How does the (*T)(nil) syntax ensure interface compliance at compile time in Camlistore?

Go Interface Compliance Compile Type Checking with (*T)(nil)

In the Camlistore codebase, the following code is used to ensure that certain types implement the necessary interfaces:

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

These statements serve as compile-time assertions, confirming that the specified types implement the required public functions of the given interfaces.

The (*T)(nil) syntax employed in this code block is known as a conversion. In this context, it represents a typed nil value. Similar to the assignment var p *T, a typed nil value represents the initial state of a pointer before any assignment.

The standard conversion syntax is T(expr). However, in the case of a pointer type, the precedence of the * operator conflicts with the expected interpretation of the conversion. This alternative syntax, (T)(expr), resolves this precedence issue.

Therefore, (*U)(expr) is the generalized form of the conversion used in Camlistore. It effectively asserts that the value of the corresponding interface pointer is nil, guaranteeing that the interface implementation is enforced during compilation.

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