Monadic Web Example

This is a very simple example of using monads in Scala to demonstrate monadic computation and how using types one can significantly reduce the opportunity for bugs while making the intent of the code much more clear.

Approach

I didn't want to get distracted by everything that is necessary in developing a Web application so I've abstracted out much of the detail. Requests are simply Scala Maps and responses can either be Values which just hold strings or Streams that signal a request to directly stream data to the client.

The Monads

In Scala objects that implement map and flatMap can be used in for-comprehensions, effectively meaning that such objects can be considered monads. The example introduces two monads: RequestMonad and RequestReader. In particular, both of these monads are example of reader monads in that they thread a common value through a chain of functions.

The RequestMonad

The RequestMonad works as an intermediary between incoming requests and handlers. Handlers are any function with the signature Request=>MaybeError. You can create a RequestMonad instance as follows:

import MaybeError._
import RequestMonad._

def foo {
  val processor = RequestMonad(handler)
}

Because RequestMonad is a monad it can be used in for-comprehensions as follows:

import MaybeError._
import RequestMonad._

def processor(handler:Request=>MaybeError) =
  for (r <- RequestMonad(handler)) yield r

In this example processor is a "fully loaded" RequestMonad all ready for request data to be passed in:

processor(Map("arg1"->"value1","arg2"->"value2"))

There's a useful monadic function defined on RequestMonads called wrapResponse that provides an example of how doing "page layout" can be done monadically:

def processor(handler:Request=>MaybeError,
              wrapper:Request=>Response=>MaybeError) =
  for {
    r1 <- RequestMonad(handler)
    r2 <- wrapResponse(r1,wrapper)
  } yield r2

Later, when we get into more detail as to how this monad is constructed, we'll see that things like error conditions are automatically handled.