Arne is a developer, public speaker, programming coach, author and open-source contributor. In this case, the largest number we passed in was 8, and that’s the number that got returned. Reducing our programs down to pure functions can drastically reduce the complexity of our programs. and understand where our audience is coming from. We’ve got perfectly good functions like Math.sqrt() built into the language. Either your function works or it doesn’t . Why bother, when the impure code works just fine? It’s totally a loophole. Mocha has a bunch of handy functions like describe and it for breaking up our tests and hooks such as before and after for setup and teardown tasks. Therefore, given the same values, it will always produce the same output. In a large, complex application, this is a Big Deal. Applying Functional Programming in React with Bit. Other functions may be relying on that cart object state to be what it was before the function was called, and now that we’ve mutated that shared state, we have to worry about what impact it will have on the program logic if we change the order in which functions have been called. Does functional purity really matter? Testability -> Because pure functions are deterministic by nature, writing unit tests for them is a lot simpler. Does it return the same value every single time? Your turn. And I find it is a little more descriptive. You can follow me on Instagram, Linkedin and Medium. Impurity is contagious. No one enjoys finding and fixing bugs in code, so becoming a better programmer is also about avoiding certain things that are error-prone. So we end up with a thin shell of impure code that wraps around a well-tested, predictable core. “Pure functions let you reason about your code,” they say. And it’s annoying to create new versions of every function. Math.random() is not pure function because it always returns new value on each call. Concat. A pure function has two main characteristics: 1. It gets annoying. In Haskell, for example, it's called pure. So, what is this mythical pure function? But I have a confession to make… The thing is, I find ap() confusing sometimes. It might look something like this: Now, if you’re paying attention, you may be wondering about map(). Program determinism is usually a desirable property in computing. Const variables. Our Effect constructor takes a function as its argument. Is fetchLoginToken a pure function? Since we’re clever engineers, we decide to store their locations in a global config object. Our returnZeroFunc() function does nothing other than return the same function, every time. It’s a coffee grinder: beans go in, powder comes out, end of story. Because they only depend on the input you feed them, you can easily reuse functions between different parts of your codebase or different projects altogether. – Eloquent JavaScript, Marijn Haverbeke. Pure functions take some input and return some output based on that input. We could enforce (by convention) that the large majority of our code base would be pure, and limit IO related / side effect bearing code to a specific part of our codebase. The answer is yes. But, we could also pass a querySelector() function like so: Now, again, you may be thinking “This is still stupid!” All we’ve done is move the impurity out of getUsernameFromDOM(). This is mostly useful as a framework of thought when creating pure functions. We could similarly write a liftA3() like so: Notice that liftA2 and liftA3 don’t ever mention Effect. These creep up all over our code base — from things like accepting user inputs, receiving an unexpected response via an http call, or writing to the file system. If you call it with those same parameters, it will return the same thing every single time. It would always say it’s the same time: the time that the function call got replaced. We could enforce (by convention) that the large majority of our code base would be pure, and limit IO related / side effect bearing code to a specific part of our codebase. Unless you have a background in functional programming these abstractions (curry, excessive use of compose and prop) are really difficult to follow, as is the flow of execution. Only values that are needed to achieve side effects are computed, the rest is ignored. So, what is this mythical pure function? When a function performs any other “action”, apart from calculating its return value, th… It helps to read the arguments to compose from right to left to understand the direction of data flow. If you want it to do something impure, you have to make it impure. Their independent nature also makes them great candidates for parallel processing across many CPUs, and across entire distributed computing clusters, which makes them essential for many types of scientific and resource-intensive computing tasks. Now, when I say they cheat, they technically follow the rules. A pure function must satisfy both of the following properties: 1. If you’re passionate about Front End development, check out the JavaScript Works job-board here! But what if you’re not doing that? For example, the following function is pure: The same input values will always map to the same output value: Many input values may map to the same output value: A pure function must not rely on any external mutable state, because it would no longer be deterministic or referentially transparent. By choosing 'I Accept', you consent to our use of cookies and other tracking technologies. As Martin Odersky (creator of Scala) puts it: non-determinism = parallel processing + mutable state. But it helps illustrate the point. This would make our debugging process a lot easier, more coherent and easier to reason about.