Type Inference

Updated: 2021/7/7 by

3 min read

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.

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let a = 1
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// `a` has type `Int`
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let b = -45
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// `b` has type `Int`
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let c = 0x02
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// `c` has type `Int`

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

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let a = 1.2
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// `a` has type `UFix64`
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let b = -1.2
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// `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.

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let integers = [1, 2]
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// `integers` has type `[Int]`
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let int8Array = [Int8(1), Int8(2)]
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// `int8Array` has type `[Int8]`
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let mixedIntegers = [UInt(65), 6, 275, Int128(13423)]
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// `mixedIntegers` has type `[Integer]`
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let nilableIntegers = [1, nil, 2, 3, nil]
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// `nilableIntegers` has type `[Int?]`
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let mixed = [1, true, 2, false]
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// `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.

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let booleans = {
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    1: true,
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    2: false
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}
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// `booleans` has type `{Int: Bool}`
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let mixed = {
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    Int8(1): true,
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    Int64(2): "hello"
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}
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// `mixed` has type `{Integer: AnyStruct}`
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// Invalid: mixed keys
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//
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let invalidMixed = {
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    1: true,
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    false: 2
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}
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// The least common super-type of the keys is `AnyStruct`.
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// 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.

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let a = true ? 1 : 2
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// `a` has type `Int`
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let b = true ? 1 : nil
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// `b` has type `Int?`
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let c = true ? 5 : (false ? "hello" : nil)
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// `c` has type `AnyStruct`

Functions

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

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let add = (a: Int8, b: Int8): Int {
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    return a + b
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}
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// `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.

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// Invalid: not possible to infer type based on array literal's elements.
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//
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let array = []
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// Instead, specify the array type and the concrete element type, e.g. `Int`.
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//
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let array: [Int] = []
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// Or, use a simple-cast to annotate the expression with a type.
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let array = [] as [Int]

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// Invalid: not possible to infer type based on dictionary literal's keys and values.
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//
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let dictionary = {}
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// Instead, specify the dictionary type and the concrete key
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// and value types, e.g. `String` and `Int`.
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//
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let dictionary: {String: Int} = {}
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// Or, use a simple-cast to annotate the expression with a type.
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let dictionary = {} as {String: Int}