I often find myself wondering what the 'killer app' of purely functional programming languages is. For the longest time I assumed they would become much more popular as multi-core cpus proliferated, but pretty much every purely functional language out there is relegated to a dusty corner.
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.
