[Haskell-cafe] Another question about unsafePerformIO

I have a program that optimizes train schedules. It employs an external solver for Integer Linear Programs. The solve function has the following type:

solve :: Constraints -> IO (Maybe Solution)

And this works. However, my external solver also behaves like a pure function from input to output. I wonder whether this guarantee should be reflected in the type system. I'd also appreciate if the compiler would be able to eliminate some calls to the solver.

solvePure :: Constraints -> Maybe Solution solvePure = unsafePerformIO . solve

Is this a good idea?