How does Go Ensure Interface Compliance at Compile Time?

Interface Compliance Compile Type Check in Go

In Go, interfaces define contracts that specify a set of methods and properties that a type must implement in order to conform to the interface. To ensure that a type implements the required methods and properties, the compiler performs type checking.

In the Camlistore code snippet provided, you observe statements that assign the zero value of the CachingFetcher and DiskCache types to the StreamingFetcher and SeekFetcher interfaces respectively:

<code class="go">var (
        _ blobref.StreamingFetcher = (*CachingFetcher)(nil)
        _ blobref.SeekFetcher      = (*CachingFetcher)(nil)
        _ blobref.StreamingFetcher = (*DiskCache)(nil)
        _ blobref.SeekFetcher      = (*DiskCache)(nil)
)</code>

The syntax (*T)(nil) represents a conversion to a typed nil, where T is the type of the interface. In this scenario, it signifies that the zero value of the CachingFetcher and DiskCache types, which is nil in Go, satisfies the StreamingFetcher and SeekFetcher interfaces.

The purpose of these statements is to ensure that the compiler verifies that CachingFetcher and DiskCache properly implement the methods and properties specified by the StreamingFetcher and SeekFetcher interfaces. This guarantees that any code using these interfaces will interact correctly with instances of these types.

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