CODEWITHSUNDEEP
haskellghccabal
preferred_anon
preferred_anon

Reputation: 547

Cabal-installed Modules won't import

There are several things I'm confused about here, so I'll try to explain each of them as clearly as I can.

I've been trying to install the diagrams package for haskell, using cabal. I've seen it suggested to install packages using sandboxes, so that's what I did. Something that's not clear to me is exactly what a sandbox is - I understand that I can initialise one with cabal sandbox init and install packages inside of it with cabal install, but I don't see how to use those packages once they're installed.

I then tried to compile a test-script using ghc, which resulted in the following error: 

diagramstutorial.lhs:3:10:
    Could not find module 'Diagrams.Prelude'
    Use -v to see a list of the files searched for.

With a similar error for another module that the script was supposed to load. These modules are definitely both included in the diagrams package, and cabal seems happy that the package is installed correctly. I expect there's something simple I just don't understand, but I don't know what it is.

Upvotes: 0

Views: 1569

Answers (2)

Arnon
Arnon

Reputation: 2237

  1. Find the sandbox directory, and locate the packages.conf.d file. For example, /home/user/.cabal-sandbox/x86_64-linux-ghc-7.8.4-packages.conf.d

  2. Rerun your GHC commands with the package-db flag:

    ghci -package-db /home/user/.cabal-sandbox/x86_64-linux-ghc-7.8.4-packages.conf.d --make diagramstutorial.lhs

Everything should now work

Upvotes: 1

duplode
duplode

Reputation: 34378

I typed ghc --make diagramstutorial.lhs to compile it

That will make GHC use the regular user package database (that is, not the sandbox one). Use cabal exec -- ghc --make diagramstutorial.lhs instead, so that GHC runs in the context of your sandbox.

You can also use GHCi within the sandbox with cabal repl. And naturally, if/when you start preparing a cabal package, all cabal commands (cabal build, etc.) will use the sandbox if you are within its directory.

Something that's not clear to me is exactly what a sandbox is

A set of packages with an accompanying database local to the directory. Beyond the cabal.sandbox.config configuration file there is also a hidden directory .cabal-sandbox, in which diagrams and the other packages you installed lie.

Upvotes: 2

Related Questions