Why is the package hidden by default? And how can I “display” it?

Question

I am running Ubuntu 10.10 and I have a transformers module installed through the Ubuntu libghc6-transformers-dev package. For some reason, this package is hidden by default:

 ghc --make -i./src/ src/fastcgi.hs -o myapp.fcgi src/MyApp/Webapp.hs:6:7: Could not find module `Control.Monad.IO.Class': It is a member of the hidden package `transformers-0.2.1.0'. Use -v to see a list of the files searched for. 

So my first question is “why?”. And my second question: what is the proper way to “display” this module (without having to explicitly specify the module through the command line)? And is this a good / bad idea?

Note. I can get ghc to compile by passing the package name explicitly, for example:

 ghc --make -package transformers -i./src/ src/fastcgi.hs -o myapp.fcgi 

Answer 1

Use the ghc-pkg tool from the command line:

 ghc-pkg expose transformers 

Why it was hidden by default, I do not know. This may be something to discuss with the accompanying Ubuntu packages.

Besides,

 ghc-pkg help 

will tell you much more about this program.