I don't think IO is unidiomatic at all, although 7 years ago I might have agreed. First if we really need to we can write haskell that looks a lot like C. Secon

I don't think IO is unidiomatic at all, although 7 years ago I might have agreed. First if we really need to we can write haskell that looks a lot like C. Secondly if I find myself in a situation where I need to write a little block of code that just doesn't lend itself to being written in Haskell I have no problem writing it in C. If I was implementing some numerical algorithm this is what I would do. Indeed haskell bindings to things like BLAS to just this. The benefit of marking all side effects with IO is an easy to read codebase, easy refactoring and of course Software Transactional Memory!