absent or null

Tags: option, toml, json, ron, choosing an engine

Features: json, toml, ron

What each engine writes for a value that is there and holds nothing.

{} and null are different documents, and an absent key is a third thing again. Which of the three an engine produces for None decides what a schema can say about it: a property that may be null is not a property that may be missing, and the two are written differently.

The engines answer in two ways, and the run below says which is which.

Note what the two that do have a null still do differently: ron spells the Option in the document, so a hand-edited file has to say Some(...), while json writes the value bare and lets null carry the absence.

TOML has no null, so a key holding nothing is a key that is not written. That is how every TOML config expresses an optional setting, and it means the format answers set(None) and delete with the same document - a distinction the other engines keep and this one cannot.

What was done

let held = Held::new_with(&store)?;

held.note().durable().set(Some("here".to_string()))?;
let with_value = std::fs::read_to_string(path.path())?;

held.note().durable().set(None)?;
let with_nothing = std::fs::read_to_string(path.path())?;

let read: Option<Option<String>> = store.get(note_path())?;

What the run printed

json - a value

held.note().durable().set(Some("here".to_string()))?;
let with_value = std::fs::read_to_string(path.path())?;

{
  "maybe": {
    "note": "here"
  }
}

json - the same key set to nothing

held.note().durable().set(None)?;
let with_nothing = std::fs::read_to_string(path.path())?;

{
  "maybe": {
    "note": null
  }
}

get::<Option<String>> reads back Some(None).

ron - a value

held.note().durable().set(Some("here".to_string()))?;
let with_value = std::fs::read_to_string(path.path())?;

{
    "maybe": {
        "note": Some("here"),
    },
}

ron - the same key set to nothing

held.note().durable().set(None)?;
let with_nothing = std::fs::read_to_string(path.path())?;

{
    "maybe": {
        "note": None,
    },
}

get::<Option<String>> reads back Some(None).

toml - a value

held.note().durable().set(Some("here".to_string()))?;
let with_value = std::fs::read_to_string(path.path())?;

[maybe]
note = "here"

toml - the same key set to nothing

held.note().durable().set(None)?;
let with_nothing = std::fs::read_to_string(path.path())?;

[maybe]

get::<Option<String>> reads back None.