Swift Foundation contains a lot of useful functions on collection types. In the context of this blog post we are interested in map(_:) and prefix(while:). Map is used for transforming collection elements and prefix(while:) for getting a sequence containing initial elements until the predicate is true. In some cases the predicate used in the prefix(while:) can be expensive, or we just want to combine the information in prefix and map functions. One of such examples is when we use NSRegularExpression. More specifically, let’s take an example of processing a list of strings while the regular expression has matches and then extracting a range from the string. A concrete example could be parsing Fastlane’s Fastfile for visualization purposes.
Fastlane is used a lot in the iOS community for automating development related tasks. Lanes are added to a Fastfile where every individual lane has a name and optionally a description.
If we would like to extract the lane name and description from the Fastfile then we can use regular expressions. The flow could be something like this: firstly, we can read the Fastfile contents, divide the file into lines and match lines with regular expressions. Second step is finding lines which contain a lane keyword. Then we could loop over preceding lines and collect lines which contain description. All in all, the logic for getting description could look like this:
Now when we have seen a case where prefixMap can be useful, it is time to look into how it is implemented. The transform passed into the prefixMap function can return nil and the nil value means that the looping should be stopped and all the transformed elements should be returned. And yes, the implementation is pretty straight-forward.
Foundation types can be extended with new functions easily. Although we could use prefix(while:) first followed with a map(_:) but sometimes we’ll just need to combine functionalities into a single function.
PrefixMapPlayground (Xcode 12.4)