What is the Purpose of the Blank Identifier '_' in Go Variable Declarations?

The Mysterious "_," in Go Variable Declarations

In Go, it's common to come across variable declarations like this:

_, prs := m["example"]

What is the purpose of this _, construct, and why is it used instead of the more intuitive prs := m["example"]?

The Blank Identifier: Disregarding Return Values

The _ represents the "blank identifier." It allows developers to discard return values from a function. In this case, the m["example"] expression returns a tuple containing two values: the element at the "example" key and a boolean indicating whether the key exists. By using _,, we're only interested in the presence of the key, not the value.

Preventing Invalid Declarations

Go strictly enforces variable declarations. If we declared prs without a value, the compiler would raise an error. Instead, we use the blank identifier to tell Go that we acknowledge the return value but don't intend to use it.

Efficiency and Code Readability

Using the blank identifier avoids declaring unneeded variables, improving code efficiency. It also enhances readability by making it clear which return values are being ignored.

Common Use Cases

• Map key existence checks: _, present := timeZone[tz]
• Channel read validity: _, err := os.Read(f)
• Discarding loop iteration values: for _, value := range array { ... }

Custom Naming Conventions

Some developers adopt conventions for naming variables used as blank identifiers, such as "ok" or "err." This further improves readability and consistency.

Example

Consider the following code snippet:

if _, err := os.Stat(path); os.IsNotExist(err) {
    fmt.Printf("%s does not exist\n", path)
}

In this example, we use the blank identifier to test if a file exists at the specified path without caring about the file's attributes.

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