add F* funcs that allows use `func() []Fields` as arguments

[negasus]

This PR add new public funcs with signature (msg string, fields ...FieldsFunc), where FieldsFunc is type FieldsFunc func() []Field

• FDebug
• FInfo
• FWarn
• FError
• FDPanic
• FPanic
• FFatal

This approach allows you to perform the calculation of field values only when the message is actually displayed in the log.

An example:

func main() {
	logger, _ := zap.NewProduction()

	logger.FDebug("hello world", fields)
}

func fields() []zap.Field {
	// Here we do any heavy operations
	// and add fields for logging
	// This function is not called if the level is not appropriate
	
	return []zap.Field{
		// ...
	}
}

[rabbbit]

Just to verify my assumptions - have you considered doing the same with https://pkg.go.dev/go.uber.org/zap#Logger.Check?

I see how FInfo is a nicer/shorter format, but then to realistically make it work you'd likely need to pass in a closure, which also is not very pretty?

so the example might look like this, which is not much easier than the Check API, right?

func main() {
	logger, _ := zap.NewProduction()

        a := someStruct....
        // some work
	logger.FDebug("hello world", fields(a))
}

func fields() func() []zap.Field {
    	return func(arg someStruct) []zap.Field {
		// Here we do any heavy operations
		// and add fields for logging
		// This function is not called if the level is not appropriate
	
		return []zap.Field{
			// use arg
		}
        }
}

Note I have no impact/relation to zap maitenance, I'm asking out of my personal curiosity.

[negasus]

Yes, under the hood these functions use log.check. You can think of an API as syntactic sugar

[prashantv]

Rather than increasing the API surface for each level, I think it would be nicer to support func Lazy(fieldFunc... func() []Field) Field. The call sites would look like logger.Info("my message", zap.Lazy(field1Func, field2Func))