Design of Context interface

Question

My question is about a design choice for the Context interface. If I want to create a child context from a parent I can do something like:

child, cancel := context.WithTimeout(parent, timeout)

Wouldn't it be nicer if WithTimeout was part of the interface, so that we could simply write:

child, cancel := parent.WithTimeout(timeout)

It seems so much cleaner to me. It is shorter, and there is no need to import context.

Why are the functions to produce child contexts not part of the Context interface?

Answer 1

This is the context.Context type:

type Context interface {
    Deadline() (deadline time.Time, ok bool)
    Done() <-chan struct{}
    Err() error
    Value(key interface{}) interface{}
}

It's simple. If you were to write an implementation of it, could you do it? Yes, easily. As there are no "setter" methods, each method can just return a default / zero value, and it's a "valid" implementation. That's exactly what the background and TODO contexts do (context.Background() and context.TODO()).

If you would add the functions that derive new context from an existing one (e.g. context.WithCancel(), context.WithDeadline() etc.) as part of the Context interface itself, that would require to provide a (valid) implementation for all, which is not feasible; and also all of them at the same time is rarely needed, so it would be a waste of resources.

Extensions are responsible to add the implementations. If you look at how the context package is implemented: context/context.go, you'll see different context.Context implementations for the different "derivatives" or "extensions":

// An emptyCtx is never canceled, has no values, and has no deadline. It is not
// struct{}, since vars of this type must have distinct addresses.
type emptyCtx int


// A cancelCtx can be canceled. When canceled, it also cancels any children
// that implement canceler.
type cancelCtx struct {
    Context

    done chan struct{} // closed by the first cancel call.

    mu       sync.Mutex
    children map[canceler]bool // set to nil by the first cancel call
    err      error             // set to non-nil by the first cancel call
}

// A timerCtx carries a timer and a deadline. It embeds a cancelCtx to
// implement Done and Err. It implements cancel by stopping its timer then
// delegating to cancelCtx.cancel.
type timerCtx struct {
    cancelCtx
    timer *time.Timer // Under cancelCtx.mu.

    deadline time.Time
}

// A valueCtx carries a key-value pair. It implements Value for that key and
// delegates all other calls to the embedded Context.
type valueCtx struct {
    Context
    key, val interface{}
}

Obviously we can make up other useful extensions for context.Context which are not in the context package. If you have a new idea, would you also add that to the Context interface? That would break all existing implementations, as obviously your new idea is not implemented in others' current implementations.