Koka is a function-oriented language that separates pure values from
side-effecting computations. In particular, the potential side-effects
of a function are apparent in its type signature. In contrast to most
effect systems that are meant for internal compiler optimizations, our
system is designed to be used by the programmer. The type and effect
of expressions can be inferred automatically using a polymorphic type
inference system based on Hindley-Milner style inference. A novel
feature of our effect system is that we support polymorphic effects
through row-polymorphism which leads to types that are constraint free
and easier to understand and write. The system has been used
successfully to infer effect on various small to medium-sized sample
programs.

Try it out live at: http://www.rise4fun.com/koka/tutorial