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Unfortunately this isn't possible. Haddock can produce documentation with hyperlinks to entities in another package, but it cannot include full documentation for entities which are re-expored from a different package. This would require essentially keeping a copy of the entire documentation for the package in the interface file.

So what is stored in a Haddock interface file?

For each module, a list of the identifiers that are exported and their original names. The same information is stored in GHC interface files, but it would be a fair bit of work to make Haddock read GHC's interface format.

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Are there standard Haddock-interfaces for the standard libraries?

A GHC distribution comes with Haddock interfaces for all the packages that it ships with.

Where? In the Debian package ghc 6.2-0.woody.1 I can only find the .hi files, and that's not the same thing, is it?

Perhaps the Debian packages don't include the files? You can grab them from the Linux binary tarball anyway. Cheers, Simon