The absolute killer app is compilers, static analyzers, and similar. Other languages don't even come close for anything in that space.
However, I think that purely functional languages are useful for basically everything. Haskell is my general purpose language of choice, unless libraries or system constraints force me to another language.
However, I think that purely functional languages are useful for basically everything. Haskell is my general purpose language of choice, unless libraries or system constraints force me to another language.