simplex-lang

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18 days ago

SimplEx - simple expression language

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SimplEx

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SimplEx — a zero-dependency TypeScript compiler that turns expression strings into safe, sandboxed JavaScript functions.

Why SimplEx?

SimplEx evaluates user-provided expressions safely — for ETL pipelines, business rules, templates, and spreadsheet-like UIs. Expressions run in a fully sandboxed environment with no access to globals, prototype chains, or host APIs; users only see data and functions you explicitly provide.

Every expression computes a value — no statements, no assignments, no loops — so expressions are easy to reason about and safe to store in configs or databases. Familiar JS-like syntax, runtime type safety, full customizability, zero dependencies.

Quick Start

npm install simplex-lang

import { compile } from 'simplex-lang'

compile('a + b')({ a: 2, b: 3 }) // 5

// Pass custom functions via globals, data at runtime
const fn = compile('clamp(score, 0, 100) * weight', {
  globals: { clamp: (v, lo, hi) => Math.max(lo, Math.min(hi, v)) }
})

fn({ score: 150, weight: 0.5 }) // 50

// Pure data expression — no globals needed
const expr = compile('price * quantity * (1 - discount)')

expr({ price: 100, quantity: 5, discount: 0.1 }) // 450

Typical Expression

A canonical mid-sized example showing the common idioms — let-bindings, lambdas, the :: extension operator, pipe with topic reference %, spread, and stdlib calls in both namespace and extension styles:

import { compile } from 'simplex-lang'
import { createStdlib } from 'simplex-lang/stdlib'

const { globals, extensions } = createStdlib()

const fn = compile(
  `
  let visible = users::filter(u => u.enabled),
      formatted = visible::map(u => {
        ...u,
        fullName: u.firstName & " " & u.lastName
      }),
  formatted
    | Arr.map(%, u => u.fullName)
    | Arr.join(%, ", ")
  `,
  { globals, extensions }
)

fn({
  users: [
    { firstName: 'Ada',  lastName: 'Lovelace', enabled: true },
    { firstName: 'Alan', lastName: 'Turing',   enabled: false },
    { firstName: 'Bob',  lastName: 'Smith',    enabled: true }
  ]
}) // "Ada Lovelace, Bob Smith"

A few things to notice:

let introduces local bindings; the last comma-separated expression is the body.
users::filter(...) is the same as Arr.filter(users, ...) — both styles are equivalent.
u => { ...u, fullName: ... } returns an object literal (no block syntax in SimplEx).
| chains the value through stages with % referring to the value at each step.
& is string concatenation; + is numeric addition only.

Playground

Try SimplEx in the browser — edit expressions, inspect the AST, and see results instantly:

SimplEx Playground

Like JS, but…

SimplEx syntax is intentionally close to JavaScript. If you know JS, you can start writing SimplEx immediately. Here are the key differences:

| Concept | JavaScript | SimplEx | Why | |---|---|---|---| | String concatenation | "a" + "b" | "a" & "b" | + is reserved for numeric addition only | | Conditional | x ? a : b | if x then a else b | Readable keyword syntax | | Modulo | a % b | a mod b | % is the topic reference in pipes | | Exponentiation | a ** b | a ^ b | Shorter syntax | | Logical NOT | !x | not x | Word operator | | Logical AND/OR | a && b returns a or b | a and b returns boolean | &&/\|\| also available, but return booleans too | | Equality | === / !== | == / != | Always strict — no loose equality exists | | Optional chaining | obj?.prop | obj.prop | Null-safe by default — null.x returns undefined | | Optional call | fn?.() | fn() | Calling null/undefined returns undefined | | Pipe | Stage 2 proposal | x \| % + 1 | Built-in with % as topic reference | | Partial application | — | fn(#, 3) | # creates a curried function | | let | Statement | let x = 5, x + 1 | Expression that returns a value | | in operator | Checks prototype chain | Checks own keys only | Works with objects, arrays, and Maps |

Everything else works as you'd expect from JavaScript: arrow functions (x => x + 1), template literals (`hello ${name}`), tagged templates, arrays, objects, spread operators, dot/bracket property access, nullish coalescing (??), typeof, and comments (//, /* */).

