I'm not particularly into Haskell or functional languages, but I came here to say it warms my heart to hear that people in finance actually embrace it (also J/APL). It seems like a good choice for banking.

The Mercury site also looks way better than most other banks I have ever used (load speed is also very good.) On the danger of seeming like a shill (I'm not), I'm tempted to try them out.

Do it. You won’t be disappointed. I’ve been using them for about 5 years now.