append() gotcha!

[DrGo]

Hello,

The following compiles as well as evades scrutiny by go Vet (and many a human reviewer) resulting in perplexing bugs. What purpose calling append() with only one argument serves? Shouldn't it be banned?

```

var byteArray= []byte{'A', 'B',}

func main() {

buf:=[]byte {'C'}

buf = append(byteArray)  //<--- gotche meant: buf = append(buf, byteArray)

fmt.Println(string(buf))

}

```

https://play.golang.org/p/jP8NGiEN9A

[Nate Finch]

This is an unfortunate side effect of using ... (a variadic) in append.  Zero is a valid number of arguments to pass to it.

However, I think this is a 100% valid thing to put into go vet.  There's never a time when someone writes this code on purpose instead of just writing buf = byteArray.

[Val]

Agreed, append with only one argument doesn't look good and should be vetted.

As a superset of this problem, any use of

  a = append(b, c)
with b != a (i.e. not assigning back to the same slice variable) 

looks either broken or cryptic me.

This holds whether c is zero or one or more variadic elements.

I mean, while there are a few usages where it is written on purpose, it makes reading and reasoning much more difficult.

Val

[Nate Finch]

Wrote it up as an issue: https://github.com/golang/go/issues/21482

and I agree that using append to assign to a different variable than the one in the append call is almost always going cause surprising behavior.... but it's less clearly a mistake than append with only a single argument.

[xingtao zhao]

Sometimes, I use:

a := []int{ 1, 2, 3 }

b : = append([]int(nil), a...)

to copy a slice.

[Lucio]

On Wednesday, 16 August 2017 05:16:45 UTC+2, Nate Finch wrote:

This is an unfortunate side effect of using ... (a variadic) in append.  Zero is a valid number of arguments to pass to it.

But append is not a normal function, it is an intrinsic, so the compiler could implement a check? It would have to be an error to have a single argument, which I presume would be acceptable? Are there likely to be buggy programs out there that would break if it did?

Lucio. 

[DrGo]

Thanks Nate,