Go Details & Tips 101 (20240316) by Tapir Liu

Author:Tapir Liu
Language: eng
Format: epub

The famous := trap

Let's view a simple program.

package main import "fmt" import "strconv" func parseInt(s string) (int, error) { n, err := strconv.Atoi(s) if err != nil { fmt.Println("err:", err) b, err := strconv.ParseBool(s) if err != nil { return 0, err } fmt.Println("err:", err) if b { n = 123 } } return n, err } func main() { fmt.Println(parseInt("true")) }

We know that the call strconv.Atoi(s) will return a non-nil error, but the call strconv.ParseBool(s) will return a nil error. Then, will the call parseInt("true") return a nil error, too? The answer is it will return a non-nil error. The outputs of the program is shown below:

err: strconv.Atoi: parsing "true": invalid syntax err: <nil> 123 strconv.Atoi: parsing "true": invalid syntax

Wait, isn't the err variable is re-declared in the inner code block and its value has been modified to nil before the parseInt("true") returns? This is a confusion many new Go programmers, including me, ever encountered when they just started using Go.

The reason why the call parseInt("true") returns a non-nil error is a variable declared in an inner code block is never a re-declaration of a variable declared in an outer code block. Here, the inner declared err variable is set (initialized) as nil. It is not a re-declaration (a.k.a. modification) of the outer declared err variable. The outer one is set (initialized) as a non-nil value, then it is never changed later.

There is the voice to remove the ... := ... re-declaration syntax form from Go. But it looks this is a too big change for Go. Personally, I think explicitly marking the re-declared variables out is a more feasible solution.

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