What if TypeScript
was purely functional?

typelang is a strict functional subset of TypeScript with algebraic effects, enforced purity, and zero runtime errors. Build reliable software with effect tracking and exhaustive pattern matching.

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Zero Dependencies 100% TypeScript Effect Tracking Subset Enforced

// Counter with Console + State
const tick = () =>
  seq()
    .let(() => State.get<{count: number}>())
    .let((s) => ({count: s.count + 1}))
    .do((next) => Console.op.log(`Count: ${next.count}`))
    .do((next) => State.put(next))
    .return((next) => next.count);

Click "Run" to see the handlers execute live.

Traditional TypeScript

// Mutation everywhere
let count = 0;
function increment() {
  count++;  // Side effect!
  console.log(count);  // Another side effect!
  return count;
}

❌ Hidden mutation makes testing hard
❌ Side effects not tracked in types
❌ No guarantee of execution order
❌ Cannot safely parallelize

→

typelang

// Pure transformation
const increment = () =>
  seq()
    .let(() => State.get<{count: number}>())
    .let((state) => ({count: state.count + 1}))
    .do((next) => Console.op.log(`${next.count}`))
    .do((next) => State.put(next))
    .return((next) => next.count);