To reproduce:

1) Look for "data IO" on Hoogle, e.g. http://haskell.org/ghc/docs/6.12.1/html/libraries/base-4.2.0.0/Prelude.html#t%3AIO

2) Click "source", which redirects to

http://haskell.org/ghc/docs/6.12.1/html/libraries/base-4.2.0.0/src/GHC-Types.html#IO

3) Observe a "Not Found" page

Is there anyone interested in fixing this?

Bye,

Lenny