Why Does `fmt.Println()` Prefer `Error()` Over `String()` for Custom Types?

Why Error() Method Overrules String() Method in fmt.Println()

When you implement both the String() and Error() methods for a custom type, you may encounter a situation where fmt.Println() prioritizes the Error() method over the String() method.

This is because the fmt package employs a hierarchy when choosing which method to call for formatting an object. According to the package documentation, the following order of operations is applied:

1. If the object implements the Formatter interface, its Format() method will be invoked.
2. If the %#v format is used and the object implements the GoStringer interface, its GoString() method will be called.
3. If the object implements the error interface, its Error() method will be used to convert the object to a string.
4. If the object implements the String() method, its String() method will be called.

Since the error interface is ranked higher than the String() interface in this hierarchy, fmt.Println() will prioritize the Error() method if both methods are implemented for a given object. This is because errors typically require more urgent attention and should be communicated promptly in a clear and concise manner.

In the example provided, the Person type implements both the String() and Error() methods. However, the Error() method simply returns the string "Failed", while the String() method provides a more informative description of the person. When you print out instances of the Person type using fmt.Println(), you will only see the "Failed" message because the Error() method takes precedence over the String() method due to the aforementioned hierarchy.

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