133 lines
2.3 KiB
Markdown
133 lines
2.3 KiB
Markdown
---
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layout: post
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title: "go: `:=` operator causes accidental shadowing"
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date: 2016-01-14 21:13
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comments: true
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categories:
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- go
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---
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Go provides `:=` operator to make declaring variables easier. It is a [shorthand to declare and set a value of a variable](https://golang.org/ref/spec#Short_variable_declarations). for example,
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```go
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var x int
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x = 42
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```
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can be written as
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```go
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x := 42
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```
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But if not careful, this can accidently shadow variable bindings. Let's look at the fictitious piece of code.
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```go
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package main
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import "fmt"
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func fictitiousFunc() (int, error) {
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return 42, nil
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}
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func main() {
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x := 10;
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x, err := fictitiousFunc()
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if err != nil {
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fmt.Println("I'll never print")
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}
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fmt.Println("value of x: ", x)
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}
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```
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This produces following output
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```
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value of x: 42
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```
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While, this following piece of code will fail to compile
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```go
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package main
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import "fmt"
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func fictitiousFunc() (int, error) {
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return 42, nil
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}
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func main() {
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x := 10
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// replace :=
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var x int
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var err error
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x, err = fictitiousFunc()
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if err != nil {
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fmt.Println("I'll never print")
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}
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fmt.Println("value of x: ", x)
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}
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```
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output:
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```
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prog.go:12: x redeclared in this block
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previous declaration at prog.go:10
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```
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So we can see that the operator is somewhat intelligent, and does not redeclare the variables.
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Now what if we push it down a scope? See the following code
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```go
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package main
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import "fmt"
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func fictitiousFunc() (int, error) {
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return 42, nil
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}
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func main() {
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someCondition := true
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x := -1;
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if someCondition {
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x, err := fictitiousFunc()
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if err != nil {
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fmt.Println("I'll never print")
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}
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fmt.Println("value of x inside: ", x)
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}
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fmt.Println("value x outside: ", x)
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}
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```
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This produces,
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```go
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value of x inside: 42
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value x outside: -1
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```
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At line: 16, since the immediate scope (line:15-32) does not have variable `x` declared, `:=` is redeclaring the variable. a.k.a the __variable `x` gets shadowed__.
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Only workaround I can think of is not to use `:=`, i.e change the code to
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```go
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if someCondition {
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var err error
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x, err = fictitiousFunc()
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if err != nil {
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fmt.Println("I'll never print")
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}
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fmt.Println("value of x inside: ", x)
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}
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```
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If you know something better let me know.
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