blogng/blog/2016-01-14-golang-shorthand-operator-allows-accidental-shadowing-of-variable.markdown
Dhananjay Balan 58b20109cf Convert from old categories to tags
import sys
import yaml

with open(sys.argv[1]) as fp:
    data = fp.read()

if not data.find("---") == 0:
    # no head
    print("NO YAML HEAD FOUND")
    sys.exit(-1)

data = data[3:]
head_end = data.find("---")

head = data[0:head_end]
data = data[head_end+3:]

metadata = yaml.safe_load(head)

cats = metadata.pop('categories', None)
if cats != None:
    if type(cats) == list:
        tags = cats
    elif type(cats) == str:
        tags = cats.split()

    tags = list(map(lambda t: t.lower(), tags))

    metadata["tags"] = ", ".join(tags)
    new_data = f"---\n{yaml.dump(metadata, default_flow_style=False)}---{data}"
    # write it
    print(f"coverted: categories to tags: {tags} - {sys.argv[1]}")
    with open(sys.argv[1], "w") as fp:
        fp.write(new_data)
    sys.exit(0)

if not metadata.get("tags", None):
    metadata["tags"] = "untagged"
    new_data = f"---\n{yaml.dump(metadata, default_flow_style=False)}---{data}"
    print(f"untagged: {sys.argv[1]}")
    # write it
    with open(sys.argv[1], "w") as fp:
        fp.write(new_data)
    sys.exit(0)

print("No changes needed")
2019-01-28 17:16:27 -05:00

2.2 KiB

comments date layout tags title
true 2016-01-14 21:13 post go go: `:=` operator causes accidental shadowing

Go provides := operator to make declaring variables easier. It is a shorthand to declare and set a value of a variable. for example,

var x int
x = 42

can be written as

x := 42

But if not careful, this can accidently shadow variable bindings. Let's look at the fictitious piece of code.

package main

import "fmt"


func fictitiousFunc() (int, error) {
	return 42, nil
}

func main() {
	x := 10;
	x, err := fictitiousFunc()
    if err != nil {
		fmt.Println("I'll never print")
    }
	fmt.Println("value of x: ", x)
}

This produces following output

value of x:  42

While, this following piece of code will fail to compile

package main

import "fmt"

func fictitiousFunc() (int, error) {
    return 42, nil
}

func main() {
    x := 10
    // replace :=
    var x int
    var err error
    x, err = fictitiousFunc()
    if err != nil {
        fmt.Println("I'll never print") 
    }
    fmt.Println("value of x: ", x)
}

output:

prog.go:12: x redeclared in this block
    previous declaration at prog.go:10

So we can see that the operator is somewhat intelligent, and does not redeclare the variables.

Now what if we push it down a scope? See the following code

package main

import "fmt"


func fictitiousFunc() (int, error) {
	return 42, nil
}

func main() {
	someCondition := true
	
	x := -1;
	
	if someCondition {
		x, err := fictitiousFunc()
		
		if err != nil {
			fmt.Println("I'll never print")
		}
		
		fmt.Println("value of x inside: ", x)
	}
	
	fmt.Println("value x outside: ", x)
	
}

This produces,

value of x inside:  42
value x outside:  -1

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.

Only workaround I can think of is not to use :=, i.e change the code to

	if someCondition {
        var err error
        x, err = fictitiousFunc()
		
		if err != nil {
			fmt.Println("I'll never print")
		}
		
		fmt.Println("value of x inside: ", x)
	}

If you know something better let me know.