A Go error handling idiom

There's yet another common Go pattern at play in our code:

err := d.Stroke(se, strokeSupply, pond)
if err != nil {
       log.Fatal(err)  // the duck died!
}

Errors should be handled once and as soon as possible.

Some consider this as an antipattern that litters code with if err != nil blocks. We'll overlook that sentiment, for now, in favor of its simplicity and pragmatism.

Next, we'll define a Capabilities struct that embeds both behavior interfaces and all the important strokes fields. The Capabilities type defines what the duck can do. It has a number of strokes that it can use to cross each pond and two behaviors--one that increases its stroke count and the other that reduces the count but helps it to get close to its next source of food:

type Capabilities struct {
       StrokeBehavior
       EatBehavior
       strokes int
}

In Go, any method or field of an embedded/inner interface is accessible to the outer interface. Note that we're not saying parent or child, as that might imply inheritances. What we have is called inner type promotion, not inheritance. As long as an inner field or method name begins with a capital letter, it will be accessible to the outer object.