Yesterday (as well as in earlier emails) I mentioned multiple-assignments in a single statement. Today we see how they’re defined to work. As such, there’s not really an new material here, but we’ll cover it just the same.

Assignment statements

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A tuple assignment assigns the individual elements of a multi-valued operation to a list of variables. There are two forms. In the first, the right hand operand is a single multi-valued expression such as a function call, a channel or map operation, or a type assertion. The number of operands on the left hand side must match the number of values. For instance, if f is a function returning two values,

x, y = f()

assigns the first value to x and the second to y. In the second form, the number of operands on the left must equal the number of expressions on the right, each of which must be single-valued, and the nth expression on the right is assigned to the _n_th operand on the left:

one, two, three = '一', '二', '三'

So the two forms are:

• Multiple return values from a single function
• Multiple single-value expressions, separated by commas.

And you can’t mix them.

That is:

one, two, three := foo(), "three"

is invalid. You need to break this into two statements:

one, two := foo()
three := "three"

The blank identifier provides a way to ignore right-hand side values in an assignment:

_ = x       // evaluate x but ignore it
x, _ = f()  // evaluate f() but ignore second result value

By far the most common use of the blank identifier is to discard return values you don’t care about from a multi-value function.

As a simple example that is likely familiar if you’ve done any REST programming:

ct, _, err := mime.ParseMediaType(req.Header.Get("Content-Type"))

The other example usage is to evaluate, but ignore a variable. This is only really useful as a way to fool the compiler into letting you keep (hopefully temporarily) unused variables in your code.

var x = "Something interesting"

_ = x // Without this, the compiler will complain that `x` is defined, but not used

Quotes from The Go Programming Language Specification Language version go1.22 (Feb 6, 2024)