Yup that worked. Sort of an obscure error message to communicate "package not found"....

Is there a convention so I know which package I need for which module? My guess is sometimes (but randomly) you need to add a package on the command line corresponding to the first part of the qualified module name. e.g. I just figured out that

-package text worked to get me Text.ParserCombinators.Parsec

OTOH, if this is the rule, it seems strange that GHC can't go and find the packages itself.

As far as it can, GHC does go and find the packages itself. There's one situation it can't: when doing batch linking from the command line, because then GHC doesn't know which modules were imported by the program. If you use GHCi or --make, then GHC figures out the package dependencies for itself. To answer your question, the package for each module is listed in the documentation: each module has the package name in the title at the top, and the contents page also lists the package name alongside each module. Incedentally, -package parsec is the right way to get Text.ParserCombinators.Parsec. Cheers, Simon