Go: Extending unnamed types such as []string

Question

I am a bit confused in regards to type aliases in Go.

I have read this related SO question - Why can I type alias functions and use them without casting?.

As far as I understand, unnamed and named variables are assignable to each other if the underlying structure is the same.

What I am trying to figure out, is can I extend unnamed types by naming them - something like this:

type Stack []string

func (s *Stack) Print() {
    for _, a := range s {
        fmt.Println(a)
    }
}

This gives me the error cannot range over s (type *Stack)
Tried casting it to []string, no go.

I know the below code works - is this the way I should do it? If so, I would love to know why the above is not working, and what is the use of declarations such as type Name []string.

type Stack struct {
    data []string
}

func (s *Stack) Print() {
    for _, a := range s.data {
        fmt.Println(a)
    }
}

s is a pointer to Stack and thus a pointer to []string and in Go you cannot range over pointers. A range *s would do. This has nothing to do with "named" or "unnamed" types or aliases, it is just a result of static typing and your s having the wrong type for range-ing. — Volker
– Volker, Commented Oct 14, 2014 at 8:01
In Go, there is no such thing as a type alias. The type keyword introduces new named types. They are not aliases. (This is one of Go's important strengths compared to various other languages) — Rick-777
– Rick-777, Commented Oct 16, 2014 at 12:43

Jarod · Accepted Answer · 2014-10-14 08:02:05Z

7

You should dereference the pointer s

type Stack []string

func (s *Stack) Print() {
    for _, a := range *s {
        fmt.Println(a)
    }
}

answered Oct 14, 2014 at 8:02

Jarod

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2 Comments

DannyB Over a year ago

Excellent! So simple. For some reason, I looked for the complicated solution. I guess it is because the Go compiler spoiled me - it usually lets me know when *type is used as type. Thanks a lot.

siritinga Over a year ago

Francesc Campoy (Google) has a great presentation where he summarizes in which cases you can use *T or T and when it is different. speakerdeck.com/campoy/things-i-learned-teaching-go starting in page 38.

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