nil maps

[Alex Dvoretskiy]

Hello, Golang Nuts!

I have an interesting question about maps. What is the possible usage of nil maps, which can be declared like "var m map[string]int"? You can't write to nil map but have an option to create it.

Perhaps there is no use at all and this is just language specific feature?

Thank you.

[Dan Kortschak]

If you only need the map conditionally, you need to declare it and then
conditionally make it.

```

var m map[string]int

if needMap {
    m = make(map[string]int)
}
```

[Christian Himpel]

The spec defines: "A nil map is equivalent to an empty map except that no elements may be added." https://golang.org/ref/spec#Map_types

You could return a nil map, if you have no elements to add, and a caller would just need to read.

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[Rob Pike]

A nil map is like a regular map but it has nothing in it, cannot grow, and takes zero space.

-rob

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[Thomas Bushnell, BSG]

Here's a case that comes up for me:

type table map[string]map[string]string

func (t table) add(x, y, z string) {

  if t[x] == nil {

    t[x] = make(map[string]string)

  }

  t[x][y] = z

}

func (t table) get(x, y string) string {

  return t[x][y]

}

The fact that t[x] can be indexed even if it hasn't been created makes this much simpler.

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[Alex Dvoretskiy]

Looks fun!: https://play.golang.org/p/8mqzb-1H7f

[roger peppe]

Or taking it a bit further: https://play.golang.org/p/g4uF2QjiJQ

[Alex Dvoretskiy]

wow, super impressed!