Does type assertion change the value in go?

Question

Go newbie here.

I have a map where the key arguments should be []string.

However, if I try to use the value directly arguments := m["arguments"] it doesn't seem to be the right type. When used later to append to another slice with arguments... I get Cannot use 'arguments' (type interface{}) as type []string.

I fixed this by chaning the assignment to a type check arguments, _ := m["arguments"].([]string). That works, but I'm not sure why. Is type assertion doing conversion as well?

The full example is below:

import (
    "github.com/fatih/structs"
    "strings"
)

var playbookKeyDict = map[string]string{
    "Playbook": "",
    "Limit" : "--limit",
    "ExtraVars" : "--extra-vars",
}

type Playbook struct {
    Playbook string `json:"playbook" xml:"playbook" form:"playbook" query:"playbook"`
    Limit string `json:"limit" xml:"limit" form:"limit" query:"limit"`
    ExtraVars string `json:"extra-vars" xml:"extra-vars" form:"extra-vars" query:"extra-vars"`
    Arguments []string `json:"arguments" xml:"arguments" form:"arguments" query:"arguments"`
    Args []string
}

func (p *Playbook) formatArgs() {
    // is it worth iterating through directly with reflection instead of using structs import?
    // https://stackoverflow.com/questions/21246642/iterate-over-string-fields-in-struct
    m := structs.Map(p)

    // direct assignment has the wrong type?
    // arguments := m["arguments"]
    arguments, _ := m["arguments"].([]string)
    delete(m, "arguments")

    for k, v := range m {
        // Ignore non-strings and empty strings
        if val, ok := v.(string); ok && val != "" {
            key := playbookKeyDict[k]
            if key == "" {
                p.Args = append(p.Args, val)
            } else {
                p.Args = append(p.Args, playbookKeyDict[k], val)
            }
        }
    }
    p.Args = append(p.Args, arguments...)
}

Himanshu · Accepted Answer · 2018-03-20T23:44:41.797

Type assertion is used to get a value wrapped around using interface.

m := structs.Map(p)

Map(v interface{}){}

Map function is actually taking interface as its argument in the case stated. It is wrapping the type which is []string and its underlying value which is slice. The type can be checked using Relection reflect.TypeOf().

func TypeOf(i interface{}) Type

According to Russ Cox blog on Interfaces

Interface values are represented as a two-word pair giving a pointer to information about the type stored in the interface and a pointer to the associated data.

As specified in Golang spec

For an expression x of interface type and a type T, the primary expression
x.(T)
asserts that x is not nil and that the value stored in x is of type T. The notation x.(T) is called a type assertion.

For the error part:-

Cannot use 'arguments' (type interface{}) as type []string

We first needs to get the underlying value of type []string from interface using type assertion.

Does type assertion change the value in go?

1 Answers1