#15129: Expose ghc-pkg internals as a library -------------------------------------+------------------------------------- Reporter: ryanreich | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: 8.6.1 Component: Core | Version: 8.2.2 Libraries | 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: -------------------------------------+------------------------------------- The module [https://downloads.haskell.org/~ghc/latest/docs/html/libraries /ghc-boot-8.4.1/GHC-PackageDb.html GHC.PackageDb], which is in the `ghc- boot` package and therefore shipped alongside `base`, and which exposes the crucial data type `InstalledPackageInfo a b c d e f g` and functions `readPackageDbForGhc`, `readPackageDbForGhcPkg`, and `writePackageDb`, is nonetheless crippled outside of GHC: - The first function requires instances of the `BinaryStringRep` and `DbUnitIdModuleRep` classes, which do not have any in any public package - The second one uses a polymorphic parameter that, in order to run without error, actually needs to be [https://downloads.haskell.org/~ghc/latest/docs/html/libraries/Cabal-2.2.0.0 /Distribution-InstalledPackageInfo.html Distribution.InstalledPackageInfo], defined in the completely different `Cabal` package neither mentioning nor mentioned by `ghc-boot` - The third one takes two arguments of which the first (a list of `InstalledPackageInfo a b c d e f g`) is redundant because it can be constructed from the second, but no conversion function is defined. While it is possible to write both the instances and conversion function by hand, it is extremely awkward and I worry that it is also fragile, given the internal nature of this entire module. The full functionality of this package is only available within the [https://github.com/ghc/ghc/blob/master/utils/ghc-pkg/Main.hs Main] module of `ghc-pkg`, which is highly monolithic despite containing such [https://github.com/ghc/ghc/blob/master/utils/ghc-pkg/Main.hs#L1226-L1307 generally useful code] as the above instances and conversion function. Since `GHC.PackageDb` provides so much of `ghc-pkg`'s essential functionality as a library, it seems reasonable to request that this code be brought out of the opaque `Main` module and into a public library. I understand from the comments that `ghc-boot` is not the place for this, but perhaps one of the following is: a. `Cabal`, because it already has the `InstalledPackageInfo` (no parameters) type used by `ghc-pkg`; however, `Cabal` seems committed to calling `ghc-pkg` as an executable, not providing linkage to its internals. b. a separate `ghc-pkg` library containing, possibly, a more carefully chosen selection of that executable's code. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15129 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler