I do research into parsing algorthms, and find that I can convert the theory into practice most easily in Haskell. A lot of the material is just (careful) transliteration of the original definitions. And more generally I go along with Vincenzo (aka Nick Name),

I use haskell when I have to write a program myself and quickly.

but add to that "and robustly". I do write the occasional GUI in it, but mainly as an exercise to show I can do it in Haskell. In practice I write GUIs in C. But, even when I don't write in Haskell, I still use habits I've picked up from Haskell in other code. I think that practice in Haskell can improve clarity of programming in other languages. Regards, Bruce.