#15391: Maybe ghc-pkg register should unregister packages with "incompatible" signatures -------------------------------------+------------------------------------- Reporter: ezyang | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: ghc-pkg | Version: 8.4.3 Keywords: backpack | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Consider the following situation: 1. I register package 'p-0.1-inplace' instantiated with 'P = base:Data.Map' 2. I register package 'p-0.1-inplace' instantiated with 'P = base:Control.Monad' Even though the IPID of these two packages is the same, we clearly want to keep both of them in the database. This thus motivates the following lines in ghc-pkg {{{ removes = [ RemovePackage p | not multi_instance, p <- packages db_to_operate_on, mungedId p == mungedId pkg, -- Only remove things that were instantiated the same way! instantiatedWith p == instantiatedWith pkg ] }}} OK... now consider the following situation 1. I register package 'p-0.1-inplace' instantiated with 'P = base:Data.Map' 2. I register package 'p-0.1-inplace', instantiated with nothing (I have edited 'p' such that it has no more signatures) So... ghc-pkg is going to keep both of these packages (per the logic above). But is that really what we want? Now the package database is in a half-consistent state, where the instances of 'p-0.1-inplace' are not consistent with the number of holes they are supposed to have. So let us suggest an invariant: INVARIANT: All packages which have the same IPID in a package database are self-consistent with each other. OK, so given this invariant, what do we have to do in step 2? It would seem that when we get a prospective package to register, we must *find all packages which are incompatible with it*, and remove them from the package database. So, given a package with a set of holes H, we must remove all packages with holes H' such that H = H' I'm not completely convinced this is a good idea. Usually ghc-pkg only removes a single package in response to a new registration, and this behavior could result in lots of packages getting unregistered, more than you might expect. On the other hand, if you think of the package database in a more hierarchical manner, it makes sense that changing the root registration would invalidate all the children. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15391 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler