[oftenwrong]

If you application is interacting with the outside world, you will be dealing with side-effects, and sometimes error conditions will happen.

For example, today I am working on a service that:

- uses a database

- calls external APIs

- publishes and consumes from a message broker

- interacts with local and remote filesystems over a variety of protocols

All of these things entail error conditions, most of which throw exceptions in the corresponding libraries. Sometimes they are converted to Either/Maybe, and sometimes they are wrapped in "native" checked exceptions.

The important bits:

1. The type system and compiler make sure the programmer has to deal with the error conditions at some point. From a programmer's perspective, a checked exception bubbling up the stack is not very different from returning a monadic object up the stack.

2. The core of the application is entirely pure. No exceptions (in both senses of the word). All side effects are pushed to the boundaries.