#13068: GHC should not allow modules to define instances of abstract type classes -------------------------------------+------------------------------------- Reporter: ezyang | Owner: (none) Type: bug | Status: patch Priority: low | Milestone: 8.2.1 Component: Compiler (Type | Version: 8.1 checker) | Resolution: | Keywords: backpack Operating System: Unknown/Multiple | Architecture: Type of failure: GHC accepts | Unknown/Multiple invalid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D3254 Wiki Page: | -------------------------------------+------------------------------------- Changes (by ezyang): * status: new => patch * differential: => Phab:D3254 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13068#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13068: GHC should not allow modules to define instances of abstract type classes -------------------------------------+------------------------------------- Reporter: ezyang | Owner: (none) Type: bug | Status: closed Priority: low | Milestone: 8.2.1 Component: Compiler (Type | Version: 8.1 checker) | Resolution: fixed | Keywords: backpack Operating System: Unknown/Multiple | Architecture: Type of failure: GHC accepts | Unknown/Multiple invalid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D3254 Wiki Page: | -------------------------------------+------------------------------------- Changes (by bgamari): * status: patch => closed * resolution: => fixed Old description:

hs-boot files permit a type class to be given "abstractly", in which case any implementation of the type class is permissible. But it does not reject instances defined for such a class.

{{{ -- A.hs-boot module A where class C a

-- B.hs module B where import {-# SOURCE #-} A instance C Int where

-- A.hs module A where import B class C a where f :: a

-- Main.hs import A main = print (f :: Int) }}}

I get this when I build with `--make`:

{{{ ezyang@sabre:~$ ghc-head --make C.hs -fforce-recomp [1 of 4] Compiling A[boot] ( A.hs-boot, A.o-boot ) [2 of 4] Compiling B ( B.hs, B.o ) [3 of 4] Compiling A ( A.hs, A.o ) [4 of 4] Compiling Main ( C.hs, C.o ) Linking C ... ./B.o:(.data+0x0): undefined reference to `A_CZCC_con_info' collect2: error: ld returned 1 exit status }}}

New description: hs-boot files permit a type class to be given "abstractly", in which case any implementation of the type class is permissible. But it does not reject instances defined for such a class. {{{#!hs -- A.hs-boot module A where class C a -- B.hs module B where import {-# SOURCE #-} A instance C Int where -- A.hs module A where import B class C a where f :: a -- Main.hs import A main = print (f :: Int) }}} I get this when I build with `--make`: {{{ ezyang@sabre:~$ ghc-head --make C.hs -fforce-recomp [1 of 4] Compiling A[boot] ( A.hs-boot, A.o-boot ) [2 of 4] Compiling B ( B.hs, B.o ) [3 of 4] Compiling A ( A.hs, A.o ) [4 of 4] Compiling Main ( C.hs, C.o ) Linking C ... ./B.o:(.data+0x0): undefined reference to `A_CZCC_con_info' collect2: error: ld returned 1 exit status }}} -- Comment: This was resolved in bba004f2a0642d3bb8c8876543aaa1a48a2f9a43. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13068#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler