Whenever I write appends and I'm splicing slices together, I often get an
error saying the second slice is the wrong type (it wants the slice element
type). So, doesn't that mean the trailing ellipsis is like an iterator
feeding out one element at a time? Is there some reason this is needed,
because if slice types match this unrolling is implicit, I mean, the
programmer obviously intends the two slices be spliced into one new one...
On Friday, 3 May 2019 19:13:02 UTC+2, Ian Lance Taylor wrote:
>
> On Fri, May 3, 2019 at 7:57 AM Louki Sumirniy
> <louki.sumi...@gmail.com <javascript:>> wrote:
> >
> > Ellipsis makes the parameter type into a slice, but in append it makes
> the append repeat for each element, or do I misunderstand this?
> >
> > There is a syntactic distinction between them too. Parameters it is a
> prefix to the type, append it is a suffix to the name. It neatly alludes to
> the direction in which the affected variable is operated on - inside the
> function name ...type means name []type and for append, we are splitting
> the slice into a tuple (internally), at least as I understand it, and the
> parameter is the opposite, tuple to slice.
> >
> > I sometimes lament the lack of a tuple type in Go (I previously worked a
> lot with Python and PHP), but []interface{} isn't that much more difficult
> and the ellipsis syntax is quite handy for these cases - usually loading or
> otherwise modifying essentially a super simple container array.
>
> For any function F and some type T declared as
>
> func F(x ...T) {}
>
> within F x will have type []T. You can call F with a slice s of type []T
> as
>
> F(s...)
>
> That will pass the slice s to F as the final parameter. This works
> for any variadic function F.
>
> The append function is implicitly declared as
>
> func append(to []byte, add ...byte)
>
> You can call it as
>
> append(to, add...)
>
> Here F is append and T is byte.
>
> There is a special case for append with an argument of type string,
> but other than that append is just like any other variadic function.
>
> Ian
>
>
>
> > On Friday, 3 May 2019 16:44:47 UTC+2, Ian Lance Taylor wrote:
> >>
> >> On Fri, May 3, 2019 at 7:34 AM Louki Sumirniy
> >> <louki.sumi...@gmail.com> wrote:
> >> >
> >> > The ellipsis has two uses in Go, one is in variadic parameters, the
> other is in the slice append operator. It is essentially an iterator that
> takes a list and turns it into a slice (parameters) or takes a slice and
> turns it into a recursive iteration (append). Parameters with the ellipsis
> are addressed inside the function as a slice of the type after the
> ellipsis.
> >>
> >> Note that there is nothing special about append here, it's just like
> >> passing a slice to any other variadic parameter. See
> >> https://golang.org/ref/spec#Passing_arguments_to_..._parameters .
> >>
> >> Ian
> >
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