NEW: Learn SwiftUI with my free YouTube video series! >>

How to fix the error “Expression was too complex to be solved in reasonable time”

Swift version: 5.0

Paul Hudson    @twostraws   

Swift’s use of type inference makes our code shorter and easier to read, but it also chews up a lot of CPU time. Sometimes a value could be one of several types, and if it gets used with another things that could be one of several types then the amount of work Swift has to do multiplies. If Swift encounters something with so many possibilities that it simply can’t figure it out after about 15 seconds, it throws an error instead: “Expression was too complex to be solved in reasonable time; consider breaking up the expression into distinct sub-expressions.”

This is something the Swift team are working to improve with every new version of Swift, and there’s no real fixed cut-off for when the compiler will throw this error. Fortunately, this error message tells you exactly what you need to do to fix the problem: break up the expression into multiple subexpressions.

For example, this kind of code takes almost 2 seconds to compile on a modern Mac:

let sum = [1, 2, 3].map { String($0) }.compactMap { Int($0) }.reduce(0, +)

On older Macs Swift would really struggle, so you’d be wise to break it up into multiple subexpressions like this:

let numbers = [1, 2, 3]
let stringNumbers = numbers.map { String($0) }
let intNumbers = stringNumbers.flatMap { Int($0) }
let sum = intNumbers.reduce(0, +)

LEARN SWIFTUI FOR FREE I wrote a massive, free SwiftUI tutorial collection, and also have a growing list of free SwiftUI tutorials on YouTube – get started today!

Available from iOS 8.0

Similar solutions…

About the Swift Knowledge Base

This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.

MASTER SWIFT NOW
Buy Testing Swift Buy Practical iOS 12 Buy Pro Swift Buy Swift Design Patterns Buy Swift Coding Challenges Buy Server-Side Swift (Vapor Edition) Buy Server-Side Swift (Kitura Edition) Buy Hacking with macOS Buy Advanced iOS Volume One Buy Advanced iOS Volume Two Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with Swift Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Beyond Code

Was this page useful? Let us know!

Average rating: 4.8/5