hsson/once

A re-implementation and drop-in replacement of the standard Go (Golang) sync.Once, with added support for return values (using generics)! This package exports three additional Once-like primitives, in addition to the standard once.Once:

once.Error returns an error value

Do(f func() error) error

once.Value[T] returns a value

Do(f func() T) T

once.ValueError[T] returns a (value, error) tuple

Do(f func() (T, error)) (T, error)

These three primitives have the behavior that, like with the standard Once, the function passed is ever only executed once. However, they also return the value returned by that one execution to all subsequent callers.

Example usage

var o once.ValueError[string]
val, err := o.Do(func() (string, error) {
  return "Hello!", nil
})
if err != nil {
  // Do something
}
fmt.Println(val) // prints "Hello!"