You actually don't need types or classes to do this. You could use design by contract, which is what I've done in Python and Perl, neither of which have very fancy type systems compared to something like Haskell.
With design by contract you can put in whatever fancy constraints you want on function parameters and return values, and those will be enforced.
As far as objects go, they're much more useful for me as just a means of passing state. Rather than using a bunch of global variables or having to pass in a ton of function arguments, I can just use an object which contains all the state I need.
Of course, having lots of state can usher in its own set of problems, and there's something to be said for trying to make your code as stateless as possible. But sometimes you need state, and maybe even a lot of it.
With design by contract you can put in whatever fancy constraints you want on function parameters and return values, and those will be enforced.
As far as objects go, they're much more useful for me as just a means of passing state. Rather than using a bunch of global variables or having to pass in a ton of function arguments, I can just use an object which contains all the state I need.
Of course, having lots of state can usher in its own set of problems, and there's something to be said for trying to make your code as stateless as possible. But sometimes you need state, and maybe even a lot of it.