Imported from majiayu000/claude-skill-registry (skills/data/east/SKILL.md). Install upstream with npx skills add majiayu000/claude-skill-registry --skill east. Copyright stays with the author.

East Language

A statically typed, expression-based programming language embedded in TypeScript. Write programs using a fluent API, compile to portable IR.

Quick Start

// Types and helpers are direct imports (NOT East.IntegerType)
import { East, IntegerType, StringType, ArrayType, NullType } from "@elaraai/east";

// 1. Define platform functions (East.platform)
const log = East.platform("log", [StringType], NullType);
const platform = [log.implement(console.log)];

// 2. Define East function (East.function)
const sumArray = East.function([ArrayType(IntegerType)], IntegerType, ($, arr) => {
    const total = $.let(arr.sum());
    $(log(East.str`Sum: ${total}`));
    $.return(total);
});

// 3. Compile and execute (East.compile)
const compiled = East.compile(sumArray, platform);
compiled([1n, 2n, 3n]);  // logs "Sum: 6", returns 6n

Decision Tree: What Do You Need?

Task → What do you need?
    │
    ├─ Create East Expressions (East.*)
    │   ├─ From TypeScript value → East.value(tsValue) or East.value(tsValue, Type)
    │   ├─ String interpolation → East.str`Hello ${name}!`
    │   └─ Inside function body → $.let(value), $.const(value)
    │
    ├─ Define a Type (import from package, NOT East.*)
    │   ├─ Primitive → IntegerType, FloatType, StringType, BooleanType, DateTimeType, BlobType, NullType
    │   ├─ Collection → ArrayType(T), SetType(K), DictType(K, V), RefType(T)
    │   ├─ Compound → StructType({...}), VariantType({...}), RecursiveType(...)
    │   ├─ Function → FunctionType<I, O>, AsyncFunctionType<I, O>
    │   └─ Patch → PatchType(T) (compute patch type for any East type)
    │
    ├─ Create TypeScript Values for East Types
    │   ├─ NullType → null
    │   ├─ BooleanType → true, false
    │   ├─ IntegerType → 42n (bigint literal)
    │   ├─ FloatType → 3.14 (number literal)
    │   ├─ StringType → "hello"
    │   ├─ DateTimeType → new Date("2025-01-01T00:00:00Z")
    │   ├─ BlobType → new Uint8Array([...])
    │   ├─ ArrayType(T) → [1n, 2n, 3n]
    │   ├─ SetType(K) → new Set([1n, 2n, 3n])
    │   ├─ DictType(K, V) → new Map([["a", 1n], ["b", 2n]])
    │   ├─ StructType({...}) → { field1: value1, field2: value2 }
    │   ├─ VariantType({...}) (use helpers from package)
    │   │   ├─ Option type → some(value), none  ← preferred for Option/Maybe
    │   │   └─ Other variants → variant("caseName", value)
    │   └─ RefType(T) (use helper from package) → ref(value)
    │
    ├─ Write a Function (East.*)
    │   ├─ Synchronous → East.function([inputs], output, ($, ...args) => { ... })
    │   └─ Asynchronous → East.asyncFunction([inputs], output, ($, ...args) => { ... })
    │
    ├─ Compile and Execute (East.*)
    │   ├─ Sync function → East.compile(fn, platform)
    │   └─ Async function → East.compileAsync(fn, platform)
    │
    ├─ Use Platform Effects (East.*)
    │   ├─ Sync effect → East.platform("name", [inputs], output).implement(fn)
    │   └─ Async effect → East.asyncPlatform("name", [inputs], output).implement(fn)
    │
    ├─ Block Operations ($)
    │   ├─ Variables → $.let(value), $.const(value), $.assign(var, value)
    │   ├─ Execute → $(expr), $.return(value), $.error(message)
    │   ├─ Control Flow → $.if(...), $.while(...), $.for(...), $.match(...)
    │   └─ Error Handling → $.try(...).catch(...).finally(...)
    │
    ├─ Expression Operations
    │   ├─ Boolean
    │   │   ├─ Logic → .and($=>), .or($=>), .not(), .ifElse($=>,$=>)
    │   │   ├─ Bitwise → .bitAnd(), .bitOr(), .bitXor()
    │   │   └─ Compare → .equals()/.equal()/.eq(), .notEquals()/.notEqual()/.ne()
    │   ├─ Integer
    │   │   ├─ Math → .add()/.plus(), .subtract()/.sub()/.minus(), .multiply()/.mul()/.times(), .divide()/.div(), .remainder()/.mod()/.rem()/.modulo(), .pow(), .abs(), .sign(), .negate(), .log()
    │   │   ├─ Convert → .toFloat()
    │   │   └─ Compare → .equals()/.equal()/.eq(), .notEquals()/.notEqual()/.ne(), .lessThan()/.less()/.lt(), .greaterThan()/.greater()/.gt(), .lessThanOrEqual()/.lessEqual()/.lte()/.le(), .greaterThanOrEqual()/.greaterEqual()/.gte()/.ge()
    │   ├─ Float
    │   │   ├─ Math → .add()/.plus(), .subtract()/.sub()/.minus(), .multiply()/.mul()/.times(), .divide()/.div(), .remainder()/.mod()/.rem()/.modulo(), .pow(), .abs(), .sign(), .negate()
    │   │   ├─ Advanced → .sqrt(), .exp(), .log(), .sin(), .cos(), .tan()
    │   │   ├─ Convert → .toInteger()
    │   │   └─ Compare → .equals()/.equal()/.eq(), .notEquals()/.notEqual()/.ne(), .lessThan()/.less()/.lt(), .greaterThan()/.greater()/.gt(), .lessThanOrEqual()/.lessEqual()/.lte()/.le(), .greaterThanOrEqual()/.greaterEqual()/.gte()/.ge()
    │   ├─ String
    │   │   ├─ Transform → .concat(), .repeat(), .substring(), .upperCase(), .lowerCase(), .trim(), .trimStart(), .trimEnd()
    │   │   ├─ Replace → .replace(), .replaceAll(), .split()
    │   │   ├─ Query → .length(), .startsWith(), .endsWith(), .contains(), .indexOf(), .charAt()
    │   │   ├─ Parse → .parse(), .parseJson()
    │   │   ├─ Encode → .encodeUtf8(), .encodeUtf16()
    │   │   └─ Compare → .equals()/.equal()/.eq(), .notEquals()/.notEqual()/.ne(), .lessThan()/.less()/.lt(), .greaterThan()/.greater()/.gt(), .lessThanOrEqual()/.lessEqual()/.lte()/.le(), .greaterThanOrEqual()/.greaterEqual()/.gte()/.ge()
    │   ├─ DateTime
    │   │   ├─ Components → .getYear(), .getMonth(), .getDayOfMonth(), .getDayOfWeek(), .getHour(), .getMinute(), .getSecond(), .getMillisecond()
    │   │   ├─ Arithmetic → .addDays(), .subtractDays(), .addHours(), .subtractHours(), .addMinutes(), .addSeconds(), .addMilliseconds(), .addWeeks()
    │   │   ├─ Duration → .durationDays(), .durationHours(), .durationMinutes(), .durationSeconds(), .durationMilliseconds(), .durationWeeks()
    │   │   ├─ Convert → .toEpochMilliseconds(), .printFormatted()
    │   │   └─ Compare → .equals()/.equal()/.eq(), .notEquals()/.notEqual()/.ne(), .lessThan()/.less()/.lt(), .greaterThan()/.greater()/.gt(), .lessThanOrEqual()/.lessEqual()/.lte()/.le(), .greaterThanOrEqual()/.greaterEqual()/.gte()/.ge()
    │   ├─ Blob
    │   │   ├─ Read → .size(), .getUint8()
    │   │   ├─ Decode → .decodeUtf8(), .decodeUtf16(), .decodeBeast(), .decodeCsv()
    │   │   └─ Compare → .equals()/.equal()/.eq(), .notEquals()/.notEqual()/.ne()
    │   ├─ Array
    │   │   ├─ Read → .size(), .length(), .has(), .get(), .at(), .tryGet(), .getKeys()
    │   │   ├─ Mutate → .update(), .pushLast(), .popLast(), .pushFirst(), .popFirst(), .append(), .prepend(), .clear(), .sortInPlace(), .reverseInPlace()
    │   │   ├─ Transform → .copy(), .slice(), .concat(), .sort(), .reverse(), .map(), .filter(), .filterMap(), .flatMap()
    │   │   ├─ Search → .findFirst(), .findAll(), .firstMap(), .isSorted()
    │   │   ├─ Reduce → .reduce(), .every(), .some(), .sum(), .mean(), .maximum(), .minimum(), .findMaximum(), .findMinimum()
    │   │   ├─ Convert → .stringJoin(), .toSet(), .toDict(), .flattenToSet(), .flattenToDict(), .encodeCsv()
    │   │   ├─ Group → .groupReduce(), .groupSize(), .groupSum(), .groupMean(), .groupMinimum(), .groupMaximum(), .groupToArrays(), .groupToSets(), .groupToDicts(), .groupEvery(), .groupSome()
    │   │   └─ Compare → .equals()/.equal()/.eq(), .notEquals()/.notEqual()/.ne()
    │   ├─ Set
    │   │   ├─ Read → .size(), .has()
    │   │   ├─ Mutate → .insert(), .tryInsert(), .delete(), .tryDelete(), .clear(), .unionInPlace()
    │   │   ├─ Set Ops → .copy(), .union(), .intersection(), .difference(), .symmetricDifference(), .isSubsetOf(), .isSupersetOf(), .isDisjointFrom()
    │   │   ├─ Transform → .filter(), .filterMap(), .map(), .forEach(), .firstMap()
    │   │   ├─ Reduce → .reduce(), .every(), .some(), .sum(), .mean()
    │   │   ├─ Convert → .toArray(), .toSet(), .toDict(), .flattenToArray(), .flattenToSet(), .flattenToDict()
    │   │   ├─ Group → .groupReduce(), .groupSize(), .groupSum(), .groupMean(), .groupToArrays(), .groupToSets(), .groupToDicts(), .groupEvery(), .groupSome()
    │   │   └─ Compare → .equals()/.equal()/.eq(), .notEquals()/.notEqual()/.ne()
    │   ├─ Dict
    │   │   ├─ Read → .size(), .has(), .get(), .tryGet(), .keys(), .getKeys()
    │   │   ├─ Mutate → .insert(), .insertOrUpdate(), .update(), .merge(), .getOrInsert(), .delete(), .tryDelete(), .pop(), .swap(), .clear(), .unionInPlace()
    │   │   ├─ Transform → .copy(), .map(), .filter(), .filterMap(), .forEach(), .firstMap()
    │   │   ├─ Reduce → .reduce(), .every(), .some(), .sum(), .mean()
    │   │   ├─ Convert → .toArray(), .toSet(), .toDict(), .flattenToArray(), .flattenToSet(), .flattenToDict()
    │   │   ├─ Group → .groupReduce(), .groupSize(), .groupSum(), .groupMean(), .groupToArrays(), .groupToSets(), .groupToDicts(), .groupEvery(), .groupSome()
    │   │   └─ Compare → .equals()/.equal()/.eq(), .notEquals()/.notEqual()/.ne()
    │   ├─ Struct → .fieldName (direct property access)
    │   ├─ Variant → .match(), .unwrap(), .hasTag(), .getTag(), .equals()/.equal()/.eq(), .notEquals()/.notEqual()/.ne()
    │   └─ Ref → .get(), .update(), .merge()
    │
    ├─ Standard Library (East.*)
    │   ├─ Integer → East.Integer.printCommaSeperated(), .roundNearest(), .printOrdinal()
    │   ├─ Float → East.Float.roundToDecimals(), .printCurrency(), .printCompact()
    │   ├─ DateTime → East.DateTime.fromComponents(), .roundDownDay(), .parseFormatted()
    │   ├─ Array → East.Array.range(), .linspace(), .generate()
    │   ├─ Set → East.Set.generate()
    │   ├─ Dict → East.Dict.generate()
    │   ├─ Blob → East.Blob.encodeBeast(), blob.decodeCsv(), array.encodeCsv()
    │   └─ String → East.String.printJson(), East.String.printError()
    │
    ├─ Comparisons (East.*)
    │   ├─ Equality → East.equal()/.equals()/.eq(), East.notEqual()/.notEquals()/.ne()
    │   ├─ Ordering → East.less()/.lessThan()/.lt(), East.greater()/.greaterThan()/.gt()
    │   ├─ Bounds → East.lessEqual()/.lte()/.le(), East.greaterEqual()/.gte()/.ge()
    │   ├─ Identity → East.is() (reference equality for mutable types)
    │   └─ Utilities → East.min(), East.max(), East.clamp()
    │
    ├─ Conversion (East.*)
    │   └─ To string → East.print(expr)
    │
    ├─ Patch Operations (East.*)
    │   ├─ Compute diff → East.diff(before, after) → returns patch
    │   ├─ Apply patch → East.applyPatch(value, patch) → returns patched value
    │   ├─ Compose patches → East.composePatch(first, second, type) → returns combined patch
    │   └─ Invert patch → East.invertPatch(patch, type) → returns undo patch
    │
    └─ Serialization
        ├─ IR → fn.toIR(), ir.toJSON(), EastIR.fromJSON(data).compile(platform)
        └─ Data → East.Blob.encodeBeast(value, 'v2'), blob.decodeBeast(type, 'v2')

Reference Documentation

• API Reference - Complete function signatures, types, and arguments
• Examples - Working code examples by use case

Type System Summary

Key Patterns

TypeScript Values vs East Expressions

TypeScript values and East expressions are different. East methods only work on East expressions.

// WRONG - Cannot call East methods on TypeScript values
const THRESHOLD = 100n;  // This is a TypeScript bigint
East.function([IntegerType], BooleanType, ($, x) => {
    $.return(x.greaterThan(THRESHOLD));  // ERROR: THRESHOLD has no .greaterThan()
});

// CORRECT - Wrap TypeScript values with East.value()
const THRESHOLD = 100n;
East.function([IntegerType], BooleanType, ($, x) => {
    $.return(x.greaterThan(East.value(THRESHOLD)));  // OK
});

// ALSO CORRECT - Use $.const() inside the function
East.function([IntegerType], BooleanType, ($, x) => {
    const threshold = $.const(THRESHOLD);  // Creates East expression
    $.return(x.greaterThan(threshold));    // OK
});

Rule: Function parameters are already East expressions. External TypeScript values must be wrapped with East.value() or bound with $.const().

Creating Variant Values

import { variant, some, none, ref } from "@elaraai/east";

// Option types - use some() and none helpers (preferred)
const hasValue = some(42n);      // { type: "some", value: 42n }
const noValue = none;            // { type: "none", value: null }

// AVOID: variant("some", ...) and variant("none", ...) for Option types
// const hasValue = variant("some", 42n);  // works but use some() instead
// const noValue = variant("none", null);  // works but use none instead

// Other variant types - use variant()
const success = variant("ok", "done");
const failure = variant("error", "failed");

// Mutable references - use ref()
const counter = ref(0n);

String Interpolation

// East.str`` creates a StringExpr from a template
// Only East expressions can be interpolated, NOT plain TypeScript values

const MY_CONSTANT = 100n;  // TypeScript value

East.function([IntegerType], StringType, ($, x) => {
    // CORRECT - x is already an East expression (function parameter)
    const msg1 = $.let(East.str`Value: ${x}`);

    // CORRECT - wrap TypeScript constant with East.value()
    const msg2 = $.let(East.str`Threshold: ${East.value(MY_CONSTANT)}`);

    // CORRECT - use $.const() to create East expression first
    const threshold = $.const(MY_CONSTANT);
    const msg3 = $.let(East.str`Threshold: ${threshold}`);

    // WRONG - cannot interpolate plain TypeScript values
    // const msg4 = $.let(East.str`Threshold: ${MY_CONSTANT}`);  // ERROR

    $.return(msg1);
});

Error Handling

$.try($ => {
    $.assign(result, arr.get(index));
}).catch(($, message, stack) => {
    $.assign(result, -1n);
}).finally($ => {
    // cleanup
});

Platform Function Implementations

// Platform implementations are raw TypeScript functions that receive East values
// (bigint for Integer, number for Float, string for String, etc.)

// Sync platform - regular TypeScript function
const log = East.platform("log", [StringType], NullType);
const timeNs = East.platform("time_ns", [], IntegerType);

// Async platform - async TypeScript function
const fetchStatus = East.asyncPlatform("fetch_status", [StringType], StringType);

// Implementations receive/return TypeScript values matching East types
const platform = [
    log.implement(console.log),                          // (msg: string) => void
    timeNs.implement(() => process.hrtime.bigint()),     // () => bigint
    fetchStatus.implement(async (url: string) => {       // async function OK
        const response = await fetch(url);
        return `${response.status}`;
    }),
];

// In East code: NO await needed - async is handled automatically
const myFn = East.asyncFunction([StringType], NullType, ($, url) => {
    const t1 = $.let(timeNs());
    const status = $.let(fetchStatus(url));  // no await, just call it
    const t2 = $.let(timeNs());
    $(log(East.str`Fetched in ${t2.subtract(t1)} ns, status: ${status}`));
});

// Compile async functions with East.compileAsync
const compiled = East.compileAsync(myFn, platform);
await compiled("https://example.com");  // await at the outer TypeScript level