# typed-format-version: load format.version.{major,minor} from a structured file.

This module tries to parse a format.version "section" in some raw data that
may have been loaded from a configuration file, and determines whether that
section contains valid "major" and "minor" integer values. The caller can
then choose the correct schema to validate the loaded data against, e.g. by
using the `typedload` library with the correct top-level dataclass definition.

The most commonly used function will probably be `get_version()`: it takes
a raw data dictionary and returns a `Version` object with a `major` and `minor`
integer attributes, if the data contained a valid "format" dictionary with
a "version" dictionary within it. Optionally the `get_version()` function can
remove the top-level "format" member, if a true value is passed for the `pop`
argument.

## Python examples

Load some data from a file, make sure it is in the correct format:

    try:
        raw = json.load(pathlib.Path(cfgfile).open())
        ver = typed_format_version.get_version(raw)
    except (OSError, ValueError) as err:
        sys.exit(f"Invalid data format for {cfgfile}: {err}")
    if ver.as_version_tuple() != (0, 2):
        sys.exit("Only config format 0.2 supported right now")
    cfg = typedload.load(raw, ConfigData)

Determine the best version to validate against, allowing more fields to be
added in minor versions that we do not know about yet:

    SCHEMAS = {
        (0, 1): ConfigTop_0_1,
        (0, 2): ConfigTop_0_2,
        (1, 0): ConfigTop_1_0,
    }
    try:
        raw = json.load(pathlib.Path(cfgfile).open())
        exact_ver = typed_format_version.get_version(raw)
        mver = typed_format_version.determine_version_match(exact_ver, SCHEMAS)
    except (OSError, ValueError) as err:
        sys.exit(f"Invalid data format for {cfgfile}: {err}")
    
    # Either load the data directly...
    cfg = typedload.load(raw, SCHEMAS[mver.version], failonextra=mver.failonextra)
    
    # ...or do something with mver.version, possibly examining ver further and
    # "upgrading" the loaded configuration from earlier versions by e.g.
    # adding default values for fields or reshaping the data.

## Rust examples

Load some data from a file, make sure it is in the correct format:

    use std::fs;
    
    use expect_exit::ExpectedWithError;
    
    let contents = fs::read_to_string(&infile).or_exit_e(|| format!("Could not read {}", infile.display()));
    let fver = typed_format_version::get_version_from_str(&contents, serde_json::from_str)
        .or_exit_e(|| format!("Could not parse format.version from {}", infile.display()));
    if (fver.major(), fver.minor()) != (0, 2) {
        expect_exit::exit("Only config format 0.2 supported right now");
    }
    let cfg: ConfigData = serde_json::from_str(&contents)
        .or_exit_e(|| format!("Could not parse {}", infile.display()));

Upgrade from an earlier versions of the data format:

    let cfg = match fver.major() {
        0 => {
            let cfg_0: ConfigData_0 = serde_json::from_str(&contents)
                .or_exit_e(|| format!("Could not parse {}", infile.display()));
            upgrade_from_version_0(cfg_0)
        },
        1 => serde_json::from_str::<ConfigData>(&contents)
            .or_exit_e(|| format!("Could not parse {}", infile.display())),
        _ => expect_exit::exit(&format!("Unexpected major format version {}", fver.major()),
    };

## Contact

The `typed-format-version` library is developed in
[a GitLab repository](https://gitlab.com/ppentchev/typed-format-version).
It was written by [Peter Pentchev](mailto:roam@ringlet.net).