How to use IsDefined with array of tables?

[gorankor]

I don't know if this is an issue or just me not knowing how to properly call it, so I apologize if I opened a new issue which is not an issue at all.

I have a config file where I want to specify one section several times:

[[methods]]
foo = "method1"
namespace = "ns1"

[[methods]]
foo = "method2"
namespace = "ns2"`

...

Here's a complete example:

package main

import (
        "fmt"
        "os"
        "github.com/BurntSushi/toml"
)

type confMethod struct {
    Foo         string     `toml:"foo,omitempty"`
    NameSpace   string     `toml:"namespace,omitempty"`
}

// Methods struct - used in include files
type confMethods struct {
    Methods []confMethod     `toml:"methods,omitempty"`
}

func main() {
    var mConf confMethods

    c := `[[methods]]
foo = "method1"
namespace = "root/cimv2"

[[methods]]
foo = "method2"
namespace = "root/cimv2"`

    md, err := toml.Decode(c, &mConf)
    if err != nil {
        fmt.Fprintf(os.Stderr, "Error: %s\n", err)
        os.Exit(1)
    }

    fmt.Printf("keys=%s\n", md.Keys())

    if md.IsDefined("methods", "namespace") {
        fmt.Printf("defined\n")
    } else {
        fmt.Printf("not defined\n")
    }
}

This returns "not defined". How can one use IsDefined() in this case? Is it possible at all?

I have tried different usages as the commend above IsDefined() in decode.go says:

md.IsDefined("methods", "namespace")
md.IsDefined("methods", "methods.namespace")
md.IsDefined("methods", "method", "namespace")
md.IsDefined("namespace")

but nothing works.

My real example is a bit more complex, and config parser works as expected when I just blindly assign values from config struct, but I'd like to only do that when something is present in the config file. I can't rely on what Keys() returns as that one is not specific enough, but it at least shows that the keys are there as expected (as the example prints it).

If I look at what is going on in decode.go and compare working (where I use just a simple struct) vs. non-working example, when it works, it gets this type:

map[string]interface {}

But for this failing example it gets:

[]map[string]interface {}

And then it fails here on second (might be third - depends how it gets called obviouslly) loop iteration:

if hash, ok = hashOrVal.(map[string]interface{}); !ok

[cespare]

Yeah, there's no way at the moment. The API doesn't offer a clear way to add this feature, either.

Super hacky strawman proposal: md.IsDefined("methods", "0", "namespace"). Pretty awful.

Probably we should add a new method instead. I'm not even sure what the API should be, though. @BurntSushi any thoughts?

[BurntSushi]

I don't have any strong opinions, but it would be nice to make IsDefined("methods", "namespace") work as expected.

[cespare]

@BurntSushi what do you expect, though?

[BurntSushi]

I suppose I'd expect it to return true because there exists a namespace key in at least one methods table. Now that I write it out, I see why that's potentially problematic, since the check would need to look at all entries in methods.

[gorankor]

If you look at it that way, it certainly is not well defined.

Suppose I walk over methods this way:

for _, m := range mConf.Methods {

Then having an index into array would tell us which one to look at. I am thinking of what @cespare said earlier:

Super hacky strawman proposal: md.IsDefined("methods", "0", "namespace"). Pretty awful.

If I try that, it doesn't work for me now, but it would certainly be more than good enough.

More info:

When searching for some issue about using Cgo I stumbled upon a post which describes when to use string vs. *string in Go and it nicely gives out one reason to use the latter: you should probably use a *string for struct properties when deserializing json or yaml (or anything) into a structure.

I was not aware of this distinction, so I could simply convert my struct like so:

type confMethod struct {
    Foo         *string     `toml:"foo,omitempty"`
    NameSpace   *string     `toml:"namespace,omitempty"`
}

and then with the above for loop I can say:

if m.Foo != nil

So if you feel like not bothering with this, I am more than OK with closing this issue and I'll change my code to work this way.