unregister dup packages in cabal store

instructions.md

Installing a library explicitly via cabal v2-install lib:blah installs it in the package-db and exposes it. If there are multiple exposed instances of a library available, GHC chooses an unspecified instance (ref). Therefore it is sometimes necessary to manually hide or unregister conflicting versions of packages.

Look for library package-id in cabal install output:

Installing library in /home/ftweedal/.cabal/store/ghc-8.4.4/incoming/new-8790/home/ftweedal/.cabal/store/
ghc-8.4.4/purebred-icu-0.1.0.0-c850b6a0c804167cfc4542bce128a8714de643dcfd6a7114ff42f7e11986c81d/lib

Run ghc-pkg to find all registered packages of the given name:

% ghc-pkg --package-db ~/.cabal/store/ghc-8.4.4/package.db list --verbose purebred-icu            
GHC package manager version 8.4.4 
Timestamp 2019-08-08 02:10:49.304936504 UTC for /home/ftweedal/.cabal/store/ghc-8.4.4/package.db/package.cache 
using cache: /home/ftweedal/.cabal/store/ghc-8.4.4/package.db/package.cache 
Timestamp 2019-05-14 06:58:23.128264739 UTC for /usr/lib64/ghc-8.4.4/package.conf.d/package.cache
using cache: /usr/lib64/ghc-8.4.4/package.conf.d/package.cache
db stack: ["/home/ftweedal/.cabal/store/ghc-8.4.4/package.db","/usr/lib64/ghc-8.4.4/package.conf.d"]
flag db stack: ["/home/ftweedal/.cabal/store/ghc-8.4.4/package.db"]
/home/ftweedal/.cabal/store/ghc-8.4.4/package.db                                                         
    purebred-icu-0.1.0.0 (purebred-icu-0.1.0.0-c850b6a0c804167cfc4542bce128a8714de643dcfd6a7114ff42f7e11986c81d) 
    purebred-icu-0.1.0.0 (purebred-icu-0.1.0.0-dfff5a8c4748d5ee4ab94c008689f30f78a3bf23a528dcf65e8520b89d4064ae)

Unregister or hide the "old" versions of the package:

% ghc-pkg --package-db ~/.cabal/store/ghc-8.4.4/package.db hide --unit-id \
    purebred-icu-0.1.0.0-dfff5a8c4748d5ee4ab94c008689f30f78a3bf23a528dcf65e8520b89d4064ae

Hiding a package makes it unavailable for import (during compilation), but it will continue to work if it is a dependency of some other package. The --unit-id flag is essential. Without it, the hide or unregister operation will ignore the unit hash and affect every instance of the given package and version.