[GHC] #15043: Expand type synonym

[GHC]

#15043: Expand type synonym
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        Reporter:  domenkozar        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.2.2
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by domenkozar):

 I can reproduce the problem using servant-generic. A minimal example to
 reproduce:

 {{{#!hs
 #!/usr/bin/env nix-shell
 #!nix-shell -i "runghc --ghc-arg=-fprint-expanded-synonyms" -p
 "haskellPackages.ghcWithPackages (pkgs: with pkgs; [servant-server
 servant-auth-server servant-generic])"

 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE DeriveGeneric #-}

 module ApiType where

 import Data.Text
 import GHC.Generics
 import Servant
 import Servant.Auth.Server
 import Servant.API
 import Servant.Generic


 type Get302 (cts :: [*]) (hs :: [*]) = Verb 'GET 302 cts
   (Headers (Header "Location" String ': hs) NoContent)

 type API = ToServant (GenericAPI AsApi)

 data GenericAPI route = GenericAPI
     { root :: route :- Get302 '[JSON] '[ Header "Set-Cookie" SetCookie]
     } deriving Generic

 server :: Server API
 server = toServant endpoints

 endpoints :: GenericAPI AsServer
 endpoints = GenericAPI
   { root = handler
   }

 handler :: Handler (Headers '[ Header "Location" Text
                              , Header "Set-Cookie" SetCookie]
                     NoContent)
 handler = undefined

 main :: IO ()
 main = undefined
 }}}

 Yields:

 {{{
 ghc15043.hs:33:12: error:
     • Couldn't match type ‘Text’ with ‘[Char]’
       Expected type: AsServer
                      :- Get302 '[JSON] '[Header "Set-Cookie" SetCookie]
         Actual type: Handler
                        (Headers
                           '[Header "Location" Text, Header "Set-Cookie"
 SetCookie] NoContent)
     • In the ‘root’ field of a record
       In the expression: GenericAPI {root = handler}
       In an equation for ‘endpoints’:
           endpoints = GenericAPI {root = handler}
    |
 33 |   { root = handler
    |            ^^^^^^^

 }}}

 Note that without using servant-generic, I do get type synonyms expanded,
 although it's not particulary helpful in this case:

 {{{
 works-ghc15043.hs:24:10: error:
     • Couldn't match type ‘Text’ with ‘[Char]’
       Expected type: Server API
         Actual type: Handler
                        (Headers
                           '[Header "Location" Text, Header "Set-Cookie"
 SetCookie] NoContent)
       Type synonyms expanded:
       Expected type: ServerT API Handler
         Actual type: Handler
                        (Headers
                           '[Header "Location" Text, Header "Set-Cookie"
 SetCookie] NoContent)
     • In the expression: handler
       In an equation for ‘server’: server = handler
    |
 24 | server = handler
    |          ^^^^^^^

 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15043#comment:3>
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