How to Correctly Format Errors with Variable Arguments in Go?

Formatting Errors: Alternative to errors.New with Variable Arguments

In an effort to enhance error handling, users may seek to create a customized version of the errors.New function that accepts multiple arguments, mimicking the behavior of fmt.Sprintf. While attempts have been made, challenges arise in correctly interpreting variable arguments within the custom function. This article explores the solution and provides guidance on implementing a robust error formatting mechanism.

One such attempt involves defining a function as follows:

<code class="go">func NewError(format string, a ...interface{}) error {
    return errors.New(fmt.Sprintf(format, a))
}</code>

However, this approach falls short when attempting to pass variable arguments to fmt.Sprintf, resulting in the arguments being treated as a single array. To resolve this issue, it is essential to use the ... operator after the variable arguments (a) in the custom function, as demonstrated in the corrected code below:

<code class="go">func NewError(format string, a ...interface{}) error {
    return errors.New(fmt.Sprintf(format, a...))
}</code>

By employing the ... operator, fmt.Sprintf can correctly interpret the variable arguments and format the string accordingly. This allows users to create custom error formatting functions that leverage the powerful formatting capabilities of fmt.Sprintf.

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