Question

I’m currently trying out web development frameworks for haskell and I recently came across yesod. It seemed pretty interesting so I installed it using cabal, however I’m not able to run the development server. Following their getting started instructions here’s the result:

$ yesod init
$ cd mysite
$ yesod devel

Configuring mysite-0.0.0...
Testing files...
Rebuilding app
yesod: bind: resource busy (Address already in use)
Preprocessing library mysite-0.0.0...
Preprocessing executables for mysite-0.0.0...
Building mysite-0.0.0...

Controller.hs:16:7:
    Could not find module `Data.Dynamic':
      It is a member of the hidden package `base'.
      Perhaps you need to add `base' to the build-depends in your .cabal file.
      It is a member of the hidden package `base-3.0.3.2'.
      Perhaps you need to add `base' to the build-depends in your .cabal file.
      Use -v to see a list of the files searched for.
Testing files...
Testing files...
^^ above line just keeps repeating...

I’m assuming it has something to do with the Data.Dynamic module but I don’t know how to go about fixing it.

Additional Info

Running Ubuntu 10.10 Maverick

ghc version:

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 6.12.1

Answer 1

I haven’t run into this specific issue, but the error message looks like it’s a simple question of GHC being unable to find version 3.0.3.2 the package “base.” This version has been buildable since GHC 6.9, so you should have it. Try running the following command:

ghc-pkg check

This will tell you if there is something wrong with your packages. Cabal can be a bit of a nightmare for dependencies — partly, it seems, because a lot of Haskell developers underestimate the extent to which their underlying libraries will shift in the future. So they will define a dependency as “>= [version of package x]” without limiting the max version to the one presently available. Or they just leave out version-limiting altogether.

Yesod, I’m happy to say, doesn’t fall into this trap. But several of the libraries it depends on do. When you start developing in Haskell, learn this lesson: never assume that future versions of a library won’t break your code. They will. A lot.

If ghc-pkg comes up with broken packages, you may need to clean up/uninstall/reinstall these packages until they are either cleaned up or hidden. (Just do ghc-pkg hide [package name] to tell ghc to ignore that package.

Your next problem is that hidden base package. Try the following:

ghc-pkg list | grep base

If you see brackets around the library, that means it’s hidden. The package base-3.0.3.2 might show up as hidden (although that’s a bit unlikely, as that’s where the backward-compatible Prelude lives). If it is hidden, try to unhide it with the following command:

ghc-pkg expose base-3.0.3.2

Now try re-running yesod devel and see how it goes. Best case scenario is that it works. If not, let us know.