For example, the implementation of the fmt package needs the functionality of unicode.IsSpace as part of fmt.Scanf. To avoid creating an undesirable dependency, fmt does not import the unicode package and its large tables of data; instead, it contains a simpler implementation, which it calls isSpace. To ensure that the behaviors of fmt.isSpace and unicode.IsSpace do not drift apart, fmt prudently contains a test. It is an external test, and thus it cannot access isSpace directly, so fmt opens a back door to it by declaring an exported variable that holds the internal isSpace function. This is
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