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Iterator Adapters in Rust

An Iterator that takes another iterator and returns a new one is called an iterator adapter. The name "adapter" comes from one of the GoF's design patterns, the adapter pattern. However, in reality, it corresponds more to the decorator pattern, so if you pay too much attention to the name, you might get confused about its purpose. So it's better not to worry too much about the name. Enough complaining about the name, what does an iterator adapter do?

An iterator adapter adds a task to be performed when the iterator iterates. This will be easier to understand when you see an example. The map function is one of the famous adapters. The iterator returned by the map function for those who have used functional languages iterates over new values transformed from the original values.

Besides, various adapters are already implemented in the standard library. Among them, the most frequently used are those that are convenient to use with loops. Examples include the filter that only returns values that meet the condition, take that returns only a specified number of values, skip that omits a few values and returns the rest, and step_by that iterates by skipping a few values. Using these adapters with loops can easily express complex condition handling.

In addition, cycle that makes it iterate forever without end, and inspect that only causes side effects without giving any transformation to the iterating values are also frequently used.

There is one thing to be careful about when using adapters. What the adapter returns is also an iterator. It does not perform the tasks specified by the adapter until the iterator is actually consumed. In other words, it performs lazy evaluation, which postpones execution until the value is actually needed. Therefore, the following code does not output any value on the screen because it does not use the iterator returned by inspect.

Then, there might be a question here. If the iterator spit out by the iterator adapter performs lazy evaluation, what happens to the iterator used as input to the adapter? In other languages which also implement the concept of iterator adapter, this can be a big problem. Fortunately, Rust prevents these kinds of problems from occurring using ownership. An iterator adapter is a function that takes an iterator and returns a new one. In other words, it takes ownership of the iterator. If it was a function that takes a reference to an iterator, it would have said that it borrows the iterator to create a new one. Therefore, the caller of the adapter can no longer use the iterator that was put into the adapter as an argument.

In this article, we introduced what an iterator adapter is and some commonly used adapters. In reality, there are many more adapters implemented in the standard library. At first glance, you might wonder where this will be used. But take a good look at the program you're creating now. There are surprisingly many cases where iterators are used. And among them, there are many cases where the code becomes cleaner when using an iterator adapter. Moreover, in Rust, problems arising from using iterator adapters in other languages do not occur, so there's no need to worry.

This article is translated from my post written in Korean. Since the post was written two years ago, there might be more sophisticated explanations for iterator adapters nowadays. However, this article seems still be sufficient to help understand iterator adapters.


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