Convert named-type map[string]string to plain one in Golang

Question

I have a map of type set, which is actually a map[string]string. However, passing it to a function which accepts a map[string]string fails, because Go does not recognize set as one.

However, I fail to convince the compiler it is one. Is there any way to resolve this, without looping and copying?

package main

import (
    "fmt"
)

type name string
type field string

type set map[name]field      // map[string]string after all
type plain map[string]string // also map[string]string

func main() {
    var typed = set{"hi": "ho"} // map[string]string?

    back := plain(typed)  // cannot convert typed (type set) to type plain
    back := typed.(plain) // invalid type assertion: typed.(plain) (non-interface type set on left)

    echo(back)
}

func echo(in map[string]string) {
    fmt.Println(in)
}

Those types aren't equivalent. You can't convert between map[name]field and map[string]string. — Adrian
– Adrian, Commented Jun 7, 2019 at 14:02
Both error messages clearly state the problem. In Go defining a type like your field is only sensible if you need to attach methods or this type is a major, standalone one. In your code you will have to copy the map. — Volker
– Volker, Commented Jun 7, 2019 at 14:09
Loop and copy it is. If you think this is wasteful: Don't use different types set and plain. Dead simple. — Volker
– Volker, Commented Jun 7, 2019 at 14:19
Yes, I dislike the different types as well. But this is being done here github.com/prometheus/common/blob/… so I need to deal with it. — shorez
– shorez, Commented Jun 7, 2019 at 14:21

eugenioy · Accepted Answer · 2019-06-07 15:14:00Z

2

You could do this using the unsafe package.

Note: I don't think this would necessarily be a good idea, and probably the right way would be to just iterate and copy, but since it does answer the question asked...

var typed = set{"hi": "ho"} // map[string]string?
p := unsafe.Pointer(&typed)
var back plain
back = *(*plain)(p)

Playground: https://play.golang.org/p/yienSuJSnQU

answered Jun 7, 2019 at 15:14

eugenioy

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1 Comment

shorez Over a year ago

Okay, wow! I didn't think of unsafe. While this is probably a bad way to use, it answers the original question. Thanks! Nevertheless, loop and copy seems to be the way to go.

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