I spent 7 months trying to build a json api using yesod with a friend. We had nothing but headaches. Its insanely hard to find example code or search issues when using yesod. Hardly anything exists about it on stack overflow already so any time I had an issue I had to post it on SO and wait a day for someone to answer it which meant I could only do about an hour of programming a day. I ended up giving up used RoR and replicated more than the 7 months worth of work in a few weeks.

There is a very good reason RoR is far more popular than haskell web apps. Its just so easy to get started with rails, there is a near infinite amount of information online.