What does "i.(string)" actually mean in golang syntax?

Question

I recently started looking for functional go examples and I found this function:

mapper := func (i interface{}) interface{} {
    return strings.ToUpper(i.(string))
}
Map(mapper, New(“milu”, “rantanplan”))
//[“MILU”, “RANTANPLAN”]

Now in this function, as you can see the return value of mapper is: strings.ToUpper(i.(string)).

But, what does this i.(string) syntax mean? I tried searching, but didn't find anything particularly useful.

When googling for punctuation, it helps to spell it out. I found the duplicate by searching "go dot parentheses". — sepp2k, Dec 02 '18 at 06:31
@sepp2k oh okay. I'll use that next time. thanks for the tip! — Daksh M., Dec 02 '18 at 06:32

score 9 · Accepted Answer · answered Dec 02 '18 at 06:29

i.(string) casts (or attempts at least) i (type interface{}) to type string. I say attempts because say i is an int instead, this will panic. If that doesn't sound great to you, then you could change the syntax to

x, ok := i.(string)

In this case if i is not a string, then ok will be false and the code won't panic.

score 2 · Answer 2 · answered Dec 02 '18 at 06:29

2

i.(string) means converting i(interface{} type) to string type.

answered Dec 02 '18 at 06:29

nightfury1204

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What does "i.(string)" actually mean in golang syntax?

2 Answers2