06 July, 2015

A Haskell reddit bot.

I am one of many many moderators on reddit's r/LondonSocialClub. This is a place for organising social gatherings in London.

Post titles usually take the form [DD/MM/YY] Event @ Place. Other moderators have fiddled with the CSS for this subreddit to give us a big red TODAY sticker next to today's events, and grey out events that are in the past. This uses reddit's flair mechanism, which allows assigning of labels to posts, and CSS styling based on a post's flair.

Unfortunately, this was not entirely automated - some sucker or other had to go in each day and adjust flair on the relevant posts to match up with reality. This bothered me as being a manual process that should be fairly easily automated. Eventually it bothered me enough that I wrote a bot, lsc-todaybot, to do it. Now the moderation logs make it look like I come home from the pub every day and move everything around before going to sleep.

Another motivation for writing this bot was it seemed small enough in scope that it would be achievable, but give me a chance to learn a few new APIs: several new Haskell libraries, and the reddit REST API.

HTTP: I've previously used HTTP when hacking at cabal. This doesn't do HTTPS (I think) and the maintainer told me to not use it. So I tried wreq. It was easy enough to get going and there was a tutorial for me to rip off.

Configuration: I used yaml to parse a YAML configuration file.

Lenses: I still haven't got a good grasp on what is happening with lenses but I used them in a few places, and it has developed my understanding a little bit: lsc-todaybot extracts fields from reddit's JSON responses using aeson-lens. yaml exposes the parsed configuration file as JSON, so the same lenses can be used for extracting configuration details. wreq also uses lenses for setting HTTP header values and the like.

Strings: I seem to have ended up using several different string classes, which is icky - ByteString, Text and String at least. I've made the source code for that more generic by using the generic monoid <> operator to concatenate them which makes things a bit less horrible looking.


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