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fmt: Empty slice and nil slice are printed both as empty slice #12780

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krnowak opened this issue Sep 29, 2015 · 3 comments
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fmt: Empty slice and nil slice are printed both as empty slice #12780

krnowak opened this issue Sep 29, 2015 · 3 comments

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@krnowak
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@krnowak krnowak commented Sep 29, 2015

Please see following snippet (also saved in http://play.golang.org/p/bTc5CWT--P):

package main

import "fmt"
import "reflect"

func main() {
    var nilArray []string = nil
    emptyArray := []string{}
    if !reflect.DeepEqual(nilArray, emptyArray) {
        fmt.Printf("They are different as expected, let's see if this is going to be helpful message, like you would like to receive in some test\n")
        fmt.Printf("Expected:\n%+v\nGot:\n%+v\n", nilArray, emptyArray)
    }
}

I would expect it to print something like:

Expected:
<nil>
Got:
[]

but instead, I'm getting:

Expected:
[]
Got:
[]

Tracking what actually is different might be a PITA in some more complex data structures.

@cznic
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@cznic cznic commented Sep 29, 2015

When the dictinction matters, the # verb modifier can be used to show it: http://play.golang.org/p/smJjDhLM-H

@krnowak
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@krnowak krnowak commented Sep 29, 2015

@cznic: Thanks for the hint. But the point probably still stands.

@ianlancetaylor ianlancetaylor changed the title fmt: Empty array and nil array are printed both as empty array. fmt: Empty slice and nil slice are printed both as empty slice Sep 29, 2015
@ianlancetaylor
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@ianlancetaylor ianlancetaylor commented Sep 29, 2015

This is intended behaviour and even documented behaviour.

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