[MetaCosm]

No, io.Copy was just the example that everyone would recognize in Go, and likely the first interface you will touch.

My point was implicit, exceptional simple interfaces are sorta magic. They are effortless, and because they are implicit, there is no harm in creating a 1 function interface (or 10 of them). Because your caller is never going to have to do an ... Implements ThingA, ThingB, ThingC, ThingD, ThingE, Thi... it implicitly supports an interface if it fulfills the signature.

So your interfaces end up being tiny (just what you need) and pervasive. This means the way your system ends up working tends to be far more aligned along interfaces than anything else.

When I speak of composition (in this case the composition of functions) I am speaking of the ability to use functions together rather freely and with little effort. This is a bit hard to explain, but it feels a bit like a modern shell, you can wire together lots of commands (functions), from lots of places that have no awareness of each other with the pipe | operator. In Go, simple, minimalistic interfaces act as the glue and let you compose lots of diverse functions together exceptionally quickly.