Do calls the function f if and only if Do is being called for the first time for this instance of Once. In other words, given

var once Once

if once.Do(f) is called multiple times, only the first call will invoke f, even if f has a different value in each invocation. A new instance of Once is required for each function to execute.

Do is intended for initialization that must be run exactly once. Since f is niladic, it may be necessary to use a function literal to capture the arguments to a function to be invoked by Do:

config.once.Do(func() { config.init(filename) })

If f causes Do to be called, it will panic.

If f panics, Do considers it to have returned; future calls of Do return without calling f.

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