Yulang

Yulang is a language for writing direct, straight-line programs while defining control flow in libraries. This page shows how lifted comparison and nondeterministic search grow from that design, then points to the rest of the guide.

if all [1, 2, 3] < any [4, 5, 6]:
    "found a pair"
else:
    "nothing"

That if compares every element of one list against every element of another. There is no loop, no comprehension, no helper — all and any are first-class values, and the comparison lifts over them. The same machinery powers nondeterministic search:

{
    my a = each 1..
    my b = each a<..
    my c = each b<..
    guard: a * a + b * b == c * c
    (a, b, c)
} .once

Three each bindings explore an infinite three-dimensional grid. guard prunes the search. .once runs it and returns the first Pythagorean triple. The code reads top to bottom like an imperative script, but underneath it is branching and backtracking.

Yulang is built on this principle: control flow is library code. Mutation, nondeterminism, lifted comparison, early return, typed errors, custom backtracking — none of them are parser builtins. They are all ordinary functions sitting on top of algebraic-effect machinery you can extend yourself. Surface code stays short and straight-line; the shape underneath can be anything you want.

In the box

• Algebraic effects with handlers. act declares an operation, catch op, k -> ... handles it. The handler receives the captured continuation k, so resuming, retrying, and aborting are all just choices of what to do with k.
• Roles instead of typeclasses. role Add 'a: declares an interface, impl Add int: provides it. Methods read as ordinary dot calls.
• Mutation that stays pure. my $x = 0, read with $x, write with &x = v. Compiles down to a handled var effect — the semantics stays pure, the syntax stays familiar.
• Patterns everywhere bindings live. my (a, b) = pair. Optional named arguments fall out of record patterns: my area {width = 1, height = 2} = width * height.
• A parens-light surface. Bare application f x y, colon application f: ..., layout blocks driven by : and indentation.

Playground

Every example on this page runs in the Playground.

Where to go next

• Installation — install the CLI and run a first .yu file
• Tour — a guided walkthrough of the language
• Cookbook — task-oriented recipes
• Cache — local CLI artifact cache and route labels
• Reference — syntax and feature details