#13292: Type checker rejects ambiguous top level declaration. -------------------------------------+------------------------------------- Reporter: jeiea | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: x86_64 | (amd64) Type of failure: Other | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by Phyx-): * cc: bgamari (added) * failure: None/Unknown => Other * component: Compiler => Compiler (Type checker) * os: Windows => Unknown/Multiple Comment: Hi, Thanks for the report. I am not sure that this is a bug or at least not a new one. It seems more like a limitation of the type inferencing. The confusing error message comes from the handling of packages in the `cabal` file. The error simplified is: {{{ $ ~/ghc/inplace/bin/ghc-stage2.exe Main.hs [1 of 2] Compiling Lib ( Lib.hs, Lib.o ) Lib.hs:6:12: error: * Ambiguous type variable `m0' arising from a use of `return' prevents the constraint `(Monad m0)' from being solved. Relevant bindings include someFunc :: m0 () (bound at Lib.hs:6:1) Probable fix: use a type annotation to specify what `m0' should be. These potential instances exist: instance Monad IO -- Defined in `GHC.Base' instance Monad Maybe -- Defined in `GHC.Base' instance Monad ((->) r) -- Defined in `GHC.Base' ...plus two others (use -fprint-potential-instances to see them all) * In the expression: return () In an equation for `someFunc': someFunc = return () | 6 | someFunc = return () | ^^^^^^^^^ }}} Essentially (I think) because there's so little context for the type inference it can't disambiguate between the possible types `someFunc` can have. So it can't chose which type is the most general type for `someFunc` simply because they're all valid and can be the most general type. So it needs to be told, either by adding a type signature, or using `someFunc` in a way that allows it to determine what the most general type should be. e.g. {{{ module Lib ( someFunc ) where -- someFunc :: Monad m => m () someFunc = moreFunc moreFunc :: IO () moreFunc = someFunc }}} But I'm not that familiar with GHC's typechecker so I'll reclassify and leave it for someone more knowledgeable in this area. I also don't know why `stack repl` would have worked. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13292#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13292: Type checker rejects ambiguous top level declaration. -------------------------------------+------------------------------------- Reporter: jeiea | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: x86_64 | (amd64) Type of failure: Other | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by jeiea): This is the command which causes the panic. {{{ ghc --interactive -i -isrc -hide-all-packages -package-id=base-4.9.1.0 -- in ghci :add Lib app/Main.hs }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13292#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13292: Type checker rejects ambiguous top level declaration. -------------------------------------+------------------------------------- Reporter: jeiea | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: x86_64 | (amd64) Type of failure: Other | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Phyx-): Oh sorry, I completely missed the panic! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13292#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13292: Type checker rejects ambiguous top level declaration. -------------------------------------+------------------------------------- Reporter: jeiea | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: x86_64 | (amd64) Type of failure: Other | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Simon Peyton Jones ): In [changeset:"3c62b1d6b672e7727ea5fa56c69bf43e43d0fd8f/ghc" 3c62b1d6/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="3c62b1d6b672e7727ea5fa56c69bf43e43d0fd8f" Gather constraints locally in checkMain Wiwth -fdefer-type-errors we were generating some top-level equality constraints, just in a corner of checkMain. The fix is easy. Fixes Trac #13292 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13292#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler