Adding prefixMap for expensive operations in Swift

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.

desc 'Build and upload app store build'
desc 'Captures screenshots, builds the app, uploads to app store, and posts a message to slack'
lane :appstore do
view raw Fastfile hosted with ❤ by GitHub

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:

let descExpression = try! NSRegularExpression(pattern: "^\\s*desc [\"']{1}([^\"]*)[\"']{1}", options: [])
let description = (0..<lineIndex)
.prefixMap({ index -> String? in
// Get a line
let line = lines[index]
// Use regular expression
guard let match = descExpression.firstMatch(in: line) else { return nil }
// Regular expression contains a capture group, therefore looking for 2 ranges
guard match.numberOfRanges >= 2 else { return nil }
// When there are matches then extract description
guard let range = Range(match.range(at: 1), in: line) else { return nil }
return String(line[range])
.joined(separator: "\n")

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.

// Swift Foundation
func prefix(while predicate: (Self.Element) throws -> Bool) rethrows -> Self.SubSequence
func map<T>(_ transform: (Self.Element) throws -> T) rethrows -> [T]
// prefixMap (new)
extension Collection {
@inlinable func prefixMap<T>(_ transform: (Self.Element) throws -> T?) rethrows -> [T] {
var result = [T]()
for element in self {
if let transformedElement = try transform(element) {
else {
return result


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.

If this was helpful, please let me know on Mastodon@toomasvahter or Twitter @toomasvahter. Feel free to subscribe to RSS feed. Thank you for reading.


PrefixMapPlayground (Xcode 12.4)

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