#13180: Confusing error when hs-boot abstract data implemented using synonym -------------------------------------+------------------------------------- Reporter: ezyang | Owner: Type: bug | Status: new Priority: low | Milestone: Component: Compiler | Version: 8.1 (Type checker) | Keywords: hs-boot | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This example is a little goofy (that's why I've marked it low priority), but here goes: {{{ -- A.hs-boot module A where data T f :: T -> T -- B.hs module B(f) where import {-# SOURCE #-} A -- A.hs module A(T, f) where import qualified B type T = Int f :: T -> T f x = B.f x }}} When compiled, we get a very interesting error: {{{ ezyang@sabre:~$ ghc-head --make A.hs -fforce-recomp [1 of 3] Compiling A[boot] ( A.hs-boot, A.o-boot ) [2 of 3] Compiling B ( B.hs, B.o ) [3 of 3] Compiling A ( A.hs, A.o ) A.hs-boot:2:1: error: Type constructor ‘T’ has conflicting definitions in the module and its hs-boot file Main module: type T = Int Boot file: abstract T | 2 | data T | ^^^^^^ A.hs-boot:3:1: error: Identifier ‘f’ has conflicting definitions in the module and its hs-boot file Main module: f :: T -> T Boot file: f :: T -> T The two types are different | 3 | f :: T -> T | ^^^^^^^^^^^ }}} The first error is legitimate, but the second is puzzling. I believe the problem has to do with the fact that when we load the hs-boot file for checking, we knot-tie it against itself. Thus, the T referenced in "Boot file" is the one WITHOUT the type synonym unfolding, while the main module *does* have the type synonym unfolding. Pretty-printing is horribly confusing in these situations, so it would be nice if we could give some better guidance here. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13180 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler