#12518: Allow customizing immutable package dbs by stacking -------------------------------------+------------------------------------- Reporter: harendra | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: Package | Version: 8.0.1 system | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- = Package Selection Across Multiple Package DBs = As explained by ezyang, when there are multiple package dbs, GHC chooses the packages to use in the following manner. I might have misunderstood so please correct me if I am wrong. * If there are multiple packages with different versions the latest non- broken version is chosen. * If the there are multiple packages with the same version the behavior is unspecified. * If there are multiple packages with the same package-id (shadowing) then the one which comes first in `GHC_PACKAGE_PATH` or which comes first on command line (`-package` flag) will be used. = The Problem = The build tool `Stack` implements stacked package databases. It uses a base package database and then stacks another package database on top of it to customise it further without modifying. The behavior is such that the package db on top of the stack completely overrides the ones below. That means you choose a package from top of the stack even if the version is older. Stack implements this by passing explicit package-ids of the packages to GHC. This scheme works well for cabal projects where we know ALL the packages used by the project in advance. But it does not work for scripts run using runghc. In that case we do not know the packages required by the script in advance and therefore cannot pass the package-ids to GHC. That means we cannot make GHC use the packages in the right way. GHC will choose the latest version even though we want it to choose a possibly older version from the top of the db stack. = Proposed Solution = Implement a new CLI option, something like `--stacked-pkg-dbs`. If this option is used GHC will use `GHC_PACKAGE_PATH` or the -package-db options as a stack of dbs. The first db in the path or the first CLI option will be considered the top of the stack. GHC will choose a package from the first db from the top of the stack irrespective of the version of the package. If the package is broken it should report error rather than silently choosing from the next db. This will allow us to modify an immutable package db by stacking another db on top. Implementing this as a separate option will keep the existing behavior so as to remain backward compatible. This has been discussed in a stack issue on github [https://github.com/commercialhaskell/stack/issues/1957 here]. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12518 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler