How Can I Distinguish Between Empty and Missing JSON Fields When Unmarshalling in Go?

Unveiling the Mysteries of Void Values and Unspecified Fields in Go Unmarshalling

In Go, when unmarshalling JSON into structs, it can be challenging to distinguish between void values and unspecified field values. This can lead to ambiguous program behavior. Here's how to resolve this conundrum:

Differentiating Void and Unspecified Values

Void values occur when a JSON field is present but has an empty value (e.g., an empty string), while unspecified values occur when a field is completely omitted from the JSON. To distinguish between these, modify the field type to use a pointer:

type Category struct {
    Name        string
    Description *string
}

Now, if the JSON field contains an empty string, it will be set to a pointer pointing to an empty string. However, if the field is not present, it will remain nil.

Example Usage

Consider the following JSON:

[
    {"Name": "A", "Description": "Monotremata"},
    {"Name": "B"},
    {"Name": "C", "Description": ""}
]

With the modified field type, the output will be:

[{Name:A Description:0x1050c150}, {Name:B Description:<nil>}, {Name:C Description:0x1050c158}]

This allows you to differentiate between the unspecified Description field in Category B and the empty Description field in Category C. You can now handle them separately within your program.

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