I'm currently using Haskell to do supply chain optimization at Target. It's pretty awesome! The language lets us express our models in ways that feel natural.

I'm currently using Haskell to do supply chain optimization at Target. It's pretty awesome! The language lets us express our models in ways that feel natural. We're both solving some specific problems (ie inventory control, network/transport optimization) and building a pretty general framework for expressing, analyzing and solving large stochastic optimization problems. Haskell is a great fit because it's extremely expressive and lets us write and reason about our problems in a natural, expressive way. It helps that we have a very mathy team in a mathy field :). Our team also has a bunch of people with programming language research backgrounds, and our long-term remit is to apply programming language ideas to operations research. Turns out that—for a whole class of PL problems and PL people—Haskell is the perfect language :). I also know people use Haskell at Facebook (already mentioned), Google (Ganeti and some unnamed X project), a few banks (JP Morgan, Barclays and Standard Chartered) as well as a whole bunch of startups and small companies.