[dsymonds]

That's true, but the standard library also has strictures about backward compatibility that have ossified some bad choices. The crypto examples you give are arguably exactly this; even the later ones are probably a mistake, but are at least consistent with what can't be changed.

[tptacek]

context also returns context.Context (an interface), and sql returns driver.Value, an interface.

[dsymonds]

context.Context is an interface because it is known there are multiple implementers (several in the context package itself). The sql driver.Value is actually the other way around: the sql package is the consuming package (for the driver).