Go - Inconsistent Evaluation of Deferred Functions -

i experimenting go , seeing unexpected behaviour deferred functions. consider following program increments global variable given amount.

package main  import "fmt"  var z = 1  func main() {      defer increasez(10)     defer fmt.println("z =", increasez(20), "deferred value 1")     defer fmt.println("z =", increasez(30), "deferred value 2")      fmt.println("z =", z, "main value") }  func increasez(y int) int {     z += y     println("z =", z, "inside increase function")     return z } 

when run in go playground, outputs:

z = 21 inside increase function z = 51 inside increase function z = 61 inside increase function z = 51 main value z = 51 deferred value 2 z = 21 deferred value 1 

if switch order of deferred functions, has effect:

defer fmt.println("z =", increasez(20), "deferred value 1") defer fmt.println("z =", increasez(30), "deferred value 2") defer increasez(10) 

outputs:

z = 21 inside increase function z = 51 inside increase function z = 51 main value z = 61 inside increase function z = 51 deferred value 2 z = 21 deferred value 1 

the go documentation states:

the deferred call's arguments evaluated immediately, function call not executed until surrounding function returns.

so arguments being evaluated, may explain why main value returned 51 , not 61, since fmt.println statements taking increasez argument, defer increasez(10) not called until after main function returns.

however, not explain why in first example increasez(10) outputting before main has completed, , after main has completed in second example.

i grateful if me understand happening here, since looks fertile ground difficult diagnose bugs further down line.

you being inconsistent in print destination.

stdout: fmt.println  stderr: println 

write same print destination.

package main  import "fmt"  var z = 1  func main() {      defer increasez(10)     defer fmt.println("z =", increasez(20), "deferred value 1")     defer fmt.println("z =", increasez(30), "deferred value 2")      fmt.println("z =", z, "main value") }  func increasez(y int) int {     z += y     fmt.println("z =", z, "inside increase function")     return z } 

output:

z = 21 inside increase function z = 51 inside increase function z = 51 main value z = 51 deferred value 2 z = 21 deferred value 1 z = 61 inside increase function 

or,

package main  import (     "fmt"     "os" )  var z = 1  func main() {      defer increasez(10)     defer fmt.fprintln(os.stderr, "z =", increasez(20), "deferred value 1")     defer fmt.fprintln(os.stderr, "z =", increasez(30), "deferred value 2")      fmt.fprintln(os.stderr, "z =", z, "main value") }  func increasez(y int) int {     z += y     println("z =", z, "inside increase function")     return z } 

output:

z = 21 inside increase function z = 51 inside increase function z = 51 main value z = 51 deferred value 2 z = 21 deferred value 1 z = 61 inside increase function