> ## Documentation Index > Fetch the complete documentation index at: https://villagesql.com/docs/llms.txt > Use this file to discover all available pages before exploring further. # Rust Extension Examples > Learn from the vsql_rot13 and vsql_rational reference implementations using the Rust SDK Two reference extensions ship in the Rust SDK repository — a minimal function-only example and a complete custom type with arithmetic, ordering, and hashing. **Source:** `examples/` in [vsql-rust-sdk](https://github.com/villagesql/vsql-rust-sdk/tree/main/examples) *** ## vsql\_rot13 — Function-Only Extension The simplest possible Rust extension: one VDF that takes a STRING and returns a STRING. **Usage:** ```sql theme={null} INSTALL EXTENSION vsql_rot13; SELECT rot13('Hello, World!'); -- 'Uryyb, Jbeyq!' SELECT rot13(rot13('Hello, World!')); -- 'Hello, World!' (rot13 is its own inverse) SELECT rot13(NULL); -- NULL ``` ### Directory Structure ``` vsql_rot13/ ├── Cargo.toml # cdylib crate, depends on villagesql ├── manifest.json # Extension metadata ├── src/ │ └── lib.rs # Implementation + extension! registration └── mysql-test/ └── t/*.test # MTR test cases ``` ### Implementation **File: `src/lib.rs`** ```rust theme={null} use villagesql::{InValue, VdfReturn}; /// SQL: rot13(s STRING) -> STRING fn rot13_impl(args: &[InValue]) -> VdfReturn { match args.first() { Some(InValue::String(s)) => VdfReturn::string(rot13(s)), Some(InValue::Null) | None => VdfReturn::null(), _ => VdfReturn::error("rot13: expected a STRING argument"), } } fn rot13(s: &str) -> String { s.chars() .map(|c| match c { 'a'..='m' | 'A'..='M' => (c as u8 + 13) as char, 'n'..='z' | 'N'..='Z' => (c as u8 - 13) as char, _ => c, }) .collect() } villagesql::extension! { funcs: [ villagesql::func!(rot13_impl, "rot13", [villagesql::Type::String] -> villagesql::Type::String), ] } ``` **Key patterns:** * VDFs receive `&[InValue]` and return `VdfReturn` — both are safe Rust enums * NULL is a first-class variant on both sides; pattern-match it directly * The `extension!` macro generates the C entry points the server calls at load time * `func!` declares the SQL signature; argument and return types use `villagesql::Type::*` ### Manifest **File: `manifest.json`** ```json theme={null} { "name": "vsql_rot13", "version": "0.1.0", "description": "Example VillageSQL extension: provides vsql_rot13(STRING) -> STRING", "author": "VillageSQL Community", "license": "GPL-2.0" } ``` *** ## vsql\_rational — Custom Type with Arithmetic A complete custom type: rational numbers stored as `(numerator, denominator)` in reduced form, with arithmetic functions, ordering, and hashing. **Usage:** ```sql theme={null} INSTALL EXTENSION vsql_rational; CREATE TABLE measurements (id INT, ratio rational); INSERT INTO measurements VALUES (1, '1/2'), (2, '2/4'), -- normalizes to '1/2' on storage (3, '-3/6'), -- normalizes to '-1/2' (4, '0/1'); SELECT id, ratio FROM measurements ORDER BY ratio; SELECT rational_add('1/2', '1/3'); -- '5/6' SELECT rational_mul('2/3', '3/4'); -- '1/2' SELECT rational_to_real('22/7'); -- 3.142857... ``` ### Binary Storage Format `rational` stores **16 bytes** (little-endian): * Bytes 0–7: numerator (`i64`) * Bytes 8–15: denominator (`i64`) Values are always stored in reduced form (GCD = 1) with a positive denominator. ### Type-System Functions **File: `src/lib.rs`** The type registers four operations: encode (string → bytes), decode (bytes → string), compare (for ORDER BY), and hash (for indexing). ```rust theme={null} pub fn rational_encode(s: &str) -> Result, String> { let (num_s, den_s) = s .split_once('/') .ok_or_else(|| format!("rational: expected 'n/d', got {s:?}"))?; let num: i64 = num_s.trim().parse().map_err(|e| format!("numerator: {e}"))?; let den: i64 = den_s.trim().parse().map_err(|e| format!("denominator: {e}"))?; let (n, d) = normalize(i128::from(num), i128::from(den)) .ok_or_else(|| "rational: zero or overflowing denominator".to_string())?; Ok(to_bytes(n, d)) } pub fn rational_decode(b: &[u8]) -> Result { if b.len() < BYTES { return Err(format!("rational: expected {} bytes, got {}", BYTES, b.len())); } let (n, d) = from_bytes(b); Ok(format!("{n}/{d}")) } pub fn rational_compare(a: &[u8], b: &[u8]) -> std::cmp::Ordering { let (n1, d1) = from_bytes(a); let (n2, d2) = from_bytes(b); // Cross-multiply; denominators are always positive after normalization let lhs = i128::from(n1) * i128::from(d2); let rhs = i128::from(n2) * i128::from(d1); lhs.cmp(&rhs) } ``` ### VDF Implementations VDFs that take a custom type receive `InValue::Custom(&[u8])` and decode the bytes themselves: ```rust theme={null} fn rational_add_impl(args: &[InValue]) -> VdfReturn { match (arg(args, 0), arg(args, 1)) { (Ok(Some((n1, d1))), Ok(Some((n2, d2)))) => { match normalize( i128::from(n1) * i128::from(d2) + i128::from(n2) * i128::from(d1), i128::from(d1) * i128::from(d2), ) { Some((n, d)) => VdfReturn::Binary(to_bytes(n, d)), None => VdfReturn::error("rational_add: overflow"), } } (Err(e), _) | (_, Err(e)) => VdfReturn::error(format!("rational_add: {e}")), _ => VdfReturn::null(), } } ``` ### Registration The `extension!` macro registers both the type and its functions in a single declaration: ```rust theme={null} villagesql::extension! { funcs: [ villagesql::func!(rational_add_impl, "rational_add", [villagesql::custom!("rational"), villagesql::custom!("rational")] -> villagesql::custom!("rational"), deterministic: true), villagesql::func!(rational_sub_impl, "rational_sub", [villagesql::custom!("rational"), villagesql::custom!("rational")] -> villagesql::custom!("rational"), deterministic: true), villagesql::func!(rational_mul_impl, "rational_mul", [villagesql::custom!("rational"), villagesql::custom!("rational")] -> villagesql::custom!("rational"), deterministic: true), villagesql::func!(rational_div_impl, "rational_div", [villagesql::custom!("rational"), villagesql::custom!("rational")] -> villagesql::custom!("rational"), deterministic: true), villagesql::func!(rational_numer_impl, "rational_numer", [villagesql::custom!("rational")] -> villagesql::Type::Int, deterministic: true), villagesql::func!(rational_denom_impl, "rational_denom", [villagesql::custom!("rational")] -> villagesql::Type::Int, deterministic: true), villagesql::func!(rational_to_real_impl, "rational_to_real", [villagesql::custom!("rational")] -> villagesql::Type::Real, deterministic: true), ], types: [ villagesql::custom_type!( type_name: "rational", persisted_length: 16, max_decode_buffer_length: 42, encode: rational_encode, decode: rational_decode, compare: rational_compare, hash: rational_hash, default: "0/1", ), ] } ``` **Key patterns:** * `villagesql::custom!("name")` references a custom type as an argument or return * `custom_type!` registers the type alongside its encode/decode/compare/hash functions * `default: "0/1"` is the intrinsic default — the server calls `encode()` on this string at type initialization, so it must be a valid value * `persisted_length` must match the byte length `encode()` returns * `deterministic: true` lets the optimizer fold constant calls *** ## Key Implementation Patterns | Pattern | Usage | | ------------------------ | -------------------------------------------------- | | **VDF signature** | `fn impl(args: &[InValue]) -> VdfReturn` | | **NULL handling** | Match `InValue::Null` and `None` explicitly | | **Error reporting** | `VdfReturn::error("message")` aborts the statement | | **Custom type encode** | Returns `Result, String>` | | **Custom type decode** | Returns `Result` | | **Type-aware arguments** | Use `villagesql::custom!("name")` in `func!` | | **Registration** | Single `extension!` block declares funcs and types | *** ## Testing Both examples use MTR (the MySQL Test Runner) just like C++ extensions: ```bash theme={null} cd /path/to/villagesql/build/mysql-test ./mysql-test-run.pl --suite=vsql_rot13 ./mysql-test-run.pl --suite=vsql_rational ``` Generate or update expected results with `--record`. *** ## Next Steps SDK installation, build, and the extension! macro Deep dive on encode, decode, compare, and hash InValue, VdfReturn, and the macro surface vsql\_rot13 and vsql\_rational complete source