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haskell-servant
AddSetCookies missing an instance for when the left tree is the same before and after the transformation
So, I've got this error
server/Server/SRV.hs:63:14: error:
• Overlapping instances for Servant.Auth.Server.Internal.AddSetCookie.AddSetCookies
('Servant.Auth.Server.Internal.AddSetCookie.S
('Servant.Auth.Server.Internal.AddSetCookie.S
'Servant.Auth.Server.Internal.AddSetCookie.Z))
((Data.Tagged.Tagged Handler Network.Wai.Application
Servant.API.Alternative.:<|> ((Data.Text.Internal.Text
-> Data.Tagged.Tagged
Handler
Network.Wai.Application)
Servant.API.Alternative.:<|> Data.Tagged.Tagged
Handler
Network.Wai.Application))
Servant.API.Alternative.:<|> (Data.Tagged.Tagged
Handler Network.Wai.Application
Servant.API.Alternative.:<|> ((Data.Text.Internal.Text
-> Data.Tagged.Tagged
Handler
Network.Wai.Application)
Servant.API.Alternative.:<|> Server.API.PdxfAPI.PdxfAPI
Flouble
(AsServerT
Handler))))
((Data.Tagged.Tagged Handler Network.Wai.Application
Servant.API.Alternative.:<|> ((Data.Text.Internal.Text
-> Data.Tagged.Tagged
Handler
Network.Wai.Application)
Servant.API.Alternative.:<|> Data.Tagged.Tagged
Handler
Network.Wai.Application))
Servant.API.Alternative.:<|> (Data.Tagged.Tagged
Handler Network.Wai.Application
Servant.API.Alternative.:<|> ((Data.Text.Internal.Text
-> Data.Tagged.Tagged
Handler
Network.Wai.Application)
Servant.API.Alternative.:<|> Data.Tagged.Tagged
Handler
Network.Wai.Application)))
arising from a use of ‘genericServeTWithContext’
Matching instances:
two instances involving out-of-scope types
instance [overlappable] (Functor m,
Servant.Auth.Server.Internal.AddSetCookie.AddSetCookies
n (m old) (m cookied),
Servant.API.ResponseHeaders.AddHeader
"Set-Cookie" Web.Cookie.SetCookie cookied new) =>
Servant.Auth.Server.Internal.AddSetCookie.AddSetCookies
('Servant.Auth.Server.Internal.AddSetCookie.S n) (m old) (m new)
-- Defined in ‘Servant.Auth.Server.Internal.AddSetCookie’
instance [overlap ok] (Servant.Auth.Server.Internal.AddSetCookie.AddSetCookies
('Servant.Auth.Server.Internal.AddSetCookie.S n) a a',
Servant.Auth.Server.Internal.AddSetCookie.AddSetCookies
('Servant.Auth.Server.Internal.AddSetCookie.S n) b b') =>
Servant.Auth.Server.Internal.AddSetCookie.AddSetCookies
('Servant.Auth.Server.Internal.AddSetCookie.S n)
(a Servant.API.Alternative.:<|> b)
(a' Servant.API.Alternative.:<|> b')
-- Defined in ‘Servant.Auth.Server.Internal.AddSetCookie’
• In the second argument of ‘($)’, namely
‘genericServeTWithContext tran srv ctx’
In a stmt of a 'do' block:
run 8088 $ genericServeTWithContext tran srv ctx
In the second argument of ‘($)’, namely
‘do let jwk = hs256jwk "aXTrwbg2peHxiY6JKAXVX8kFrcPZ2Mto"
ctx
= defaultCookieSettings :. defaultJWTSettings jwk :. EmptyContext
run 8088 $ genericServeTWithContext tran srv ctx’
|
63 | run 8088 $ genericServeTWithContext tran srv ctx
|
If we look carefully, it's saying that for this type:
AddSetCookies
('S ('S 'Z))
(
(
Tagged Handler Application
:<|>
(
(Text -> Tagged Handler Application)
:<|>
Tagged Handler Application
)
)
:<|>
(
Tagged Handler Application
:<|>
(
( Text -> Tagged Handler Application)
:<|>
PdxfAPI Flouble (AsServerT Handler)
)
)
)
(
(
Tagged Handler Application
:<|>
(
(Text -> Tagged Handler Application)
:<|>
Tagged Handler Application
)
)
:<|>
(
Tagged Handler Application
:<|>
(
( Text -> Tagged Handler Application)
:<|>
Tagged Handler Application
)
)
)
It cannot decide if it should use AddSetCookies ('S n) (m old) (m new) or AddSetCookies ('S n) (a :<|> b) (a' :<|> b').
This is because the first branch is the same e.g. AddSetCookies ('S n) (a:<|>b) (a :<|>b) and therefore a :<|> can be interpreted as m in AddSetCookies ('S n) (m old) (m new).
It's seems perfectly reasonable to me that the a branch can stay the same after the AddSetCookie transformation. I've written this orphan instance that fixed my issue.
instance {-# OVERLAPPING #-}
( AddSetCookies ('S n) a a
, AddSetCookies ('S n) b b'
)
=> AddSetCookies ('S n) (a :<|> b) (a :<|> b') where
addSetCookies cookies ( a :<|> b) = addSetCookies cookies a :<|> addSetCookies cookies b
If someone more experienced with servant could confirm that this is indeed the case and this instance should exist (and it's not just working by accident) then I'd be happy to do a pull request for this issue.
is this not solved by this - https://github.com/haskell-servant/servant/pull/1531 ?
@silky I'm not sure. #1531 was merged in February, but I still experienced this problem in August. :/
i see
well, i'm no expert, but your solution certainly seems reasonable to me :)
A colleague and I hit this problem with one of our API types and spent a good amount of time looking at the proposed new instance, as well as reading the GHC description of overlapping instance pragmas. I'm pretty sure this is correct, and #1653 is probably not what's wanted.
@endgame well, I guess that's enough encouragement for me to open a PR then :). I'll try to do it over the weekend.
