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

Swift version: 5.1

Paul Hudson @twostraws May 28th 2019

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, +)