#9242: Implement {-# OVERLAPPABLE #-} and {-# INCOHERENT #-} pragmas -------------------------------------+------------------------------------- Reporter: simonpj | Owner: diatchki Type: feature | Status: new request | Milestone: Priority: normal | Version: 7.8.2 Component: Compiler | Keywords: Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: | Related Tickets: None/Unknown | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by dterei): I'd like to propose we remove the `OVERLAPS` pragma. Why? == Long Version == https://ghc.haskell.org/trac/ghc/wiki/SafeHaskell/NewOverlappingInstances#Ne... --a.k.aInstanceSpecificPragmas == Short Version (kind-of) == The security implications of OVERLAPPABLE vs. OVERLAPPING are fairly different. Remember, in Safe Haskell we apply a policy of only allowing instances from a module M compiled with `-XSafe` to overlap other instances from module M. If it overlaps (and is the most specific overlap) instances from modules other than M then we don't allow this to succeed. This is done to ensure that untrusted code compiled with `-XSafe` can't alter the behavior of existing code, some of which may be part of the TCB and security critical. Brining the new finer grained pragmas into the story we get the following: * OVERLAPPABLE is the programmer communicating that they can be overlapped, an open instance if you will. We want to relax the above restriction and allow instances from `-XSafe` modules to overlap instances from their own module AND instances declared OVERLAPPABLE that reside in any module. * OVERLAPPING is the programming simply declaring they may overlap less specific instances. We want to keep the above restriction for these instances. That is, a instance I1 from a `-XSafe` module M won't be able to overlap as the most specific instance, a instance I2 from another module if I2 is marked as OVERLAPPING. This distinction enables new encodings in Safe Haskell by allowing security library authors to distinguish how untrusted code can overlap their instances. In some way giving them open vs closed instances. This distinction is subtle and important. Having a pragma OVERLAPS that implies both glosses over this and will encourage developers to use this without much thought. ## Safe Inference We can also safely infer a module that only has OVERLAPPABLE instances as safe, while ones that contain OVERLAPPING or OVERLAPS instances must be regarded as unsafe since there is a difference in semantics of these pragmas under Safe vs Unsafe. So we also have an advantage if developers are more specific about what they want, than just defaulting to OVERLAPS. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9242#comment:30 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler