Apply the lens

A lens translates a record between two schemas. Its forward operation, get, returns the target record and a complement that retains any source information needed by the reverse operation, put.

This chapter applies the lens fetched in Chapter 1 to the following source record:

{
  "text": "hello, world"
}

Confirm the Panproto version

The runnable package pins the Panproto crates used here to 0.71.0:

panproto-lens   = { git = "https://github.com/panproto/panproto.git", tag = "v0.72.0" }
panproto-schema = { git = "https://github.com/panproto/panproto.git", tag = "v0.72.0" }

The idiolect workspace uses the same Panproto version.

Run get

From the repository root, run the supplied client:

cargo run --quiet \
  --manifest-path scripts/publish-tutorial-lens/Cargo.toml \
  --bin apply-tutorial-lens

target_record = {
  "text": "hello, world"
}

The executable creates one HTTP client, gives it to the lens and schema resolvers, and calls apply_lens:

let out = apply_lens(
    &resolver,
    &loader,
    &protocol,
    ApplyLensInput {
        lens_uri,
        source_record,
        source_root_vertex: None,
    },
)
.await?;

The JSON value is unchanged because this tutorial lens renames a schema sort from string to text; it does not rename the record's text field. The unchanged value records a successful run: the runtime resolved the lens, loaded both schemas, instantiated Panproto 0.71.0, and produced a target-schema value.

The returned out.complement belongs with this application of the lens. Passing it to apply_lens_put would reconstruct the source record. Chapter 4 checks that reconstruction over several inputs.