I'm not sure where this idea that it is "mostly used for sudoku solvers" comes from. While clearly insulting, it is also clearly false.
The breadth of code on http://hackage.haskell.org/ gives witness to this, with just on 1000 libraries written over the last 18 months, most heavily weighted towards networking, graphics, guis, databases, and so on.
Similarly, the industrial users , http://haskell.org/haskellwiki/Haskell_in_industry , aren't writing sudoku solvers, but are finance houses, defense contractors, game companies, bioinformatics places, hardware dessign, a full range of applications, for a general purpose language.
Sorry, I didn't mean to imply that Haskell is only used for writing toy programs. I meant that Haskell people gravitate toward problems that are functional in nature. Many such programs are useful, though rarely in isolation.
If you're wondering where the idea comes from, here's an example. There is a thread on Facebook titled "What have you made using Haskell?"
6. A lambda expression evaluator with alpha equivalence testing
7. A hyperbolic raytracer
8. An image regression program
9. A program that finds equidistant letter sequences
10. A boolean expression to truth table converter
11. A resolution theorem prover
12. An interpreter of a Prolog basic subset
13. A Markov Chain generator
14. An anagram generator
15. A library for doing computation inside various algebraic structures
16. A maze generator and solver
Some of these are exercises, some are toys, some look potentially useful. But except for the first two, they are all pure-functional problems, and are rather abstract. Maybe this sample isn't representative of all Haskell usage or what Haskell is capable of, but the stereotype, like most stereotypes, exists for a reason.
Some of those are pretty big companies - clearly they're not using it for everything, but have decided it's the best choice for some particular problem domain. Sure, it's a general purpose language, but where's the "sweet spot"? I could write scientific number crunching stuff in Ruby, but that's definitely not its forte.
The breadth of code on http://hackage.haskell.org/ gives witness to this, with just on 1000 libraries written over the last 18 months, most heavily weighted towards networking, graphics, guis, databases, and so on.
Similarly, the industrial users , http://haskell.org/haskellwiki/Haskell_in_industry , aren't writing sudoku solvers, but are finance houses, defense contractors, game companies, bioinformatics places, hardware dessign, a full range of applications, for a general purpose language.