swift-guide/TypeInference.md

Type Inference

Convention

Use type annotation as little as possible when declaring a constant or a variable; instead let the compiler infer the variable type. This convention does not apply to properties.

Rationale

Type inference helps reduces line noise and maintain clearer code.

Shorthand Dot Syntax

When assigning an enum or a static property to a variable or a constant, if the type of the variable or the constant is known, use the shorthand dot syntax to access the enum or static property.

Examples

Good: use shorthand dot syntax

func layout(with direction: NSUserInterfaceLayoutOrientation) {
    switch direction {
    case .horizontal:
        layoutHorizontally()
    case .vertical: 
        layoutVertically()
    }
}

Bad: unnecessarily verbose without shorthand dot syntax

func layout(with direction: NSUserInterfaceLayoutOrientation) {
    switch direction {
    case NSUserInterfaceLayoutOrientation.horizontal:
        layoutHorizontally()
    case NSUserInterfaceLayoutOrientation.vertical: 
        layoutVertically()
    }
}

Constants and Variables

Let the compiler infer the type of a constant or variable whenever possible.

Examples

Good: no left-hand-side type annotations

let horizontalMargin = 10

let font = NSFont.systemFont(ofSize: 15.0)

Bad: with unnecessary left-hand-side type annotations

let horizontalMargin: Int = 10

let font: NSFont = NSFont.systemFont(ofSize: 15.0)

Properties

Use type annotations instead of type inference for properties for API clarity.

Examples

Good: property use type annotations

class CustomLabel {
    let textColor: UIColor = .white
}

Bad: property does not use type annotations

class CustomLabel {
    let textColor = UIColor.white
}