The Reader Monad

Yet another exploration in Monadland. Like the State Monad, its sibling, Read Monad had managed to elude me until I came across an enlightening example in Debashish Gosh's excellent book, Functional and Reactive Domain Modelling. In the following example I'll describe a simple scenario where I'd usually use dependency injection and refactor it to a Reader monad using variant.

Version 1. Dependency Injection

When one tries to follow FP principles, she strives to build her application up like an onion. The core should contain pure functions, and all interaction with external services - DB, web service calls, user input, ... - , i.e. side-effects, should be confined to the outer layer. In the code above the domain logic and side-effects are undisentanglable. The next version shows an alternative.

Version 2. Higher order function

This is better. `notifyUser` is now a referentially transparent function. The actual execution of the effects is deferred to a later point, when the result function is called with the context. The Reader monad is nothing else just a convenient wrapper around such a function.

Version 3. Reader Monad

The benefit Reader monad offers over the simple HOF-solution is the monadic composability, like in the example below.


Note that inside the for comprehension the context doesn't even appear.