[Solved] Unmarshal JSON to minimum type

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Since you cannot describe your data in a struct then your options are to:

  1. Use a json.Decoder to convert the values to your desired types as they are parsed.

  2. Parse the document into a generic interface and post-process the value types.

Option #1 is the most flexible and can likely be implemented to be more performant than the other option since parsing and transformation could be performed in a single pass of the data.

Option #2 might be simpler but will require two passes over the data. Here is an example of what the post-processing step might look like:

func TransformValueTypes(o map[string]interface{}) {
  for k, v := range o {
    // Convert nil values to *string type.
    if v == interface{}(nil) {
      o[k] = (*string)(nil)
    }
    // Convert numbers to int32 if possible
    if x, isnumber := v.(float64); isnumber {
      if math.Floor(x) == x {
        if x >= math.MinInt32 && x <= math.MaxInt32 {
          o[k] = int32(x)
        }
        // Possibly check for other integer sizes here?
      }
      // Possibly check if float32 is possible here?
    }
    // Check for maps and slices here...
  }
}

So if you call TransformValueTypes(data) then your types will look like:

// my_int     -> 3     (int32)
// my_string  -> <nil> (*string)
// my_string2 -> "foo" (string)
// my_float   -> 1.23  (float64)

Of course, your transform function could also apply type transformation logic based on the key name.

Importantly, note that if your document might have additional structure not mentioned in your question (such as nested objects or arrays) then your transform function will need to account for them by more value type checking, recursive calls, and iteration.

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solved Unmarshal JSON to minimum type