serde-er7

Serde support for er7, the pipe-hat encoding that carries HL7 v2 messages between healthcare systems — so a parsed message can flow through JSON, YAML, or any other Serde data format, and come back out unchanged.

let text = "MSH|^~\\&|LAB|ACME|EHR|CLINIC|20260815120000||ORU^R01|MSG9|P|2.5\r\
            PID|1||12345^^^ACME^MR||SMITH^JOHN^Q||19800101|M\r\
            OBX|1|NM|2093-3^Cholesterol^LN||187|mg/dL|||||F";

let message = serde_er7::Message::parse(text)?;

let json = serde_json::to_string(&message)?;
let back: serde_er7::Message = serde_json::from_str(&json)?;

assert_eq!(back.to_er7(), text);

Contents

• Why this crate exists
• Install
• The shape each level serializes as
• What it does
• What it deliberately does not do
• Documentation
• Development
• License

Why this crate exists

er7 parses, queries, edits, and writes ER7 text with zero dependencies of its own — a deliberate choice for code that sits at the bottom of a stack of HL7 crates. That means er7::Message has no Serde support built in, and adding it there would cost every user of er7 a dependency they may not want.

This crate is the bridge instead: two dependencies, serde and er7, and nothing else. Every Serialize/Deserialize impl is written by hand against the low-level trait methods — the same pattern serde's own documentation walks through for a manual implementation — because the shapes below (a bare array for a field's repetitions, a bare string for a leaf) are not what #[derive(Serialize)] would produce on its own.

Install

cargo add serde-er7

The shape each level serializes as

So PID-5.1 (SMITH) is the bare JSON string "SMITH", PID-5 (SMITH^JOHN) is ["SMITH", "JOHN"], and a repeating field such as 555-1111~555-2222 is [["555-1111"], ["555-2222"]] — one array level per level of the tree, all the way up to the two objects at the top: Segment and Message itself.

What it does

• Every level of the tree: Message, Segment, Field, Repetition, Component, Subcomponent, plus Separators and Terminator — a Serde-enabled wrapper for every public type er7 exposes.
• Format-agnostic: nothing in this crate mentions JSON, YAML, or any other format by name. serde_json appears only as a dev-dependency, to test and demonstrate against.
• Round trip, the same guarantee er7 makes: every subcomponent serializes as its raw text, escape sequences included, not decoded — so Message::parse(text)? through any Serde format and back out through .to_er7() reproduces the same bytes er7::parse(text)?.to_er7() would.
• Absent, empty, and null stay distinct: a field that was never sent, a field sent as ||, and a field holding the explicit "" null serialize and deserialize as three different values, never collapsed into one.
• Ergonomic wrappers, not just trait impls: every wrapper implements Deref/DerefMut to its er7 type, plus From conversions both ways, so message.query(...), message.segments, and the rest of er7's API work directly on a Message without unwrapping it first.

What it deliberately does not do

This crate adds exactly one thing to er7: Serde support for its existing value tree. Like er7 itself, it is an encoding bridge, not a dictionary — it does not know which fields a segment should have, what data type each carries, or what any code table means. It does not validate, and it does not pick a wire format for you: that choice — serde_json, serde_yaml, anything else — is the caller's, every time.

For the HL7 v2.5 dictionary layer, see the sibling crates hl7-2-5-to-xml-using-rust and hl7-2-5-to-json-using-rust.

Documentation

Rendered API docs are at https://docs.rs/serde-er7/, or locally with cargo doc --no-deps --open.

Development

cargo test                                # unit, integration, and doc tests
cargo clippy --all-targets -- -D warnings # lint-clean
cargo fmt --check                         # formatting
cargo rustdoc --lib -- -W missing-docs    # every public item documented
cargo run --example round_trip_via_json   # try an example

Behavioural changes start in spec/, not in the code — see AGENTS/spec-driven-development.md.

License

Multi-licensed, so a downstream project can pick whichever fits: MIT, Apache-2.0, BSD-3-Clause, GPL-2.0-only, or GPL-3.0-only. See LICENSE.md.