Conclusion

• Values of type IO a describe programs with side effects, which will eventually result in a value of type a.

• While we cannot safely extract a value of type a from an IO a, we can use several combinators and syntactic constructs to combine IO actions and build more-complex programs.

• Do blocks offer a convenient way to run and combine IO actions sequentially.

• Do blocks are desugared to nested applications of bind operators ((>>=)).

• Bind operators, and thus do blocks, can be overloaded to achieve custom behavior instead of the default (monadic) bind.

• Under the hood, IO actions are stateful computations operating on a symbolic %World state.

What's next

Now, that we had a glimpse at monads and the bind operator, it is time to in the next chapter introduce Monad and some related interfaces for real.