How Does Go\'s Compiler Ensure Interface Compliance at Compile Time?

Golang Interface Compliance Compile Type Check

In Go, the compiler ensures that a type implements the public functions of an interface. This is known as compile time type checking. The code example from camlistore demonstrates this concept.

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

The _ prefix indicates that no variable is being created. Instead, the statements are used to specify that the named types implement the corresponding interfaces.

The right-hand side (RHS) portion uses the conversion syntax (*T)(nil). This syntax creates a typed nil value. A nil value is a special value that represents the absence of a value. A typed nil value is a nil value that has a specific type.

In this case, (*T)(nil) creates a nil value that has the type *T. This is equivalent to declaring a pointer variable and assigning it to nil. The nil value can then be used to assign to a pointer of the same type.

For example, the following code declares a pointer variable f of type StreamingFetcher:

var f blobref.StreamingFetcher

The following code then assigns a nil value of type *CachingFetcher to f:

f = (*CachingFetcher)(nil)

This assignment is valid because the CachingFetcher type implements the StreamingFetcher interface.

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