One word says more than a thousand pictures: Vim http://www.vim.org/. (well, okay, I'm sure Emacs will do just as well, and some of the more recent IDEs seem to be catching up;-) plus plugins, of course!-) - unfolding definitions: if you really want that, it is in the domain of program transformation systems and refactorers (HaRe, the Haskell refactorer, has been mentioned - it worked on Haskell'98 sources, plugging into Vim or Emacs; it would really be great to have funding for porting that to a modern GHC/Cabal-based environment, but if you're happy with Haskell'98, and have all the sources, the old HaRe should still do the job once you get it to build with recent GHCs/libraries) - looking up definitions: that is supported in various ways in Vim/Emacs and the like - I'll talk about some Vim examples, as that is what I use. - tag files (generated by running tools like 'ghc -e :ctags', hasktags,.. over the sources) are a simple database linking identifiers to definition sites. Based on these, one can jump from identifiers to definitions (keeping a stack of locations, so one can go back easily), or open split windows on the definition sites. See the "Moving through programs" section in Vim's help, also online at: http://vimdoc.sourceforge.net/htmldoc/usr_29.html . - the haskellmode plugins for Vim support documentation lookup (opening the haddocs for the identifier under cursor in a browser), and the documentation provides source links, if the docs themselves aren't sufficient. Useful for all those sourceless package installations. - the haskellmode plugins also support type tooltips (or, if you don't like tooltips, or are working in a terminal without gui, type signatures can be displayed in the status line, or added to the source code). This is currently based on GHCi's :browse!, though, so you can only get the types of toplevel definitions that way. One of the insertmode completions also displays types. - if you explain Haskell's import syntax to Vim, you can also search in (local) imported files, using Vim's standard keyword search, for instance ([I). The haskellmode plugins for Vim are currently in the process of moving to http://projects.haskell.org/haskellmode-vim/ . Which made me notice that I hadn't updated the publicly available version in quite some time (announcement to follow when that process has settled down somewhat). Claus