruk]

@fieldstrength I know this is not as neat as DerivingVia, but you can always do this:

instance ToSchema Example where
  declareNamedSchema _ = declareNamedSchema (Proxy @ (APIEncoding SnakeCaseDropPrefix Example))

See the full gist (based on yours).

@phadej how much code would we break if we switch from proxy to Proxy? I don't really see the benefit of proxy anymore, do you?

[fieldstrength]

Thanks for the feedback and pointers!

I was encouraged @fizruk that you seem positive towards the change so I went ahead and made the PR. Happily, it is a quite small patch. I have also sanity-checked that it does indeed allow DerivingVia to work.