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haskell-servant / servant

Servat is a Haskell DSL for describing, serving, querying, mocking, documenting web applications and more!
https://docs.servant.dev/
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WithStatus is unusable with class-based MimeRender instances #1356

Closed intolerable closed 3 years ago

intolerable commented 3 years ago

Almost any use of the new UVerb WithStatus combinator fails with an Overlapping instances error, due to the (in the case of JSON) MimeRender ctype a => MimeRender ctype (WithStatus status a) and ToJSON a => MimeRender JSON a instances. It's possible to work around this issue by writing an explicit HasStatus instance, but this makes UVerbs usage much more verbose.

Due to this issue, the UVerb cookbook page doesn't build at all.

The rest of the new UVerb work is incredible and super helpful, keep up the great work!

arianvp commented 3 years ago

Thanks. @fisx @maksbotan lets have a look at this before release.

We should at least make sure the cookbook works =)

arianvp commented 3 years ago

It sounds to me like we should remove the 

MimeRender ctype a => MimeRender ctype (WithStatus status a)

instance completely and keep the

ctype a => ctype  (WithStatus status a)

instances for each ctype we support.

Then the instances do not overlap; but still have the correct behaviour

arianvp commented 3 years ago

I think the default JSON instance for WithStatus is very dubious too:

instance (GHC.Generic (WithStatus n a), ToJSON a) => ToJSON (WithStatus n a)

instance (GHC.Generic (WithStatus n a), FromJSON a) => FromJSON (WithStatus n a)

This should use newtype deriving instead. It requiring a Generic constraint is very unflexible

intolerable commented 3 years ago

This still doesn't work correctly in HEAD, unfortunately. There are also no tests in servant-server for WithStatus (which I assume is why this wasn't caught).

Adding a new test for the WithStatus combinator (and turning on -fprint-potential-instances) still results in

/home/fraser/Code/servant/servant-server/test/Servant/ServerSpec.hs:866:16: error:
    • Overlapping instances for Servant.API.ContentTypes.MimeRender
                                  JSON (WithStatus 203 AnimalResponse)
        arising from a use of ‘serve’
      Matching instances:
        two instances involving out-of-scope types
          instance Servant.API.ContentTypes.MimeRender ctype a =>
                   Servant.API.ContentTypes.MimeRender ctype (WithStatus _status a)
            -- Defined in ‘Servant.API.UVerb’
          instance [overlappable] ToJSON a =>
                                  Servant.API.ContentTypes.MimeRender JSON a
            -- Defined in ‘Servant.API.ContentTypes’
    • In the second argument of ‘($)’, namely
        ‘serve (Proxy :: Proxy UVerbApi) server’
      In the first argument of ‘with’, namely
        ‘(pure $ serve (Proxy :: Proxy UVerbApi) server)’
      In the expression:
        with (pure $ serve (Proxy :: Proxy UVerbApi) server)
    |       
866 |   with (pure $ serve (Proxy :: Proxy UVerbApi) server) $ do
    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
arianvp commented 3 years ago

Correct. I just noticed this as well. Tracking here: https://github.com/haskell-servant/servant/issues/1367

Would you mind opening a PR with the failing test? I'll then push the fix on top of that