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Appending slices unreliable behaviour. #29710

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Armatorix opened this issue Jan 12, 2019 · 7 comments
Closed

Appending slices unreliable behaviour. #29710

Armatorix opened this issue Jan 12, 2019 · 7 comments
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@Armatorix
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Armatorix commented Jan 12, 2019 •
edited

What version of Go are you using (go version)?

1.11.1

What did you do?

I've append two slices.
https://play.golang.org/p/SV9LKgUV496

What did you expect to see?

ID: [0 1 2 3 4 5 6 7 8 9]
IDX: [0 1 0 1 2 3 4 5 6 7 8 9]
ID: [0 1 5 6 7 8 9 7 8 9]
IDX: [0 1 5 6 7 8 9]

What did you see instead?

ID: [0 1 2 3 4 5 6 7 8 9]
IDX: [0 1 0 1 2 3 4 5 6 7 8 9]
ID: [0 1 2 3 4 5 6 7 8 9]
IDX: [0 1 5 6 7 8 9]

what is the purpose?

So IMO this function is not reliable, because when there is enough space in the allocated memory we're changing first arguments value, and if there is not enough we're not doing it. So we're ending with a various result of this function.
Second thing is if I've got return statement with result slice, I'm not expecting to change arguments values.
@tomor
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tomor commented Jan 12, 2019

IMO the method behaves exactly as described in the docs: https://golang.org/pkg/builtin/#append

@ianlancetaylor
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Please read https://blog.golang.org/slices .

@Armatorix
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Yes, it is working as described. But I'm wondering if it is a good approach. IMO append loses its semantic meaning when working like this.

@ianlancetaylor
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Slices work as they are documented. That is not something we are going to change at this point in the language's development.

@Armatorix
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Armatorix commented Jan 17, 2019 •
edited

If something is documented it doesn't imply that it is good. I can document that my function is doing unexpected things and implement it like so, and that would not be good at all.
So you're telling that when you would see for first time function append() which returns some slice, you're expecting that in some situations it changes it arguments values and in some not?

@ianlancetaylor
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The language acts as is documented and intended. I understand that you find it surprising, but there are good reasons for it to act this way, and more importantly, as I said, this is not something that can be changed at this point in the language's development. If you need to discuss this further, please use a forum; see https://golang.org/wiki/Questions .

@Armatorix
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Thx for the link, it will be probably better to use other discussion channel.

@golang golang locked and limited conversation to collaborators Jan 21, 2020
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@tomor @ianlancetaylor @gopherbot @Armatorix