TEXLUTION

Not everything in Go is unicorns and rainbows, like every other language, it has its gotchas. For me personally, the worst of all is nil checking, because while being aware of it, I still get bitten sometimes.

If you have been programming in Go long enough, you probably know by now that nil values do not always equal nil. Francesc Campoy talked about it at dotGo 2014. If you want to know if you really get it, I’d challenge you to answer the following quiz by writing down if each of the following variables is == nil. To check your score just run the program.

package main

import (
	"fmt"
	"reflect"
)

// Fooer is just a dummy interface to implement
type Fooer interface {
	IsFoo() bool
}

// Foo is our example type
type Foo struct {}

// IsFoo implements the Fooer interface
func (f *Foo) IsFoo() bool {
	return true
}

// NewFoo returns an uninitialized Foo pointer
func NewFoo() *Foo {
	var VarFooPointer *Foo
	return VarFooPointer
}

// NewFooNil foo returns nil as Foo pointer
func NewFooNil() *Foo {
	return nil
}

// NewFooer returns an uninitialized Foo pointer as Fooer
func NewFooer() Fooer {
	var VarFooPointer *Foo
	return VarFooPointer
}

// NewFooerNil returns nil as Fooer
func NewFooerNil() Fooer {
	return nil
}

var q = fmt.Printf // shorthand to reduce noise

func main() {
	var VarFooPointer *Foo
	q("Q01  VarFooPointer == nil?       %t \t value = %#v reflect.IsNil = %t\n",
		VarFooPointer == nil,
		VarFooPointer,
		reflect.ValueOf(VarFooPointer).IsNil())

	VarFooPointer = NewFoo()
	q("Q02  VarFooPointer == nil?       %t \t value = %#v reflect.IsNil = %t\n",
		VarFooPointer == nil,
		VarFooPointer,
		reflect.ValueOf(VarFooPointer).IsNil())

	VarFooPointer = NewFooNil()
	q("Q03  VarFooPointer == nil?       %t \t value = %#v reflect.IsNil = %t\n",
		VarFooPointer == nil,
		VarFooPointer,
		reflect.ValueOf(VarFooPointer).IsNil())

	var VarEmptyInterface interface{}
	q("Q04  VarEmptyInterface == nil?   %t \t value = %#v reflect.IsNil = %s\n",
		VarEmptyInterface == nil,
		VarEmptyInterface,
		"panics")

	VarEmptyInterface = VarFooPointer
	q("Q05  VarEmptyInterface == nil?   %t \t value = %#v reflect.IsNil = %t\n",
		VarEmptyInterface == nil,
		VarEmptyInterface,
		reflect.ValueOf(VarEmptyInterface).IsNil())

	VarEmptyInterface = NewFoo()
	q("Q06  VarEmptyInterface == nil?   %t \t value = %#v reflect.IsNil = %t\n",
		VarEmptyInterface == nil,
		VarEmptyInterface,
		reflect.ValueOf(VarEmptyInterface).IsNil())

	VarEmptyInterface = NewFooNil()
	q("Q07  VarEmptyInterface == nil?   %t \t value = %#v reflect.IsNil = %t\n",
		VarEmptyInterface == nil,
		VarEmptyInterface,
		reflect.ValueOf(VarEmptyInterface).IsNil())

	var VarFooAsserted = VarEmptyInterface.(*Foo)
	q("Q08  VarFooAsserted == nil?      %t \t value = %#v reflect.IsNil = %t\n",
		VarFooAsserted == nil,
		VarFooAsserted,
		reflect.ValueOf(VarFooAsserted).IsNil())

	var VarFooerAsserted = VarEmptyInterface.(Fooer)
	q("Q09  VarFooerAsserted == nil?    %t \t value = %#v reflect.IsNil = %t\n",
		VarFooerAsserted == nil,
		VarFooerAsserted,
		reflect.ValueOf(VarFooerAsserted).IsNil())

	var VarFooer = NewFooer()
	q("Q10  VarFooer == nil?            %t \t value = %#v reflect.IsNil = %t\n",
		VarFooer == nil,
		VarFooer,
		reflect.ValueOf(VarFooer).IsNil())

	var VarFooer2 = NewFooerNil()
	q("Q11  VarFooer2 == nil?           %t \t value = %#v reflect.IsNil = %s\n",
		VarFooer2 == nil,
		VarFooer2,
		"panics")

	var VarEmptyInterface2 interface{}
	VarEmptyInterface2 = VarEmptyInterface.(*Foo)
	q("Q12  VarEmptyInterface2 == nil?  %t \t value = %#v reflect.IsNil = %t\n",
		VarEmptyInterface2 == nil,
		VarEmptyInterface2,
		reflect.ValueOf(VarEmptyInterface2).IsNil())

	VarEmptyInterface2 = NewFooer()
	q("Q13  VarEmptyInterface2 == nil?  %t \t value = %#v reflect.IsNil = %t\n",
		VarEmptyInterface2 == nil,
		VarEmptyInterface2,
		reflect.ValueOf(VarEmptyInterface2).IsNil())

	VarEmptyInterface2 = NewFooerNil()
	q("Q14  VarEmptyInterface2 == nil?  %t \t value = %#v reflect.IsNil = %s\n",
		VarEmptyInterface2 == nil,
		VarEmptyInterface2,
		"panics")

	var VarFooer3 Fooer
	VarFooer3 = VarEmptyInterface.(*Foo)
	q("Q15  VarFooer3 == nil?           %t \t value = %#v reflect.IsNil = %t\n",
		VarFooer3 == nil,
		VarFooer3,
		reflect.ValueOf(VarFooer3).IsNil())

	VarFooer3 = NewFoo()
	q("Q16  VarFooer3 == nil?           %t \t value = %#v reflect.IsNil = %t\n",
		VarFooer3 == nil,
		VarFooer3,
		reflect.ValueOf(VarFooer3).IsNil())

	VarFooer3 = NewFooNil()
	q("Q17  VarFooer3 == nil?           %t \t value = %#v reflect.IsNil = %t\n",
		VarFooer3 == nil,
		VarFooer3,
		reflect.ValueOf(VarFooer3).IsNil())

	VarEmptyInterface = nil
	q("Q18  VarEmptyInterface == nil?   %t \t value = %#v reflect.IsNil = %s\n",
		VarEmptyInterface == nil,
		VarEmptyInterface,
		"panics")
}

Note: If you find assigning to interface{} variables very odd, notice that this is pretty much what happens when you use interface{} function parameters.