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Call Anonymous Function for Go var Assignment

Go has a specific order that code is evaluated:

1. Declarations with initial value assignment
2. Imported init functions
3. Package init functions
4. Everything in main

During the declaration and initial value assignment step you can call functions that already exist (since compilation obviously happens before all of this, something that isn’t obvious to people used to languages where stuff has to happen in order, like shell):

package main

import "fmt"

var n = multiply(3, 5)

func multiply(x, y int) int {
	return x * y
}

func main() {
	fmt.Println(n)
}

But, you cannot assign to things that were previously declared in package scope. You also cannot just call a function.

n = 14        // "expected declaration"
multiply(x,y) // "expected declaration

When you need to do this it is generally because you want to assign a value or otherwise initialize the state of the package. That’s what init() is for.

But sometimes you don’t need an independent function just to initialize a single package variable. The solution is to call an anonymous function during declaration and assignment:

// Reg points to the the internal Register. Everything that imports cmdbox
// within a single executable shares the same Register.
var Reg = func() *register {
	r := new(register)
	r.Init()
	return r
}()

This has the added advantage of ensuring that the assignment runs before any init is called allowing a sort of init before the init() meaning that all the init calls can use the initialized value of your package variable.

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