为什么 ghc (Haskell) 编译器找不到我导入的模块 Data.Vector？ 我收到错误：找不到模块“Data.Vector”

Question

vector module has been installed by "cabal install vector"

bash-3.2$ ghc-pkg list -f $HOME/.cabal/store/ghc-9.2.1/package.db 
/Users/gcolpitts/.cabal/store/ghc-9.2.1/package.db
    primitive-0.7.3.0
     vector-0.12.3.1 

but the compiler can't find it:

$ ghc prob214ff.hs
[1 of 1] Compiling Main             ( prob214ff.hs, prob214ff.o )

prob214ff.hs:14:1: error:
    Could not find module ‘Data.Vector’
    Perhaps you meant Data.Functor (from base-4.16.0.0)
    Use -v (or `:set -v` in ghci) to see a list of the files searched for.
   |
14 | import Data.Vector

Thx @n. 1.8e9-where's-my-share !

 ghc -package-db $HOME/.cabal/store/ghc-9.2.1/package.db  prob214ff.hs 

solves my problem although it is a regression from previous versions of ghc which didn't require the use of the -package-db option.

I'd love to use GHC_PACKAGE_PATH but I can't see how to make that work. Sec 5.9.5 of the ghc users guide is confusing. It states "A package database is where the details about installed packages are stored. It is a directory, usually called package.conf.d." ie not the package.db file that I gave with -package-db on the command line. But where is the package.conf.db directory with info on the vector package that I installed with cabal?

"ghc-pkg list" references /usr/local/lib/ghc-9.2.1/lib/package.conf.d but the output of the command doesn't list the vector package I installed.

"ghc-pkg list -f $HOME/.cabal/store/ghc-9.2.1/package.db" does list the vector package in it's output but doesn't tell me a package.conf.d directory that knows about the vector package.

Answer 1

This is a bit fiddly with cabal version 2. Installing packages no longer really makes sense (despite the cabal install still existing...). Instead you will probably find it easiest to create a cabal package instead, for example:

$ mkdir my-package
$ cd my-package 
$ cabal init
<Edit my-package.cabal and add vector to the build-depends line>
<Put the contents of prob214ff.hs in app/Main.hs instead>
$ cabal run

Answer 2

这将在 9.2.2 中修复，请参阅https://gitlab.haskell.org/ghc/ghc/-/issues/20660#note_391573