Convert terraform resourceData of type map[string]interface{} to struct

I'm creating a custom terraform provider and I came across this issue.
I was trying to convert a schema.TypeList field into a struct, the TypeList looks something like this:

"template": {
				Type:     schema.TypeList,
				Required: true,
				ForceNew: false,
				Elem: &schema.Resource{
					Schema: map[string]*schema.Schema{
						"lists_test": {
							Type:     schema.TypeSet,
							Required: true,
							ForceNew: false,
							Elem: &schema.Schema{
								Type: schema.TypeString,
							},
						},
						"name_test": {
							Type:     schema.TypeString,
							Required: true,
							ForceNew: false,
						},
},},

and the struct that I'm trying to align to looks something like this:

type TestStruct struct {
	NameTest string   `json:"name_test"`
	ListsTests   []string `json:"lists_test"`
}

I tried a couple of solutions, for instance I tried unmarshalling it to json. Something like below:

template := d.Get("template").([]interface{})[0].(map[string]interface{})
templateStr, err := json.Marshal(template)
templateConverted := &TestStruct{}
json.Unmarshal(template, templateConverted)

however, I'm getting an error json: unsupported type: SchemaSetFunc, which is probably because it's trying to marshal a schema.Schema type instead of map[string]interface{} type, which confuses me. I also tried to use gohcl.DecodeBody but I abandoned the idea since it's usage seems more inclined into reading direct tf files rather than *schema.ResourceData types.

Does anyone had the same experience dealing with this type of scenario? Any help or suggestion is appreciated. Thank you!

Terraform's older SDK (SDKv2) is not designed around the paradigm of decoding into a tagged structure, and instead expects you to use d.Get and manually type-assert individual values, which in your case would perhaps look something like this:

  raw := d.Get("template").([]interface{})[0].(map[string]interface{})
  t := &TestStruct{
      NameTest: raw["name_test"].(string),
      ListsTests: make([]string, len(raw["lists_test"].([]interface{})),
  }
  for i, itemRaw := range raw["lists_test"].([]interface{}) {
    t.ListsTests[i] = itemRaw.(string)
  }

The idiomatic style for most Terraform providers is to write logic like this in separate functions for each complex-typed attribute, where each returns an object of the appropriate type in the target platform's SDK. There would typically also be a matching function for going in the opposite direction: given an object from the target platform's SDK, return a map[string]interface{} that can be assigned to this attribute using d.Set.

However, just because there isn't something built in to the SDK to handle this, that doesn't mean you can't use other libraries that are more general utilities for use in any Go programs.

One example library is github.com/mitchellh/mapstructure, which is designed for exactly the goal you have in mind: to take a value of some interface type and try to use reflection to fit it onto a tagged structure type.

If you want to use that library then you would need to annotate your structure with mapstructure:, instead of the json: ones, and then pass your raw value to the mapstructure.Decode function:

  raw := d.Get("template").([]interface{})[0].(map[string]interface{})
  var t TestStruct
  err := mapstructure.Decode(raw, &t)

Since the schema.ResourceData abstraction in SDKv2 guarantees to return specific data types based on the schema you defined, you should not typically get errors from mapstructure.Decode as long as your schema and your target type match, but still a good idea to check for errors anyway because otherwise your t value may not be completely populated, causing confusing broken behavior downstream.

This is not a typical implementation style used in the official providers, but there's no real harm in writing your provider in this way if you find this style more convenient, or easier to maintain.

Alternatively, if you are not already deeply invested in SDKv2 then you may wish to consider using Terraform Plugin Framework instead. As well as being designed around the type system of modern Terraform (whereas SDKv2 was designed for Terraform v0.11 and earlier), it also supports a programming style more like what you are aiming for, with methods like tfsdk.Plan.Get and tfsdk.Plan.GetAttribute that can decode directly into an appropriately-shaped and appropriately tagged "normal" Go value.

I can't easily show an example of that because it would presume a provider written in quite a different way, but hopefully you can see from the signature of those two functions how they might be used. There's some more commentary and examples in Accessing State, Config, and Plan.