I spent the last 18 months getting the grip of functional programming, starting with learning OCaml and for some weeks now Haskell. Now I want to take the next step and implement some actual application: A simple realtime terrain editor. I've written numerous realtime terrain rendering engines, so this is a familiar topic. And the used recursive algorithms and data structures seem very fit for a functional implementation.
With this being a realtime application I'm naturally looking for the best performance I can get. Now some proponents of OCaml quite frequently complain about Haskell being slow compared to OCaml or F#. But according to the The Computer Language Benchmarks Game Haskell oftenly beats OCaml, if only by rather small fractions. There remains the problem, that this benchmark takes only very specific samples.
The right thing to do would be to, of course, implement the program in both languages and compare them, but I simply don't want to do double the work.
But maybe other people did comparable applications in OCaml and Haskell and give some statistics?
By all accounts, both OCaml and Haskell have sufficiently performant compilers and runtimes for almost anything. Picking between them on the basis of pure performance seems silly to me. You've come this far -- moving away from the obviously most low-level and performant languages (C, C++, etc.) in the name of clearer, more succinct, more expressive, higher-level code. So why, when the performance differences involved are much smaller, switch to that criteria now?
I'd go with some broader criteria -- if you want pervasive parallelism, then Haskell's the better choice. If you want genuinely pervasive mutation, then OCaml is better.
If you want only very coarse parallelism at best, and you intend to stick with mostly functional structures, then pick based on something else, like syntax (I think Haskell is much nicer here, but that's subjective) or available libraries (Haskell wins on quantity/availability, but OCaml might edge it out in the graphics department nonetheless).
I don't think you'll go wrong either way