Type Inference

If a variable or constant declaration is not annotated explicitly with a type, the declaration's type is inferred from the initial value.

Basic Literals

Decimal integer literals and hex literals are inferred to type Int.

let a = 1

// `a` has type `Int`

let b = -45

// `b` has type `Int`

let c = 0x02

// `c` has type `Int`

Unsigned fixed-point literals are inferred to type UFix64. Signed fixed-point literals are inferred to type Fix64.

let a = 1.2

// `a` has type `UFix64`

let b = -1.2

// `b` has type `Fix64`

Similarly, for other basic literals, the types are inferred in the following manner:

Literal Kind	Example	Inferred Type (x)
String literal	let x = "hello"	String
Boolean literal	let x = true	Bool
Nil literal	let x = nil	Never?

Array Literals

Array literals are inferred based on the elements of the literal, and to be variable-size. The inferred element type is the least common super-type of all elements.

let integers = [1, 2]

// `integers` has type `[Int]`

let int8Array = [Int8(1), Int8(2)]

// `int8Array` has type `[Int8]`

let mixedIntegers = [UInt(65), 6, 275, Int128(13423)]

// `mixedIntegers` has type `[Integer]`

let nilableIntegers = [1, nil, 2, 3, nil]

// `nilableIntegers` has type `[Int?]`

let mixed = [1, true, 2, false]

// `mixed` has type `[AnyStruct]`

Dictionary Literals

Dictionary literals are inferred based on the keys and values of the literal. The inferred type of keys and values is the least common super-type of all keys and values, respectively.

let booleans = {

1: true,

2: false

}

// `booleans` has type `{Int: Bool}`

let mixed = {

Int8(1): true,

Int64(2): "hello"

}

// `mixed` has type `{Integer: AnyStruct}`

// Invalid: mixed keys

//

let invalidMixed = {

1: true,

false: 2

}

// The least common super-type of the keys is `AnyStruct`.

// But it is not a valid type for dictionary keys.

Ternary Expression

Ternary expression type is inferred to be the least common super-type of the second and third operands.

let a = true ? 1 : 2

// `a` has type `Int`

let b = true ? 1 : nil

// `b` has type `Int?`

let c = true ? 5 : (false ? "hello" : nil)

// `c` has type `AnyStruct`

Functions

Functions are inferred based on the parameter types and the return type.

let add = (a: Int8, b: Int8): Int {

return a + b

}

// `add` has type `fun(Int8, Int8): Int`

Type inference is performed for each expression / statement, and not across statements.

Ambiguities

There are cases where types cannot be inferred. In these cases explicit type annotations are required.

// Invalid: not possible to infer type based on array literal's elements.

//

let array = []

// Instead, specify the array type and the concrete element type, e.g. `Int`.

//

let array: [Int] = []

// Or, use a simple-cast to annotate the expression with a type.

let array = [] as [Int]

// Invalid: not possible to infer type based on dictionary literal's keys and values.

//

let dictionary = {}

// Instead, specify the dictionary type and the concrete key

// and value types, e.g. `String` and `Int`.

//

let dictionary: {String: Int} = {}

// Or, use a simple-cast to annotate the expression with a type.

let dictionary = {} as {String: Int}