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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 KindExampleInferred Type (x)
String literallet x = "hello"String
Boolean literallet x = trueBool
Nil literallet x = nilNever?

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 `((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}
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