Synchronous Behavior and IO

Overview

We’ll ignore the function name for now and focus on how to create the Between. It should be clear that there are problems with purity when it comes to getting the start and end times because we want to show a certain range in regard to the current time. Consider the last three hours of metrics, for example. This means that the function for getting our Between returns different values depending on when it was invoked, which is impure behavior. How will we wrap it? In a Task? We could do that, but this is synchronous behavior, for which the IO monad is ideally suited. Similarly, IO is also what we normally use to wrap our logging, which is, after all, another synchronous side effect.

As noted, in a hybrid language like TypeScript (which allows other styles of programming besides FP),we can decide for ourselves which effects are important enough that we want to keep an eye on them. That decision comes down to whether the correct program behavior depends on the effect, in this case, logging. In most cases it won’t, but what if our console output is used by another program as input? In the latter case, our output is an essential part of our application and should be treated with the utmost care. So, if you like debugging with console statements and all that logging is temporary, go ahead and take advantage of the flexibility of JavaScript, which doesn’t force us to wrap logging in a monad. If we’re capturing some important logs but our application doesn’t depend on them, that seems acceptable as well. On the other hand, in some rare cases (like the first one we discussed), avoiding IO might prove to be a mistake.

Defining constants

As with several other potentially configurable values, we’ll keep it simple and only check for the last three hours. We’ll add these values to our constants.