fallback to a config file

[pacak]

API for bpaf can look like this:

1. add a key method to NamedField to try to read next value that belongs to this key
2. add enter method to Parser - this method would move a cursor inside the AST
3. add something that would add an applicative config reader to Args/State

API for applicative config reader can look like this:

1. enter(&mut self, key: &'static str) - move to a table key inside a current table
2. pop(&mut self) - go one level up
3. get(&mut self, key: &'static str) -> Option<&str> - to to read next available value from a key in a current table
4. drain(&mut self, key: &'static) - mark full key as parsed - successfully consumed item

applicative reader needs to maintain a set of cursors - one for each field, similar to command line arguments reading should succeed only once, bpaf will handle cloning for forks.

reader structure can be something like this:

struct Reader {
    payload: Rc<BTree<key, ValueOrTree>>, // config data, read only
    flat_keys: Rc<RefCell<MapPathToInt>>>, // maps path to each key to a number, mapping is persistent across the branches
    consumed: Vec<usize>, // counts how many times value at a given key was consumed
}

• What to use to read the config? toml_edit seems to allow to do it in a serde-free way.
• Config fallback interaction with user passed options, mostly collections many, some?
  • user options override possibility of having config options
  • config options are added to user options

[aeosynth]

What to use to read the config? toml_edit seems to allow to do it in a serde-free way.

could you let the user read the config and pass that as the payload?

[pacak]

could you let the user read the config and pass that as the payload?

Problem is that I need to be able to query into that payload: "get field called foo in current scope", "move current scope into table bar", etc. Currently there's a trait that lets user to represent this (see #286). User can create an impl Parser trait object that produces something that implements impl Config trait object and this supports hierarchical configuration files - like a list of examples in Cargo.toml. Problem is often you don't need all the flexibility and you just want to be able to read a config file and for that I'd like to provide something simple - you make impl Parser that gets a path to a file and as long as this file is in some common format it works as expected. For that I need to support reading config in those common formats and to implement my Config trait for some of the data types library produces. One way is serde with a few crates like serde_json - I can implement Config for a Value type provided by those crates, but after recent events - I don't really want to depend on it. This means toml_edit. Or as an alternative - make a separate crate that depends on serde and provide impl Config in there...