Error: cannot define new methods on non-local type []string

[hidsh]

Hi,

I'd like to concatenate all elements in []string.

fn mapconcat(arr []string) string {
    mut ret := ''
    for s in arr {
        ret += s
    }
    return ret
}

arr := ['aaa', 'bbb']
s := mapconcat(arr)

println(s)      // =>aaabbb

Okey. However, I prefer to use it as a method if possible.
So I tried:

fn (arr []string) mapconcat() string {
    mut ret := ''
    for s in arr {
        ret += s
    }
    return ret
}

arr := ['aaa', 'bbb']
s := arr.map(|x| x.to_upper()).mapconcat()

println(s)

But I got an error:

sample.v:1:9: error: cannot define new methods on non-local type []string
    1 | fn (arr []string) mapconcat() string {
      |         ~~~~~~~~
    2 |     mut ret := ''
    3 |     for s in arr {

I have no idea though it looks like Golang's receiver error relatives.

Does anyone have how to fix?

[Delta456]

You need to make an alias

type Alias = []string

fn (arr Alias) mapconcat() string {
    mut ret := ''
    for s in arr {
        ret += s
    }
    return ret
}

arr := Alias(['aaa', 'bbb'])
s := Alias(arr.map(|x| x.to_upper())).mapconcat()

println(s)

[hidsh]

Thank you for your quick reply.

I didn't know type Alias and casting to use it.
Thanks.

[Delta456]

I believe we should add a suggestion to the error message to help others.