How Does Go Ensure Interface Compliance Through Compile-Time Type Checking?

Go Language Interface Compliance Compile Type Check

In Go, the statement "var x interface{}" declares a variable x of type interface{}, which can hold values of any type. However, when assigning a value to an interface variable, the compiler checks whether the type of the assigned value conforms to the interface's definition.

In the example you provided:

var (
    _ blobref.StreamingFetcher = (*CachingFetcher)(nil)
    _ blobref.SeekFetcher      = (*CachingFetcher)(nil)
    _ blobref.StreamingFetcher = (*DiskCache)(nil)
    _ blobref.SeekFetcher      = (*DiskCache)(nil)
)

the statements ensure that CachingFetcher and DiskCache implement the public methods of the StreamingFetcher and SeekFetcher interfaces. The "_ blobref.StreamingFetcher" prefix indicates that the CachingFetcher type satisfies the StreamingFetcher interface. This check is important to ensure that these types can be correctly used in code that expects types implementing these interfaces.

The RHS portion of these statements uses a pointer constructor syntax with a nil parameter. In this case, "(T)(nil)" represents a typed nil value, which is used to indicate that the nil value should be assigned to a variable of type T. For example, the following statement declares a pointer to a StreamingFetcher interface with a nil value:

var fetcher *blobref.StreamingFetcher = (*blobref.StreamingFetcher)(nil)

This syntax is a convenient way to create a pointer to an interface and ensure that the nil value is assigned correctly.

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