Recently on haskell-cafe I asked if there is an easy way to get the Haddock documentation url for a given identifier in Haskell. I discovered that it’s not completely straightforward, for example if we look up String using ghc-pkg we are told that it’s defined in GHC.Base:

$ ghc-mod info foo.hs Foo String
type String = [Char]     -- Defined in `GHC.Base'

Then we can find the Haddock url for the base package:

```$ ghc-pkg find-module GHC.Base /home/carlo/opt/ghc-7.6.3_build/lib/ghc-7.6.3/package.conf.d base- /home/carlo/.ghc/x86_64-linux-7.6.3/package.conf.d

$ ghc-pkg field base- haddock-html haddock-html: /home/carlo/opt/ghc-7.6.3_build/share/doc/ghc/html/libraries/base-

so we'd expect to be able to view 

/home/carlo/opt/ghc-7.6.3_build/share/doc/ghc/html/libraries/base- ```

but this file doesn’t exist because GHC.Base is an
internal module and does not have a Haddock page. This happens with some other packages too, not just GHC.Base.

In the haskell-cafe thread, Roman Cheplyaka pointed out that I should be asking for where a thing is imported from, as opposed to defined. For the String example we would prefer to send the user to the Haddock page for the Prelude.

This turns out to be a bit tricky as well, but I think I have a reasonable prototype working now, which I have unimaginatively called ghc-imported-from. There are some very rough edges, for example parsing of language extensions and GHC options, but these should be easy to fix either by following how ghc-mod does the same thing, or perhaps by adding my code to a fork of ghc-mod. We’ll see.

Here’s a short screencast of ghc-imported-from in action, along with my forked version of ghcmod-vim:

Make sure that the video plays fullscreen in 720p otherwise the text is illegible.