GoRoutines and passing struct to original context - go

Question

I have a configuration that defines a number of instances (SomeConfigItems) which have a thing() created for each of them.
That thing is a struct returned by an included package, which contains, among other things, a Price (float64) and a nested struct. The nested struct maintains a map of trades.
The problem is that I am able to loop through the thing.Streams.Trades and see all trades happening in real time from my main()'s for{} loop. I am not able to see an updated thing.Price even though it is set in the Handler on occasion.
I am having a hard time understanding how the nested structs can contain data but not Price. I feel as though I am missing something with scoping, goroutines, or possibly pointers for instantiation of new objects.
Any help would be appreciated, I will continue reading in the meantime. I've reduced the code to what seems relevant.
main.go:
package main
import thing
var Things []thing.Handler
for _, name := range SomeConfigItems {
handler := thing.New(name)
Things = append(Things, handler)
}
for {
for _, t := range Things {
log.Info("Price: ", t.Price) // This is set to 0 every iteration, but I can actively data in thing.Streams.Trades
}
}
thing.go:
package thing
import streams
type Handler struct {
Name string
Price float64
Streams streams.Streams
}
func New(name string) (h Handler, err error) {
stream, err := streams.New(strings.ToLower(name))
h = Handler{
Name: name,
Price: "0.0"
Streams: stream,
}
go h.handler()
return h, err
}
func (bot *Handler) handler() {
var currentPrice float64
for {
currentPrice = external.GetPrice(bot.Name).Price // Validated that this returns a float64
bot.Price = currentPrice // Verified that this is updated immediately after in this context.
// Unable to see Price updated from outer context.
}
}
streams.go:
package streams
type Streams struct {
Trades
}
type State struct {
Price string `json:"p"`
Quantity string `json:"q"`
}
type Trades struct {
Trades map[float64]float64
TradeMutex sync.Mutex
Updates chan State
}
func New(name string) (s Streams, err error) {
p := newTradeStream(name)
s = Streams{
Trades: p,
}
return s, err
}
func newTradeStream(name string) (ts Trades) {
ts = Trades{}
ts.Trades = make(map[float64]float64, MaxDepth)
ts.Updates = make(chan State, 500)
// ... Other watchdog code
return ts
}
Note:
I am added some debug logging in multiple locations. From within the Bot Handler, the price was printed (successfully), then updated, and then printed (successfully) again -- Showing no gap in the setting of Price from within the handler() function.
When adding the same type of debugging to the main() for{} loop, I tried setting an incrementing counter and assigning the value of thing.Price -- Printing thing.Price on each loop results in 0, even if I set the price (and validate it gets set) in the same loop, it is back to 0 on the next iteration.
This behavior is why I think that I am missing something very fundamental.

Answer 1

In Go, arguments are passed to functions by value -- meaning what the function gets is a copy of the value, not a reference to the variable. The same is true of the function receiver, and also the return list.
It's not the most elegant description, but for the sake of explanation, let's call this the "function wall." If the value being passed one way or the other is a pointer, the function still gets a copy, but it's a copy of a memory address, and so the pointer can be used to change the value of the variable on the other side of the wall. If it is a reference type, which uses a pointer in the implementation of the type, then again a change to the thing being pointed to can cross that wall. But otherwise the change does not cross the wall, which is one reason so many Go functions are written to return values instead of just modifying values.
Here's a runnable example:
package main
import (
"fmt"
)
type Car struct {
Color string
}
func (c Car) Change() { // c was passed by value, it's a copy
c.Color = "Red"
}
func main() {
ride := Car{"Blue"}
ride.Change()
fmt.Println(ride.Color)
}
Prints "Blue"
But two small changes:
func (c *Car) Change() { // here
c.Color = "Red"
}
func main() {
ride := &Car{"Blue"} // and here
ride.Change()
fmt.Println(ride.Color)
}
And now it prints "Red". Struct is not a reference type. So if you want modifications to a struct to cross the wall without using the return list to do it, use a pointer. Of course this only applies to values being passed via argument, return list, or receiver; and not to variables that are in scope on both sides of the wall; or to modifying the underlying value behind a reference type.
See also "Pointers Versus Values" in Effective Go, and "Go Data Structures" by Russ Cox.