Serialized defaults in serde

[Alec]

Is it possible to deserialize it in such a way that if a value is not provided for an entry, it would look up the value from a different "default" entry?

For example, suppose I have a struct like this:

#[derive(Deserialize, Serialize)]
struct Entry {
    reticulate_splines: bool,
    integrate_curves: bool,
    align_covariance_matrices: bool,
    chlorinate_threadpool: bool,
}

#[derive(Deserialize, Serialize)]
struct Config {
    defaults: Entry,

    #[serde(rename = "entry")]
    entries: HashMap<String, Entry>,
}

I want to deserialize a config like the one below, where it would load the values defined in entries, and if missing, look up in "defaults", and fail if it's not in either.

[defaults]
integrate_curves = false
align_covariance_matrices = false
chlorinate_threadpool = true

[entry.Foo]
reticulate_splines = true

[entry.Bar]
reticulate_splines = false
chlorinate_threadpool = false

Is it possible? How would I go about doing it?

One way is to make all fields optional and manually merge after deserializing, but that's annoying.

I'm guessing it should be possible to implement a custom Deserialize for Config but I'm a bit lost on how to do it.

[jeffsmith82]

Not sure how you would do if from a config file but this is how to do it on the struct Default value for a field · Serde

#[derive(Deserialize, Serialize)]
struct Entry {
#[serde(default = true)]
reticulate_splines: bool,
#[serde(default = false)]
integrate_curves: bool,
#[serde(default = false)]
align_covariance_matrices: bool,
#[serde(default = false)]
chlorinate_threadpool: bool,
}

[jonasbb]

You might be able to create something like that with DeserializeSeed (or maybe serde_state).
If it is for a configuration system, maybe a specialized crate which has a concept of layers which get merged might be more useful. A quick search revealed config and twelf.

[noyez]

I use a strategy to use an intermediary struct just for deserailizing then convert that into a struct for coding.

warning: i'm sure this code doesn't compile, but i hope it gives you an idea to my approach.

#[derive(Deserialize, Serialize)]
struct EntryOption {
    reticulate_splines: Option<bool>,
    integrate_curves: Option<bool>,
    align_covariance_matrices: Option<bool>,
    chlorinate_threadpool: Option<bool>,
}

#[derive(Deserialize, Serialize)]
struct Entry {
    reticulate_splines: bool,
    integrate_curves: bool,
    align_covariance_matrices: bool,
    chlorinate_threadpool: bool,
}

#[derive(Deserialize, Serialize)]
struct ConfigOption {
    defaults: Entry,

    #[serde(rename = "entry")]
    entries: HashMap<String, EntryOption>,
}
impl ConfigOption {
  fn into_config(Self) -> Config {
     let defaults_entry: Self.defaults;
     let entries = Self.enteries.map( |(k,v)| {
        let new_entry = Entry {
             reticulate_splines = v.reticulate_splines.unwrap_or(defaults_entry.reticulate_splines)
             integrate_curves = v.integrate_curves.unwrap_or(defaults_entry.integrate_curves)
             align_covariance_matrices = v.align_covariance_matrices.unwrap_or(defaults_entry.align_covariance_matrices)
             chlorinate_threadpool = v.chlorinate_threadpool.unwrap_or(defaults_entry.chlorinate_threadpool)
         }
         (k,new_entry)
      }).collect::<Vec<_>>()
      Config {
         defaults: defaults_entry,
         entries: entries
      }
  }
}

struct Config {
    defaults: Entry,
    entries: HashMap<String, Entry>,
}

[Alec]

Thanks, DeserializeSeed looks like the building block I'm looking for.

What I'm trying to do is not so much to merge configs, as to de-duplicate repetitive entries. Basically, a #[serde(default = ...)] where I don't hard-code the default. My config can have a large number of user-defined entries, and I want to have a global section that can be used to set the defaults that would apply to each entry that doesn't have those value explicitly set.

[Alec]

Thanks, this does work, but I'm hoping to have something a bit more generic.

Here's a version that compiles:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=00146e41d80ac3c58eae4576e44de736