GSoC Hoogle: Week 3

This week I've travelled a further 600 miles by train, but am now starting to get settled for the Summer, and down to work on Hoogle.

My main focus this week has been getting Haddock to generate Hoogle databases. For Haddock 0.8 I added in a --hoogle flag to generate Hoogle databases, and a similar --hoogle flag to Cabal. Unfortunately, for Haddock 2.0, the feature was removed as most of the code got rewritten. Now I've added the feature back, making extensive use of the GHC API to reduce the amount of custom pretty-printing required, and to support more Haskell features. The code has been added to the development Haddock branch, and will be present in the next release.

Most of the challenge was working with the GHC API. It's certainly a powerful body of code, but suffers from being inconsistent in various places and poorly documented. I mainly worked with the code using :i to view the API. I got bitten by various problems such as the Outputable module exporting useful functions such as mkUserStyle :: QueryQualifies -> Depth -> PprStyle, but not exporting any functions that can create a Depth value, and therefore not actually being usable. If Hoogle and Haddock could be used over the GHC API, it would substantially improve the development experience!

I've also worked more on defining the database format. I am about to start work on the implementation today. I've also added a few more command line flags, but mainly as placeholders.

Next week: Database creation and text searches (looking back I see some similarity to last week!)

User visible changes: haddock --hoogle now works.