Defer Expensive operations

It may be better to defer expensive operations until they are needed. If some conditions are not met, that operation will not happen. This mechnism helps to avoid unnecessary expense.

Expensive Evaluation

Say you need to log some expensive value:

slog.Debug("frobbing", "value", computeExpensiveValue(arg))

Even if this line is disabled, computeExpensiveValue will be called. (Argments Evaluation happens before slog.Debug() is called)

We can avoid such an unnecessary expense by deferring this operation:

type expensive struct { arg int }

func (e expensive) LogValue() slog.Value {
    return slog.AnyValue(computeExpensiveValue(e.arg))
}

Then use a value of that type in log calls:

slog.Debug("frobbing", "value", expensive{arg})

Now computeExpensiveValue will only be called when the line is enabled. (It will “resolve” the value only when the line is enabled.)