Assigning the returning error to an underscore

Question

I've been reading some Golang code from github.com/lib/pq which provides drivers for interacting with a postgres database.

Among the code I came across this:

go func() {
    select {
    case <-done:
        _ = cn.cancel()
        finished <- struct{}{}
    case <-finished:
    }
}()

The cancel function looks like:

func (cn *conn) cancel() error

As far as I can tell, the underscore isn't being used as a static assertion about a type (and therefore the compiler doesn't evaluate any side effects as far as I can see (as in this example)) and it isn't a second parameter whereby the author may want to discard it.

In summary: Why assign the result of the cancel function (the error) to an underscore?

Answer 1

Code must be correct. To be sure that code is correct, code must be readable.

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The First Rule of Go: Check for errors.

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func (cn *conn) cancel() error

If I write

cn.cancel()

did I forget to check for errors or did I decide to discard the error value?

However, if I write

_ = cn.cancel()

I did not forget to check for errors and I did decide to discard the error value.

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The Go Programming Language Specification

Blank identifier

The blank identifier is represented by the underscore character _. It serves as an anonymous placeholder instead of a regular (non-blank) identifier and has special meaning in declarations, as an operand, and in assignments.

Assignments

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