Type assertions in Go

Pulling a concrete type back out, safely or with a panic.

type Closer interface{ Close() error }

type File struct{ name string }

func (f File) Close() error { return nil }

func assertions(value any) (int, bool, bool) {
    number, ok := value.(int)

    _, isCloser := value.(Closer)

    defer func() { _ = recover() }()
    forced := value.(string) // panics unless it is a string
    return number + len(forced), ok, isCloser
}

How it works

1. The two-result form never panics: check the bool.
2. The one-result form panics when the type is wrong.
3. Asserting to an interface asks whether extra methods exist.

Keywords and builtins used here

• any
• bool
• defer
• error
• func
• int
• interface
• len
• recover
• return
• string
• struct
• type

The run, in numbers

Lines

15

Characters to type

355

Tokens

94

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 54 seconds.

Type this snippet

Step 2 of 2 in Dynamic types, step 9 of 19 in Interfaces & generics.

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