Cannot use `[]Data` as `[]Cols` in argument even though `Data` implements `Cols`

[jorgeluismireles]

Describe the bug

I think is a bug otherwise I want to know alternatives.

• Struct Data implements interface Cols with function cols() string
• There is a function fn print_first_last(cols []Cols) { }
• An array of []Data{ len:3 } is passed to print_first_last function but is not accepted.

Nor []&Data{} is accepted.

Reproduction Steps

pub interface Cols {
	cols() string
}

pub fn print_first_last(cols []Cols) {
	// ...
}

struct Data implements Cols {
	// data fields
}

fn (c Data) cols() string { // implementation
	return '...'
}

fn main() {
	data := []Data{ len:3 }
	print_first_last(data)
}

Expected Behavior

[]Data{} passed to print_first_last(c [] Cols) should be accepted since Data implements Cols and array of implementation should be accepted also.

Current Behavior

Running code...
Can't run code. The server returned an error:
code.v:19:19: error: cannot use `[]Data` as `[]Cols` in argument 1 to `print_first_last`
   17 | fn main() {
   18 |     data := []Data{ len:3 }
   19 |     print_first_last(data)
      |                      ~~~~
   20 | }
Exited with error status 1
Please try again.

Possible Solution

No response

Additional Information/Context

No response

V version

V 0.4.9 b91bbad

Environment details (OS name and version, etc.)

https://play.vlang.io/p/d6a82f4ac2

Note

You can use the 👍 reaction to increase the issue's priority for developers.

Please note that only the 👍 reaction to the issue itself counts as a vote.
Other reactions and those to comments will not be taken into account.

[huly-for-github]

Connected to Huly®: V_0.6-22087

[JalonSolov]

Why do you think this should work? Cols knows nothing about Data. Data does know about Cols, since it implements it.

[jorgeluismireles]

Workaround:

fn main() {
	mut cols := []Cols{}
	cols << []Data{}
	cols << []Data{}
	print_first_last(cols)
}

But if we already have an array []Data and don't want to push one of one to a []Cols to call the function I wonder if is possible... Thoughs?

[JalonSolov]

Change

pub fn print_first_last(cols []Cols) {

to

pub fn print_first_last(cols []Data) {

[jorgeluismireles]

print_first_last is a helper to print three rows: the first element, second row with ... and third row the last element:

pub fn print_first_last_table_cols(cols []Cols) {
	len := cols.len
	if len == 0 { // empty array
		println('  empty')
	} else { // first element
		println('  ${0:004d}) ${cols[0].cols()}')
	}
	if len > 2 { // intermediate
		println('        ...')
	}
	if len > 1 { // last element
		println('  ${len:004d}) ${cols[len-1].cols()}')
	}
}

and knows nothing about Data content nor (there are more) implementors which want to use the function with the condition they implements Cols method cols.

Now I realize I need to copy Data elements to Cols, that I think could be prevented to reduce programmer's code.

Having a large array of Data invites to use something like this:

	mut cols := []Cols{ len:data.len } // array of interfaces not initalized
	for i, d in data {
		cols[i] = d
	}
	print_first_last(cols)

which produces:

arrays of interfaces need to be initialized right away, therefore `len:` cannot be used (unless inside `unsafe`, or if you also use `init:`)

unless we add @[heap] to Data.

As i said I think solving somehow the expectation of this issue would help programmer reducing his/her code.

Maybe this issue should be changed from Bug to Feature request

[jorgeluismireles]

Just wanted to be sure and check that Golang doesn't accept the feature:

package main

type Cols interface {
	Cols() string
}

func PrintFirstLast(cols []Cols) {
	// ...
}

type Data struct {
	// ...
}

func (d Data) Cols() string {
	return "..."
}

func main() {
	data := make([]Data, 3)
	PrintFirstLast(data)
}

./prog.go:22:17: cannot use data (variable of type []Data) as []Cols value in argument to PrintFirstLast

Please remove bug tag from this issue and maybe set to feature request.

[qtc-de]

I agree that this would be pretty useful 👍