How Can We Distinguish Between Built-in and Custom Types in Go Using Reflection?

Identifying Non-Builtin Types Using Reflection

Problem:

Differentiating between types like []byte and type A []byte using reflection proves challenging when reflect.TypeOf(A{}).Kind indicates both as slices of bytes. It becomes necessary to find a way to distinguish these types without relying on pre-defined lists of types.

Background:

Types in Go can be categorized as named (with a type name) or unnamed (using a type literal). Predeclared types (like string or int) are named, while types created using type literals (like []int or struct{i int}) are unnamed.

Solution:

Using Type.PkgPath():

Type.PkgPath() provides the package path of a named type. For predeclared or unnamed types, this path will be empty. Thus, non-builtin types can be identified by checking if Type.PkgPath() returns a non-empty string.

Handling Unnamed Types Derived from Custom Types:

Using type literals with custom types can result in unnamed types (e.g., []A). To determine if an unnamed type is derived from a custom type, check the element type using Type.Elem():

if elem := t.Elem(); elem.Kind() == reflect.Ptr || elem.Kind() == reflect.Struct {
    // Recursively check elem
}

Special Cases:

• Anonymous Struct Types: Inspect the struct fields and check if any have a custom type.
• Map Types: Examine both the key and value types of the map.

Example Implementation:

The following code defines a utility function isCustom to identify non-builtin types:

func isCustom(t reflect.Type) bool {
    if t.PkgPath() != "" {
        return true
    }
    // Handle special cases and recursion for unnamed types
    return false
}

Testing the Solution:

fmt.Println(isCustom(reflect.TypeOf(""))) // false (builtin)
fmt.Println(isCustom(reflect.TypeOf(A{}))) // true (custom)

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