FirstNonNil.md

First Non-Nil

[Source | Tests]

Returns the first non-nil result obtained from applying the given transformation to the elements of the sequence.

let strings = ["three", "3.14", "-5", "2"]
if let firstInt = strings.firstNonNil({ Int($0) }) {
    print(firstInt)
    // -5
}

Like first(where:), this method stops iterating the sequence once a match is found. Use firstNonNil(_:) to avoid having to apply a transformation twice:

let strings = ["three", "3.14", "-5", "2"]
if let firstIntAsString = strings.first(where: { Int($0) != nil }) {
    let firstInt = Int(firstIntAsString)! // :(
    // ...
}

This method's behavior can also be approximated using compactMap(_:):

let strings = ["three", "3.14", "-5", "2"]
if let firstInt = strings.compactMap({ Int($0) }).first {
    // ...
}

However, unlike firstNonNil(_:) and first(where:), compactMap(_:) does not stop iterating the sequence once the first match is found. Adding .lazy fixes this, at the cost of requiring an escaping closure that you cannot throw from:

let strings = ["three", "3.14", "-5", "2"]
if let firstInt = strings.lazy.compactMap({ Int($0) }).first {
    // ...
}

Detailed Design

The firstNonNil(_:) method is added as an extension method on the Sequence protocol:

public extension Sequence {
    func firstNonNil<Result>(
        _ transform: (Element) throws -> Result?
    ) rethrows -> Result?
}

Complexity

firstNonNil(_:) is an O(n) operation, where n is the number of elements at the start of the sequence that result in nil when applying the transformation.

Naming

Some alternative names were considered:

• compactMapFirst(_:)
• first(mapping:)
• firstSome(_:)
• firstMap(_:)

Comparison with other languages

Haskell: Haskell includes the firstJust method, which has the same semantics as the firstNonNil(_:) method.

Rust: Rust includes the find_map method, which has the same semantics as the firstNonNil(_:) method.

Scala: Scala includes the collectFirst method, which has the same semantics as the firstNonNil(_:) method.