Should I embed interface from standard library or write my own?

Solution 1:

For such common and simple interfaces I would definately embed the one from the standard lib (such as io.Closer, io.Reader and io.ByteReader).

But not any interface type. In general interfaces should be defined where they are needed. Embedding any interface defined in other packages (including the standard library) has the danger of failing to implicitly implement them if they are changed or extended.

The "owner" (definer) of the package may change it (e.g. extend it with a new method) and properly update all its types implementing it, so the package can continue to work from the outside, but obviously the package owner will not update your implementations.

For example the reflect.Type interface type had no Type.ConvertibleTo() method back in Go 1.0, it was added in Go 1.1. The same thing may happen: interfaces in the standard lib may get altered or extended in future Go versions, resulting in your existing code failing to implement them.

What's the difference between small, common interfaces and the "rest"? The bigger the interface, the weaker the abstraction – so goes the Go proverb. Small interfaces like io.Closer and io.Reader capture a tiny yet important functionality. They are so common, "every" library tries to implement them, every utility functions build upon them. I don't ever expect them to change. If there will be a reason to change / extend them, they will be rather added as new interfaces. Not like bigger interfaces where abstraction is harder to capture accurately. They have a better chance to change / evolve over time.