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Type identity

May 25, 2023

Type identity

Two types are either identical or different.

Well now that’s profound, isn’t it? LOL.

The real point is that there are not different “degrees of similarity.”

The spec continues…

A named type is always different from any other type. Otherwise, two types are identical if their underlying type literals are structurally equivalent; that is, they have the same literal structure and corresponding components have identical types.

So the rules for type identity are rather simple. Although tomorrow we’ll dive into the specific implications of the rules.

Any named type is different from all other types, regardless of the underlying type. (A caveat is type aliases, which we’ll get to later, and allows multiple identifiers to point to a single type.)

Unnamed types (such as unnamed slices, arrays, structs, interfaces, etc) are identical if they have the same recursive structure.

Therefore, these two struct types are different, even though they have the same fields and types, because their structure is different (the fields are reversed):

struct {
	Name string
	Age  int
}

struct {
	Age int
	Name string
}

Quotes from The Go Programming Language Specification Version of December 15, 2022

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Type identities of other types

Type identity … Two pointer types are identical if they have identical base types. Given: type ( P = *int Q = P R *int ) P, Q, and *int are all identical to each other, but different from *int32 and R. Two function types are identical if they have the same number of parameters and result values, corresponding parameter and result types are identical, and either both functions are variadic or neither is.

Type identities of compound types

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Type identity wrapup

Today we wrap up our discussion of type identity. This is mostly a recap of what has already been discussed the last few days. Type identity … Given the declarations type ( A0 = []string A1 = A0 A2 = struct{ a, b int } A3 = int A4 = func(A3, float64) *A0 A5 = func(x int, _ float64) *[]string B0 A0 B1 []string B2 struct{ a, b int } B3 struct{ a, c int } B4 func(int, float64) *B0 B5 func(x int, y float64) *A1 C0 = B0 D0[P1, P2 any] struct{ x P1; y P2 } E0 = D0[int, string] ) these types are identical: