Skillet — “Lightning-fast formulas, Rust-powered.”

Skillet is a tiny, embeddable expression engine (written in Rust) inspired by Excel formulas and Ruby-style chaining. It parses expressions into an AST and evaluates them with a small runtime.

This MVP supports numbers, strings, booleans, nulls, arrays, method chaining, functions (built-ins), comparisons, logical ops, ternary, array indexing/slicing, spread ..., lambdas with named parameters, and basic type casting via ::Type.

Skilled can be extended with JS, take a look at Documentation

Build

Requirements: Rust stable (2021 edition)
Build and test:

cargo build
cargo test

CLI (quick try)

A minimal CLI is included to evaluate expressions without external variables.

cargo run --bin sk -- "= [30,60,80,100].filter(:x > 50).map(:x * 0.9).sum()"

Notes:

Wrap the expression in quotes in your shell.
A leading = is optional (supported for spreadsheet-style familiarity).

Library Usage

Add to your Cargo project (from crates.io):

[dependencies]
skillet = "0.2.0"

Or with cargo-edit:

cargo add skillet@0.2.0

Evaluate expressions:

use skillet::{evaluate, evaluate_with, Value};
use std::collections::HashMap;

fn main() -> Result<(), Box<dyn std::error::Error>> {
    // Numeric
    let v = evaluate("= 2 + 3 * 4")?; // -> Value::Number(14.0)

    // Variables
    let mut vars = HashMap::new();
    vars.insert("sales".to_string(), Value::Number(5000.0));
    let v = evaluate_with("=SUM(:sales, 1000)", &vars)?; // -> 6000.0

    // Strings and chaining
    let v = evaluate("= '  john  '.trim().upper()")?; // -> "JOHN"

    // Arrays + F/M/R
    let v = evaluate("= [30,60,80,100].filter(:x > 50).map(:x * 0.9).sum()")?; // -> 216.0

    // Named lambda parameters
    let v = evaluate("= FILTER([1,2,3,4], :n % 2 == 0, 'n')")?; // -> [2,4]

    // Type casting
    let v = evaluate("= '42'::Integer")?; // -> 42
    Ok(())
}

Language Features (MVP)

Numbers, booleans (TRUE/FALSE), strings ('...' or "..."), NULL
Operators: + - * / % ^, > < >= <= == !=, AND/OR/NOT (also &&/||/!), ternary ? :
Variables: :name (provided via evaluate_with map)
Functions (subset):
- Math: SUM, AVG/AVERAGE, MIN, MAX, ROUND, CEIL, FLOOR, ABS, SQRT, POW
- Arrays: ARRAY, FIRST, LAST, CONTAINS, UNIQUE, SORT, REVERSE, JOIN, FLATTEN
- Strings: CONCAT, UPPER, LOWER, TRIM, LENGTH, SPLIT, REPLACE
- Logic: ISBLANK
- Functional: FILTER(array, expr, [param]), MAP(array, expr, [param]), REDUCE(array, expr, initial, [valParam], [accParam])
- Conditional aggregations: SUMIF(array, expr), AVGIF(array, expr), COUNTIF(array, expr)
Methods (subset): chaining with . and predicates ?
- Numbers: .abs() .round(n) .floor() .ceil(); predicates .positive? .negative? .zero? .even? .odd? .numeric?
- Arrays: .length() .size() .first() .last() .sum() .avg() .min() .max() .sort() .unique() .reverse() .compact() .flatten()
- Strings: .upper() .lower() .trim() .reverse()
Arrays: literals [1, 2, 3]; indexing arr[i] (negatives allowed); slicing arr[a:b]
Spread: ...expr inside arg lists
Casting: expr::Integer|Float|String|Boolean|Array|Currency|DateTime|Json

Examples

Arithmetic precedence: = 2 + 3 * 4 → 14
Ternary: = :score >= 90 ? 'A' : 'B'
Named lambda param: = [1,2,3,4].map(:v * 10, 'v') → [10,20,30,40]
Reduce with named params: = [1,2,3].reduce(:a + :v, 0, 'v', 'a') → 6
SUMIF: = SUMIF([1,-2,3,-4], :x > 0) → 4
FLATTEN: = FLATTEN([1,[2,[3]],4]) → [1,2,3,4]

Notes

This is an MVP; error messages and type coverage are intentionally minimal.
For variables beyond numbers/strings/arrays (e.g., dates, currency), see Value in src/types.rs.

Install Binaries

If you want the binaries such as sk, sk_server and sk_client installed system-wide:

cargo install skillet

Server Mode

Skillet includes a high-performance evaluation server that keeps the interpreter warm and eliminates per-process overhead.

Start the server: sk_server 8080 (binds to 127.0.0.1:8080)
Daemonize (Unix): sk_server 8080 -d (writes PID to skillet-server.pid in CWD)
Stop daemon: kill $(cat skillet-server.pid)

Client and benchmarks:

One-off eval: sk_client localhost:8080 '=2+3*4'
With variables: sk_client localhost:8080 '=SUM(:a,:b)' a=10 b=5
JSON vars: sk_client localhost:8080 '=:user.name' --json '{"user":{"name":"Alice"}}'
Benchmark: sk_client localhost:8080 --benchmark '=2+3*4' 10000

Scripts:

Build + run multi-test benchmark: bash scripts/benchmark_server.sh [port] [iterations] [threads]

take a look at the Server Usage Guide for more details about how to use it and consume in different langauages

Built-in Functions

Arithmetic: SUM, AVG/AVERAGE, MIN, MAX, ROUND, CEIL, CEILING, FLOOR, ABS, SQRT, POW/POWER, MOD, INT
Logical: AND, OR, NOT, XOR, IF, IFS
String: LENGTH, CONCAT, UPPER, LOWER, TRIM, SUBSTRING, SPLIT, REPLACE, REVERSE, ISBLANK, ISNUMBER, ISTEXT
Array: ARRAY, FLATTEN, FIRST, LAST, CONTAINS, IN, COUNT, UNIQUE, SORT, REVERSE, JOIN
Date/Time: NOW, DATE, TIME, YEAR, MONTH, DAY, DATEADD, DATEDIFF
Financial: PMT, DB, FV, IPMT
Statistical: MEDIAN, MODE.SNGL (MODESNGL, MODE_SNGL), STDEV.P (STDEVP, STDEV_P), VAR.P (VARP, VAR_P), PERCENTILE.INC (PERCENTILEINC, PERCENTILE_INC), QUARTILE.INC (QUARTILEINC, QUARTILE_INC)
Functional: FILTER(array, expr, [param]), MAP(array, expr, [param]), REDUCE(array, expr, initial, [valParam], [accParam]), SUMIF(array, expr), AVGIF(array, expr), COUNTIF(array, expr)

API Surface (Rust)

parse(input: &str) -> Result<Expr, Error>: parse into AST
evaluate(input: &str) -> Result<Value, Error>: evaluate without variables
evaluate_with(input: &str, vars: &HashMap<String, Value>) -> Result<Value, Error>
evaluate_with_json(input: &str, json_vars: &str) -> Result<Value, Error>
evaluate_with_custom(input: &str, vars: &HashMap<String, Value>) -> Result<Value, Error>
evaluate_with_json_custom(input: &str, json_vars: &str) -> Result<Value, Error>
Custom functions:
- register_function(Box<dyn CustomFunction>) -> Result<(), Error>
- unregister_function(name: &str) -> bool
- list_custom_functions() -> Vec<String>
Types:
- Value enum: Number(f64) | Array(Vec<Value>) | Boolean(bool) | String(String) | Null | Currency(f64) | DateTime(i64) | Json(String)
- Error with message and optional position

Tests

Run the test suite:

cargo test

Author

@zenbakiak

Github repo

License

MIT OR Apache-2.0

skillet 0.2.0