Parsing different structures depending on a field

[rasky]

Hi, I would like to parse a TOML made like this:

[connection]
type = "http"

    [connection.config]
    url = "https://foobar.com/api"
    forcessl = True

or:

[connection]
type = "tcp"

    [connection.config]
    ip = "12.12.12.12"
    port = 1234

so basically the config table changes its keys depending on connection.type. It looks like this library allows me to define a structure like this:

type Connection struct {
    Type string
    Config interface{}
}

and then the config table is read into a map[string]interface{}. Instead, I would be able to decode those configuration keys into their own structures, such as:

type ConfigHttp struct {
    Url string
    ForceSsl bool
}

type ConfigIp struct {
    Host string
    Port int
}

So, I have two questions:

1. Is it possible to first parse the config file, and later, once the Type has been parsed, further decode the map[string]interface{} into the actual structure? I couldn't find an API to do this.
2. Alternatively, I tried parsing the file two times (which would be acceptable). The first time, the map[string]interface{} is generated, but I ignore it. I then instantiate the correct configuration structure and put it into the config field, eg: conn.Config = ConfigHttp{} and proceed to run a second decoding, but unfortunately it seems to be ignored by the library, because the field's content is discarded and overwritten with a new map[string]interface{} instance. BTW this behavior doesn't match what I would expect; for instance, when I use encoding/json to decode a JSON into a structure that contains an interface{} with a reference to an actual object, the library "sees through" the interface{} and decodes into the actual structure.

Would you accept a patch that implements the roadmap presented in option 2? That is, if an interface{} field is non-nil and is a struct, decodes into it rather than just overwriting it with the map[string]interface{} instance?

[cespare]

Use toml.Primitive to delay encoding instead of interface{}. (This is like json.RawMessage.)

package main

import (
    "fmt"
    "log"

    "github.com/BurntSushi/toml"
)

type ConfigHTTP struct {
    URL      string
    ForceSSL bool
}

type ConfigIP struct {
    Host string
    Port int
}

const input0 = `
[connection]
type = "http"
    [connection.config]
    url = "https://foobar.com/api"
    forcessl = true
`

const input1 = `
[connection]
type = "tcp"
    [connection.config]
        host = "12.12.12.12"
    port = 1234
`

type connection struct {
    Connection struct {
        Type   string
        Config toml.Primitive
    }
}

func parseConfig(s string) (interface{}, error) { // or return a more specific interface
    var conn connection
    md, err := toml.Decode(s, &conn)
    if err != nil {
        return nil, err
    }
    var conf interface{}
    switch conn.Connection.Type {
    case "http":
        conf = new(ConfigHTTP)
    case "tcp":
        conf = new(ConfigIP)
    default:
        return nil, fmt.Errorf("unknown type %q", conn.Connection.Type)
    }
    if err := md.PrimitiveDecode(conn.Connection.Config, conf); err != nil {
        return nil, err
    }
    return conf, nil
}

func main() {
    for _, s := range []string{input0, input1} {
        conf, err := parseConfig(s)
        if err != nil {
            log.Fatal(err)
        }
        fmt.Printf("%#v\n", conf)
    }
}

&main.ConfigHTTP{URL:"https://foobar.com/api", ForceSSL:true}
&main.ConfigIP{Host:"12.12.12.12", Port:1234}

[rasky]

Thanks, I'm trying this approach now. I found out that, if the config section is not present, PrimitiveDecode() exits with an error: Type mismatch for main.ConfigHTTP. Expected map but found '<nil>'.. You can try by changing one of your test inputs like this:

const input0 = `
[connection]
type = "http"
`

Is this behavior expected?

[cespare]

Interesting, that should work. I filed #126.

In the meantime, you can work around this by using a *Primitive and explicitly checking the nil case yourself:

http://play.golang.org/p/fKiiL7a-uc

[cespare]

Hey @rasky, I also fixed the empty Primitive case so now my original code works if you don't have any config section.

[rasky]

Thanks! BTW I suggest you add this or a similar example of delayed encoding to the README