What are the differences between the Reader Monad and the State Monad in Haskell?

What are the differences between the Reader Monad and the State Monad in Haskell?

The Reader Monad is used for dependency injection, allowing you to pass read-only state through a computation. In contrast, the State Monad allows you to manage mutable state, enabling you to read and write state throughout your computations.

Can you give me an example of how to use each of them?

Sure! For the Reader Monad, you might have a function that reads a configuration value without modifying it. For the State Monad, you could have a function that updates a counter, reading its current value and then incrementing it.

Which one should I use for a web application?

It depends on your needs. If you need to pass around configuration or environment settings, use the Reader Monad. If you need to manage and update state, like user sessions or counters, the State Monad is more appropriate.

Are there any performance implications between the two?

Generally, the Reader Monad can be more efficient since it doesn't involve state mutation. However, the performance difference is often negligible unless you're working with a large number of state transitions.