找不到模块“Control.Parallel”

Question

I'm following this guide and I got to the "Write your first parallel Haskell program", where you have you use Control.Parallel . When i try to compile it, I get:

    A.hs:1:1: error:
    Could not find module `Control.Parallel'
    Use -v (or `:set -v` in ghci) to see a list of the files searched for.
  |
1 | import Control.Parallel
  | ^^^^^^^^^^^^^^^^^^^^^^^

Nowhere does it say that I have to install anything. Despite that, I ran cabal install --lib parallel and it installed okay but It still cannot find the package. When I run ghc-pkg list parallel is not on the list.

How do I go about solving that?

Answer 1

You can do either of two things here:

1. You can start a new project using cabal, and add parallel as a dependency. To do so, I'll refer you to Cabal's quickstart guide .
2. Alternatively, you can just skip the parallel package. In the example you listed, you need to import Control.Parallel to make use of par and pseq . However, these functions are also part of GHC.Conc , which is part of the base package. So, to get your program to work without any package management involved, simply replace import Control.Parallel by import GHC.Conc , and you should be good.

Answer 2

Do not use cabal install or stack install to manage your dependencies.

Instead use cabal or stack files to list all of those packages, then use "build" commands to fetch them into your project folder.

Both cabal and stack will then supply proper flags to your dependencies when compiling with GHC.

Alternatively, find out where did cabal downloaded that source code and pass it with flags to GHC. Thought that's unnecessary low level work.