"Conflicting family instance declaration" error with same type/definition

[parsonsmatt]

Please follow the steps below for reporting a bug:

General summary/comments (optional)

Steps to reproduce

$ git clone [email protected]:yesodweb/persistent
$ cd persistent
$ git checkout matt/stack-build-error
$ stack build --test --no-run-tests --fast

Expected

ExitSuccess, or at least an actual error.

Actual

λ ~/Projects/persistent/ matt/stack-build-error stack build --fast --test --no-run-tests
persistent-mongoDB-2.8.0: unregistering (local file changes: test/MongoInit.hs)
persistent-test-2.0.2.0: unregistering (missing dependencies: persistent-mongoDB)
persistent-2.9.2: Test running disabled by --no-run-tests flag.
persistent-mongoDB-2.8.0: configure
persistent-redis-2.5.2.2: Test running disabled by --no-run-tests flag.
persistent-sqlite-2.9.2: Test running disabled by --no-run-tests flag.
persistent-template-2.6.0: Test running disabled by --no-run-tests flag.
persistent-mongoDB-2.8.0: build
persistent-mongoDB-2.8.0: copy/register
persistent-test-2.0.2.0: configure (lib + exe)
persistent-test-2.0.2.0: build (lib + exe)
persistent-test-2.0.2.0: copy/register
persistent-mongoDB-2.8.0: configure (test)
persistent-mongoDB-2.8.0: build (test)
persistent-qq-2.9.1: configure (test)
persistent-qq-2.9.1: build (test)
persistent-qq-2.9.1: Test running disabled by --no-run-tests flag.
Log files have been written to: /home/matt/Projects/persistent/.stack-work/logs/
Progress 9/10

--  While building package persistent-mongoDB-2.8.0 using:
      /home/matt/.stack/setup-exe-cache/x86_64-linux/Cabal-simple_mPHDZzAJ_2.4.0.1_ghc-8.6.4 --
builddir=.stack-work/dist/x86_64-linux/Cabal-2.4.0.1 build test:test --ghc-options " -ddump-hi
-ddump-to-file -fdiagnostics-color=always"
    Process exited with code: ExitFailure 1
    Logs have been written to: /home/matt/Projects/persistent/.stack-work/logs/persistent-mongo
DB-2.8.0.log

    Configuring persistent-mongoDB-2.8.0...
    Preprocessing library for persistent-mongoDB-2.8.0..
    Building library for persistent-mongoDB-2.8.0..
    [1 of 1] Compiling Database.Persist.MongoDB ( Database/Persist/MongoDB.hs, .stack-work/dist
/x86_64-linux/Cabal-2.4.0.1/build/Database/Persist/MongoDB.o )
    Preprocessing test suite 'test' for persistent-mongoDB-2.8.0..
    Building test suite 'test' for persistent-mongoDB-2.8.0..
    [1 of 4] Compiling MongoInit        ( test/MongoInit.hs, .stack-work/dist/x86_64-linux/Caba
l-2.4.0.1/build/test/test-tmp/MongoInit.o )

    /home/matt/Projects/persistent/persistent-mongoDB/test/MongoInit.hs:1:1: error:
        Conflicting family instance declarations:
          BackendKey MongoDB.MongoContext
            -- Defined in ‘Database.Persist.MongoDB’
          BackendKey MongoDB.MongoContext
            -- Defined in ‘persistent-mongoDB-2.8.0:Database.Persist.MongoDB’
      |
    1 | {-# LANGUAGE ScopedTypeVariables #-}
      | ^

    /home/matt/Projects/persistent/persistent-mongoDB/test/MongoInit.hs:1:1: error:
        Conflicting family instance declarations:
          BackendSpecificFilter
            MongoDB.MongoContext record = Database.Persist.MongoDB.MongoFilter
                                            record
            -- Defined in ‘Database.Persist.MongoDB’
          BackendSpecificFilter
            MongoDB.MongoContext
            record = persistent-mongoDB-2.8.0:Database.Persist.MongoDB.MongoFilter
                       record
            -- Defined in ‘persistent-mongoDB-2.8.0:Database.Persist.MongoDB’
      |
    1 | {-# LANGUAGE ScopedTypeVariables #-}
      | ^

    /home/matt/Projects/persistent/persistent-mongoDB/test/MongoInit.hs:1:1: error:
        Conflicting family instance declarations:
          BackendSpecificUpdate
            MongoDB.MongoContext record = Database.Persist.MongoDB.MongoUpdate
                                            record
            -- Defined in ‘Database.Persist.MongoDB’
          BackendSpecificUpdate
            MongoDB.MongoContext
            record = persistent-mongoDB-2.8.0:Database.Persist.MongoDB.MongoUpdate
                       record
            -- Defined in ‘persistent-mongoDB-2.8.0:Database.Persist.MongoDB’
      |
    1 | {-# LANGUAGE ScopedTypeVariables #-}
      | ^

The location it gives are for the exact same place. It would appear that the test-suite is somehow gaining privileged access to the Database.Persist.MongoDB module. Removing the dependency in the cabal file does not fix the problem - instead, we get an error where the module is not available to import.

Gist with the command run with --verbose

Stack version

λ ~/Projects/persistent/ stack --version
Version 1.9.3, Git revision 40cf7b37526b86d1676da82167ea8758a854953b (6211 commits) x86_64 hpa
ck-0.31.1

Method of installation

• Fresh upgrade via stack upgrade

[parsonsmatt]

Note that this is a "cyclic" dependency:

• persistent-mongoDB:test depends on
• persistent-test:lib depends on
• persistent-mongoDB:lib

Could that be the reason for this?

[dbaynard]

Could that be the reason for this?

Yes, it very well could. Cyclic dependencies are not supported in this manner in 1.9.3. I believe they may be supported on master (with the changes to the component-based build). Would you be able to try that?

[qrilka]

@dbaynard maybe I'm wrong here but I don't think we have proper component-based builds in Stack yet (though I think that it should help us with some issues).

[parsonsmatt]

Seems like a description of what's going on is in this comment!

#2583 (comment)