For a feature-by-feature comparison — including everything JavaScript has that SimplEx doesn't (for/while, classes, try/catch, mutation, regex, import, …) — see the full reference: docs/js-comparison.md.

Language Reference

Literals

| Expression | Description | | --- | --- | | 42, .5, 1.2e3, 0xFF | Numbers (integer, decimal, scientific, hex) | | "hello", 'world' | Strings (supports \n, \t, \uXXXX escapes) | | true, false | Booleans | | null | Null | | undefined | Undefined (identifier, not a keyword) |

Operators

Operators listed by precedence (highest first):

| Precedence | Operators | Description | |---|---|---| | 1 | +x -x not x typeof x | Unary | | 2 | ^ | Exponentiation (right-associative) | | 3 | * / mod | Multiplicative | | 4 | + - | Additive (numbers only) | | 5 | & | String concatenation (coerces to string) | | 6 | < <= > >= in | Relational | | 7 | == != | Equality (strict) | | 8 | and && | Logical AND (short-circuit, returns boolean) | | 9 | or \|\| | Logical OR (short-circuit, returns boolean) | | 10 | ?? | Nullish coalescing | | 11 | \| \|? \|> | Pipe operators |

Runtime type enforcement:

Arithmetic (+, -, *, /, mod, ^) — operands must be finite numbers or bigints
Relational (<, >, <=, >=) — operands must be numbers or strings
& — coerces any value to string
==/!= — strict comparison, no coercion

String Concatenation

The + operator only works with numbers. Use & to concatenate strings:

| Expression | Result | | --- | --- | | "Hello" & " " & "world" | "Hello world" | | "Count: " & 42 | "Count: 42" (coerces to string) | | "Values: " & [1, 2, 3] | "Values: 1,2,3" |

Collections

Arrays:

| Expression | Description | | --- | --- | | [1, 2, 3] | Array literal | | [1, , 3] | Sparse array | | [1, ...other, 4] | Spread (arrays only) |

Objects:

| Expression | Description | | --- | --- | | { a: 1, b: 2 } | Object literal | | { "special-key": 1 } | Quoted key | | { [dynamic]: value } | Computed key | | { x, y } | Shorthand property ({ x: x, y: y }) | | { ...base, extra: true } | Spread |

Spread is the primary form of object composition. Use it whenever the field shape is known statically:

| Expression | Description | | --- | --- | | { ...base, extra: true } | Add or override one field | | { ...a, ...b } | Merge two objects (later wins) | | { ...base, name: name & " (renamed)" } | Compose a derived field | | { ...obj, [key]: value } | Set a computed key |

The standard library's Obj.assign(...) is for cases where spread can't generalize — e.g., merging a dynamic number of objects via Arr.fold(objs, Obj.assign, {}). For known field names, prefer spread.

Property Access

| Expression | Description | | --- | --- | | obj.name | Dot access (own properties only) | | obj["key"] | Bracket access (own properties only) | | arr[0] | Index access | | obj.nested.deep | Chaining | | null.anything | undefined (null-safe, no error) | | expr! | Non-null assert — throws if null/undefined | | a.b!.c.d! | Chainable non-null assertions | | foo!(args) | Assert non-null, then call |

Note: Unlike JavaScript (which has optional chaining ?. and no runtime !), SimplEx has null-safe member access by default but explicit non-null assertion via !. This is inverted from JS — more practical for an expression language working with optional data structures.

Extension Methods (`::`)

The :: operator calls extension methods registered via the extensions compile option. obj::method(args) is equivalent to methodBag.method(obj, args) — the receiver is passed as the first argument to the resolved function.

| Expression | Equivalent | |---|---| | obj::method(x) | methodBag.method(obj, x) | | null::anything() | undefined (null-safe) | | a::f()::g() | g(f(a)) (chaining) |

Extensions are matched by typeof for primitives or by constructor for objects. With the standard library, every Str.* / Num.* / Arr.* / Obj.* function is also available as an extension:

import { createStdlib } from 'simplex-lang/stdlib'

const { globals, extensions } = createStdlib()

compile('"hello"::toUpperCase()', { globals, extensions })()  // "HELLO"
compile('[3, 1, 2]::sort()', { globals, extensions })()        // [1, 2, 3]
compile(
  'users::filter(u => u.active)::map(u => u.name)',
  { globals, extensions }
)({ users: [{ name: 'A', active: true }, { name: 'B', active: false }] })
// ["A"]

Custom extensions:

const extensions = new Map([
  ['string', {
    capitalize: (s: string) => s[0].toUpperCase() + s.slice(1),
    truncate: (s: string, len: number) =>
      s.length > len ? s.slice(0, len) + '...' : s
  }]
])

compile('"hello"::capitalize()', { extensions })()        // "Hello"
compile('"long text here"::truncate(8)', { extensions })() // "long tex..."

Pipe vs :: — both compose values; choose by intent:

users | Arr.filter(%, u => u.age >= 18) | Arr.map(%, u => u.name)
users::filter(u => u.age >= 18)::map(u => u.name)

Use :: when the operation is naturally a method on the value; use | when you need an arbitrary expression with the topic reference % (e.g., a non-method call, a transform that doesn't take the value as the first argument, or a side-by-side use of the value).

Throws ExpressionError if no extensions are configured for the type or the method is not found.

Function Calls

| Expression | Description | | --- | --- | | min(1, 2) | Global function | | obj.method(x) | Method call | | fn()() | Chaining | | null() | undefined (null-safe) |

Currying with `#`

The # placeholder in function arguments creates a partially applied function:

| Expression | Equivalent | | --- | --- | | add(#, 3) | x => add(x, 3) | | add(1, #) | x => add(1, x) | | mul(#, 2, #) | (a, b) => mul(a, 2, b) | | [1, 2, 3] \| map(%, add(#, 10)) | [11, 12, 13] |

Conditionals

| Expression | Description | | --- | --- | | if score >= 90 then "A" else "B" | Conditional with else | | if active then value | Else is optional (defaults to undefined) |

Falsy values: 0, "", false, null, undefined, NaN. Everything else is truthy.

Pipe Operators

Pipes chain a value through a series of expressions. The % topic reference holds the current value:

| Expression | Result | | --- | --- | | 5 \| % + 1 | 6 | | 5 \| % * 2 \| % + 1 | 11 | | 1 \| add(%, 2) \| % * 4 | 12 | | value \|? toUpper(%) | If value is null, returns null (\|? short-circuits) |

|> (forward pipe) — reserved (not available by default). Override pipe in compile options to implement custom semantics.

Lambda Expressions

| Expression | Description | | --- | --- | | x => x + 1 | Single parameter | | (a, b) => a + b | Multiple parameters | | () => 42 | No parameters | | a => b => a + b | Curried (nested) |

Lambdas are closures — they capture the enclosing scope. Parameters shadow outer variables. The body is always a single expression (SimplEx has no block syntax — { ... } after => is an object literal, not a block).

Not supported:

| Form | Workaround | | --- | --- | | Destructuring: ({a, b}) => ..., ([x, y]) => ... | Destructure in the body: pair => let a = pair[0], b = pair[1], ... | | Default parameters: (x = 5) => ... | Use ?? in the body: x => let v = x ?? 5, ... | | Rest parameters: (...args) => ... | Pass an array: args => ... | | Named function declarations | Use let f = (...) => ..., ... (see Recursion for self-reference) |

Let Expressions

let creates local bindings and evaluates a body expression:

| Expression | Result | | --- | --- | | let x = 5, x + 1 | 6 | | let a = 1, b = a + 1, a + b | 3 | | let tax = price * 0.2, price + tax | Sequential binding |

Bindings are sequential — each initializer can reference previous bindings. The last comma-separated expression is the body. Duplicate names cause a CompileError.

Recursion

Named recursion is supported through let-bindings when the initializer is a lambda. The binding's name is captured in the lambda's closure and resolved at call time, by which point the binding is established in the scope chain:

let factorial = n => if n <= 1 then 1 else n * factorial(n - 1),
factorial(5)   // 120

let countdown = n => if n <= 0 then [] else [n, ...countdown(n - 1)],
countdown(3)   // [3, 2, 1]

Self-reference works only for lambdas. Plain expressions need their right-hand side evaluated immediately:

let x = x + 1, x   // Error: x is not defined

No mutual recursion. Two sibling let bindings cannot see each other — each let opens a new scope, and the name becomes visible only for bindings that follow it. Combine both functions into one with a selector parameter, or use the self(self) trick below.

Multi-branch recursion (Fibonacci, tree traversal). When the recursive case combines two or more recursive calls in a single expression, bind each call with let first:

let fib = n =>
  if n <= 1 then n
  else
    let a = fib(n - 1),
        b = fib(n - 2),
    a + b,
fib(10)   // 55

Anonymous recursion — for cases where a name isn't available (e.g., inside a pipe stage):

// self(self) trick
let fact = self => n => if n <= 1 then 1 else n * self(self)(n - 1),
fact(fact)(5)   // 120

// Y combinator — cleaner body, more setup
let Y = f => (x => f(y => x(x)(y)))(x => f(y => x(x)(y))),
Y(self => n => if n <= 1 then n else self(n - 1) + self(n - 2))(10)   // 55

Prefer named recursion for readability; the anonymous forms are useful when a name isn't available.

Template Literals

| Expression | Description | | --- | --- | | `Hello ${name}, you have ${count} items` | String interpolation | | `Price: ${price * (1 + tax)}` | Any expression inside ${} | | `Nested: ${`inner ${x}`}` | Nested template literals | | Multiline content | Allowed (unlike regular strings) |

Tagged template literals — any expression before a template literal calls it as a tag function:

| Expression | Description | | --- | --- | | sql`SELECT * FROM ${table}` | Tag receives (strings, ...values) | | obj.escape`user input: ${value}` | Member expression as tag |

The tag function receives an array of static string parts and the interpolated values (not coerced to strings). It can return any type.

Comments

| Syntax | Description | | --- | --- | | // comment | Single-line comment | | /* comment */ | Multi-line / inline comment |

Reserved Words

if, then, else, and, or, not, in, mod, typeof, let, true, false, null — cannot be used as identifiers.

Data and Scope

Identifiers are resolved in this order: local scope (lambda params, let bindings) -> closure -> globals -> data -> error.

// Globals — compile-time constants, always take priority
const fn = compile('x + y', { globals: { x: 10 } })
fn({ x: 999, y: 20 }) // 30 (x=10 from globals, y=20 from data)

// Data — runtime values passed when calling the compiled function
const expr = compile('firstName & " " & lastName')
expr({ firstName: 'John', lastName: 'Doe' }) // "John Doe"

Globals take priority over data. This lets you provide trusted constants and functions that user expressions cannot override.

API Reference

compile()

import { compile } from 'simplex-lang'

function compile<
  Data = Record<string, unknown>,
  Globals = Record<string, unknown>
>(
  expression: string,
  options?: CompileOptions<Data, Globals>
): (data?: Data) => unknown

Compiles a SimplEx expression string into a reusable function. The returned function accepts an optional data argument and returns the result of evaluating the expression.

CompileOptions

type CompileOptions<Data, Globals> = Partial<
  ContextHelpers<Data, Globals> &
    ExpressionOperators & {
      globals: Globals
      extensions: Map<string | object | Function, Record<string, Function>>
      errorMapper: ErrorMapper | null
    }
>

All fields are optional. You can override any combination of:

| Option | Type | Description | |---|---|---| | globals | Globals | Compile-time constants and functions available to the expression | | extensions | Map<string \| object \| Function, Record<string, Function>> | Extension methods for :: operator. Keys: typeof string or class/constructor. Values: method bags | | errorMapper | ErrorMapper \| null | Error mapping strategy. Default: auto-detected (V8). null disables mapping | | getIdentifierValue | (name, globals, data) => unknown | Custom identifier resolution | | getProperty | (obj, key, extension) => unknown | Custom property access. extension is true for :: access | | callFunction | (fn, args) => unknown | Custom function call behavior | | pipe | (head, tail) => unknown | Custom pipe operator behavior | | nonNullAssert | (val) => unknown | Custom non-null assertion for ! operator | | castToBoolean | (val) => boolean | Custom truthiness rules (affects if, and, or, not) | | castToString | (val) => string | Custom string coercion (affects & and template literals) | | ensureFunction | (val) => Function | Custom function validation | | ensureObject | (val) => object | Custom object validation (for spread) | | ensureArray | (val) => unknown[] | Custom array validation (for spread) | | unaryOperators | Record<op, (val) => unknown> | Override unary operators | | binaryOperators | Record<op, (left, right) => unknown> | Override binary operators | | logicalOperators | Record<op, (left, right) => unknown> | Override logical operators (args are thunks) |

Errors

All SimplEx errors include the original expression and source location for precise error reporting.

ExpressionError — runtime evaluation error (unknown identifier, type mismatch, invalid operation):

import { ExpressionError } from 'simplex-lang'

try {
  compile('x + 1')({}) // x is not defined
} catch (err) {
  if (err instanceof ExpressionError) {
    err.message    // "Unknown identifier - x"
    err.expression // "x + 1"
    err.location   // { start: { offset, line, column }, end: { ... } }
  }
}

CompileError — compilation error (e.g., duplicate let bindings):

import { CompileError } from 'simplex-lang'

compile('let a = 1, a = 2, a') // throws CompileError

UnexpectedTypeError — runtime type validation error:

import { UnexpectedTypeError } from 'simplex-lang'

compile('"hello" + 1')() // throws UnexpectedTypeError: expected number

Eval-free Backend (`interpret`)

compile() builds the expression with new Function(), which is blocked under a strict Content Security Policy and in some edge/sandboxed runtimes. For those environments SimplEx ships a second backend — a tree-walking interpreter that evaluates the AST directly, with no new Function / eval:

import { interpret } from 'simplex-lang/interpret'

interpret('a + b')({ a: 2, b: 3 }) // 5

Use it when new Function is unavailable:

Strict CSP — pages without 'unsafe-eval' in script-src.
MV3 browser extensions — service workers forbid eval/new Function.
Edge runtimes — Cloudflare Workers, Deno Deploy, and similar isolates that disallow dynamic code generation.

interpret() is a drop-in for compile() — same signature, same language semantics, same errors. The two backends share the runtime (runtime.ts) and the compile-time checks (validate.ts); only the evaluation strategy differs. A parity test suite runs the same cases through both and asserts identical results, error types, and messages.

import { interpret } from 'simplex-lang/interpret'
import { createStdlib } from 'simplex-lang/stdlib'

const { globals, extensions } = createStdlib()

interpret('users::filter(u => u.active)::map(u => u.name)', { globals, extensions })({
  users: [{ name: 'A', active: true }, { name: 'B', active: false }]
}) // ["A"]

Trade-offs vs compile():

| | compile() | interpret() | |---|---|---| | Mechanism | codegen → new Function() | tree-walking AST evaluation | | Works under strict CSP / no-eval | no | yes | | Per-call performance | near-native after JIT warmup | slower (AST walk per call) | | errorMapper option | yes | not applicable (errors are located from AST nodes directly) |

interpret() is a fallback for eval-free environments, not a replacement — prefer compile() whenever new Function is available. The two are import-isolated: importing simplex-lang/interpret does not pull in the codegen path or new Function, so the eval-free build tree-shakes cleanly.

Note: InterpretOptions is CompileOptions without errorMapper (the interpreter attributes errors to source nodes directly, so no stack-trace mapping is needed). Every other option — globals, extensions, operator overrides, context helpers — works identically.

Customization

Every aspect of SimplEx evaluation can be customized through compile options.

Custom operators — override or extend any operator:

import {
  compile,
  defaultBinaryOperators,
  defaultUnaryOperators
} from 'simplex-lang'

const fn = compile('not -a + b', {
  unaryOperators: {
    ...defaultUnaryOperators,
    not: val => Number(val) + 1 // redefine "not"
  },
  binaryOperators: {
    ...defaultBinaryOperators,
    '+': (a, b) => Number(a) * Number(b) // make "+" multiply
  }
})

Custom identifier resolution — control how variables are looked up:

// Use a Map instead of a plain object for globals
const fn = compile('foo', {
  globals: new Map([['foo', 'bar']]),
  getIdentifierValue(name, globals, data) {
    if (globals.has(name)) return globals.get(name)
    return data[name]
  }
})

Custom property access — intercept or transform property lookups:

const fn = compile('a.b', {
  getProperty: (obj, key, extension) => `custom:${String(key)}`
})

fn({ a: { b: 'real' } }) // "custom:b"

Custom function calls — wrap or intercept function invocations:

const fn = compile('f(1, 2)', {
  globals: { f: (a, b) => a + b },
  callFunction: (fn, args) => {
    if (args === null) return fn()
    return `intercepted:${fn(...args)}`
  }
})

fn() // "intercepted:3"

Custom pipe — implement your own pipe semantics:

const fn = compile('1 | % + 1', {
  pipe: (head, tail) => {
    let result = head
    for (const t of tail) {
      result = `piped:${t.next(result)}`
    }
    return result
  }
})

fn() // "piped:2"

Custom boolean coercion — change what counts as truthy/falsy (affects if, and, or, not):

const fn = compile('if a then "yes" else "no"', {
  castToBoolean: val => val === 'truthy'
})

fn({ a: 'truthy' }) // "yes"
fn({ a: true })     // "no" — only the string "truthy" is truthy now

Standard Library

SimplEx includes a built-in standard library with namespaced functions and extension methods:

import { compile } from 'simplex-lang'
import { createStdlib } from 'simplex-lang/stdlib'

const { globals, extensions } = createStdlib()

compile('Math.abs(x) + Str.upper(name)', { globals, extensions })({
  x: -5,
  name: 'hello'
}) // 5 + "HELLO" → uses Math and Str namespaces

Namespaces: Str, Num, Math, Arr, Obj, Json, Date + top-level utilities (empty, exists, typeOf).

Extension methods let you use method-call syntax: x::abs(), items::map(fn), name::upper().

Key conventions:

NaN → null — functions that would return NaN in JS return null instead. Use ?? to provide defaults: Math.sqrt(x) ?? 0
Immutable — array operations return new copies (no mutation)

See Standard Library Reference for the full API.

Using External Functions

Beyond the Standard Library, you can provide any custom functions via globals. This is useful for domain-specific logic. To combine stdlib with your own functions, spread them together:

import { createStdlib } from 'simplex-lang/stdlib'

const { globals, extensions } = createStdlib()

const fn = compile(
  `price * quantity * (1 - discount)
    | Math.round(%)
    | formatPrice(%)`,
  {
    globals: {
      ...globals,
      formatPrice: (val) => `$${val.toFixed(2)}`
    },
    extensions
  }
)

fn({ price: 19.99, quantity: 3, discount: 0.15 }) // "$51.00"

Domain-specific helpers:

const fn = compile(
  `
  if classify(score) == "A" then
    bonus(salary)
  else
    salary
  `,
  {
    globals: {
      classify: (score) => (score >= 90 ? 'A' : score >= 70 ? 'B' : 'C'),
      bonus: (salary) => salary * 1.2
    }
  }
)

fn({ score: 95, salary: 50000 }) // 60000

Combining with currying:

// `map` and `mul` here are *your* globals — not the stdlib.
// The stdlib equivalent is `Arr.map(%, Math.mul(#, factor))` only if `Math.mul`
// were provided; in practice you'd write: `items | Arr.map(%, x => x * factor)`.
const fn = compile('items | map(%, mul(#, factor))', {
  globals: {
    map: (arr, fn) => arr.map(fn),
    mul: (a, b) => a * b
  }
})

fn({ items: [1, 2, 3], factor: 10 }) // [10, 20, 30]

Note: the bare map/mul names work above because they are passed in as user globals. The standard library exposes its functions only via namespaces (Arr.map, Str.toUpperCase, …) or :: extensions (x::map(fn)). See Standard Library and docs/stdlib.md.

AI / LLM Integration

SimplEx is a good target for AI-generated expressions: safe by design (no globals, filesystem, or network), deterministic, simple grammar, and compilation catches errors before runtime.

const fn = compile(aiResponse.expression) // e.g., "price * quantity * (1 - discount)"
fn(data) // safe execution

Expressions are compiled once to native JS functions via new Function() — subsequent calls have near-native performance. In environments where new Function is blocked (strict CSP, MV3 extensions, edge runtimes), use the eval-free interpret() backend instead.

License

MIT