Haskell package installed but not found

Question

I installed diagrams, and it seems to be there, but GHCi doesn’t find it. I tried adding the local sandbox to the command line (-package-db), but still no luck. Any suggestions?

   C:\Users\guthrie>
   C:\Users\guthrie>cabal install diagrams
    Resolving dependencies...
    All the requested packages are already installed:
   diagrams-1.2
   Use --reinstall if you want to reinstall anyway.

I find it in:

  C:\Users\guthrie\.cabal-sandbox\i386-windows-ghc-7.6.3-packages.conf.d
        (diagrams-1.2, diagrams-contrib, -core, -lib, -svg)

But running: “cabal repl” or using the GHC(i) flag “-package-db=…” fail to find it:

C:\Users\guthrie>cabal repl
GHCi, version 7.6.3: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Prelude> :m + Diagrams.Prelude

<no location info>:
    Could not find module `Diagrams.Prelude'
   It is not a module in the current program, or in any known package.
Prelude>

To clarify; ignoring the cabal invocations, using GHC/i directly, and the program diagramsDemo.hs:

--  http://projects.haskell.org/diagrams/doc/quickstart.html
--
import Diagrams.Prelude
import Diagrams.Backend.SVG.CmdLine

main = mainWith (circle 1 :: Diagram B R2)

Gives:

C:\Users\guthrie\Desktop\xFer\Graphics>ghc --make diagramsDemo.hs

diagramsDemo.hs:7:8:
    Could not find module `Diagrams.Backend.SVG.CmdLine'
    Use -v to see a list of the files searched for.

C:\Users\guthrie\Desktop\xFer\Graphics>ghc --make diagramsDemo.hs -package-db=C:\Users\guthrie\.cabal-sandbox\i386-windows-ghc-7.6.3-packages.conf.d

diagramsDemo.hs:7:8:
    Could not find module `Diagrams.Backend.SVG.CmdLine'
    Use -v to see a list of the files searched for.

If the diagrams package isn't specified as a dependency in your .cabal file it won't be available from cabal repl (a mis-feature, IMO). You can get around this with ghci -no-user-package-db -package-db=./.cabal-sandbox/i386-windows-ghc-7.6.3-packages.conf.d, which I've made an alias for in my install of cygwin bash. — bheklilr
– bheklilr, Commented Feb 20, 2015 at 15:00
Thanks - but ignoring cabal-repl approach, it seems like ghci -package-db=... should be enough for ghc/i to find it. I have the same application on an adjacent machine, and it works fine, finds any/all installed packages. So teh Question is why the -db argument should not be enough to find it, when cabal install does. — guthrie
– guthrie, Commented Feb 20, 2015 at 19:51
When you use cabal repl, it loads the dependencies you've specified for your project. You can definitely install other packages into a sandbox, but that doesn't mean they're available to import. The -package-db flag adds an additional package database to scan when simply ghci is looking for packages, but I prefer the -no-user-package-db flag as well since it prevents conflicts with my system install of cabal. — bheklilr
– bheklilr, Commented Feb 20, 2015 at 19:54

ipsec · Accepted Answer · 2015-02-20 19:53:11Z

6

As bheklilr said, if ghci is started with cabal repl, it will only find packages specified as a dependency in the .cabal file.

However you can start it with cabal exec ghci, then it will find all packages installed in the sandbox.

The same is true for invoking ghc (cabal build vs. cabal exec ghc), but note that if you want to pass flags you have to use --, like in cabal exec ghc -- -O2 Main.hs. Alternatively you can use cabal exec bash and launch ghci or ghc in the new shell.

cabal exec was added with Cabal 1.20.

answered Feb 20, 2015 at 19:53

ipsec

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3 Comments

guthrie Over a year ago

Hmm, not sure if this really answers the issue - I am running ghc/ghci directly, not via cabal. and to have it find packages in the sandbox I added the -package-db=./.cabal-sandbox... option. I have a single pgm.hs file, and no .cabal file, and should not need one. This same procedure works on the adjacent machine, so the procedure seems correct - but the result fails on this machine.

ipsec Over a year ago

Yes, it should be possible to give some flags to ghci to find the packages in the sandbox. I don't know, why it does not work in this case. But I find it easier to use cabal exec if you are using a Cabal sandbox. This is also more robust in regard to future changes to the sandbox structure.

guthrie Over a year ago

Thanks - I did not know about "cabal exec ghc", will look more at that, but it looks to me like there is something messed up with my cabal databases in general. I find that 80% of problems and time wastes and failures with Haskell are cabal related, and I have to just periodically rebuild and reinstall all libraries, and even then lots of things fail (xxHaskell, gtk+, ...).

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