fmt: %v verb does not print nil values for maps or slices

[jhftrifork]

Steps to reproduce

package main
import "fmt"
func main() {
    var not_inited map[string]string = nil
    var inited     map[string]string = make(map[string]string)

    fmt.Printf("not_inited = %v\n", not_inited)
    fmt.Printf("inited = %v\n", inited)
}

Expected behavior

$ go run foo.go
not_inited = <nil>
inited = map[]

Actual behavior

$ go run foo.go
not_inited = map[]
inited = map[]

Notes

The nil value inhabits pointer types, function types, interface types, channel types, map types, and slice types. For all of these types, the %v verb should distinguish the nil value from all other values of the type.

Here is a larger program showing the inconsistent behavior of %v:

package main
import "fmt"
func main() {
    var pointer_not_inited    *string            = nil
    var pointer_inited        *string            = (func(s string) *string { return &s; })("foo")

    var func_not_inited       func(string)string = nil
    var func_inited           func(string)string = func(s string) string { return s; }

    var interface_not_inited  interface{}        = nil
    var interface_inited      interface{}        = "foo"

    var channel_not_inited    chan string        = nil
    var channel_inited        chan string        = make(chan string)

    var map_not_inited        map[string]string  = nil
    var map_inited            map[string]string  = make(map[string]string)

    var slice_not_inited      []string           = nil
    var slice_inited          []string           = make([]string, 0)

    fmt.Printf("pointer_not_inited = %v\n", pointer_not_inited)
    fmt.Printf("pointer_inited = %v\n", pointer_inited)

    fmt.Printf("func_not_inited = %v\n", func_not_inited)
    fmt.Printf("func_inited = %v\n", func_inited)

    fmt.Printf("interface_not_inited = %v\n", interface_not_inited)
    fmt.Printf("interface_inited = %v\n", interface_inited)

    fmt.Printf("channel_not_inited = %v\n", channel_not_inited)
    fmt.Printf("channel_inited = %v\n", channel_inited)

    fmt.Printf("slice_not_inited = %v\n", slice_not_inited)
    fmt.Printf("slice_inited = %v\n", slice_inited)

    fmt.Printf("map_not_inited = %v\n", map_not_inited)
    fmt.Printf("map_inited = %v\n", map_inited)
}

[cznic]

• This cannot be changed due to the Go 1 compatibility promise.
• It's possible to get the desired distinction using %#v: http://play.golang.org/p/AIFc50tUO7

[jhftrifork]

I just looked; the %#v verb is better. I'll stop using %v from now on then.

The issue I mentioned is serious: it cost me a lot of time debugging due to my (sensible) assumption that a value was not nil when in fact it was. If %v cannot be changed, I would suggest that the documentation mark %v as deprecated and encourage the use of %#v.

[ianlancetaylor]

I'm not sure whether we can make this change or not. Sending to Rob to decide.

[robpike]

Working as intended, with an easy workaround (%#v). Shouldn't change behavior like this.