Swift Safe Collection Access

Ever wonder why Swift crashes if you access a collection the wrong way? Especially an array? Me too here’s some extensions. 🎉

collection[orNil:]

This subscript fails gracefully on invalid input by returning nil or refusing to mutate the collection. With valid input, it behaves precisely like Swift’s builtin subscripts!

import SafeCollectionAccess



var first5Fibonacci = [1, 1, 2, 3, 5]

first5Fibonacci[orNil: 0]                       // Optional(1)
first5Fibonacci[orNil: 3] == first5Fibonacci[3] // true
first5Fibonacci[orNil: 999]                     // `nil`
first5Fibonacci[orNil: -1]                      // `nil`

first5Fibonacci[orNil: 2...4]  // Optional([2, 4])
first5Fibonacci[orNil: -2...2] // `nil`
first5Fibonacci[orNil: ..<42]  // `nil`


first5Fibonacci[orNil: 1]   = 42  // [1, 42, 2, 3, 5]
first5Fibonacci[orNil: 999] = -1  // [1, 42, 2, 3, 5]
first5Fibonacci[orNil: -1]  = 777 // [1, 42, 2, 3, 5]

first5Fibonacci[orNil: 0...2]  = [9, 5]    // [9, 5, 3, 5]
first5Fibonacci[orNil: -2...2] = [42]      // [9, 5, 3, 5]
first5Fibonacci[orNil: ..<42]  = [7, 7, 7] // [9, 5, 3, 5]

collection[clamping:]

This subscript fails gracefully on invalid input by returning or mutating the closest valid element (or, with empty collections, returning nil or refusing to mutate). With valid input, it behaves precisely like Swift’s builtin subscripts!

import SafeCollectionAccess



var first5Fibonacci = [1, 1, 2, 3, 5]

first5Fibonacci[clamping: 0]                       // 1
first5Fibonacci[clamping: 3] == first5Fibonacci[3] // true
first5Fibonacci[clamping: 999]                     // 5
first5Fibonacci[clamping: -1]                      // 1


first5Fibonacci[clamping: 1]   = 42  // [1, 42, 2, 3, 5]
first5Fibonacci[clamping: 999] = -1  // [1, 42, 2, 3, -1]
first5Fibonacci[clamping: -1]  = 777 // [777, 42, 2, 3, -1]