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Reference documentation, not a pitch

WAST

A typed source language that stays close to the WebAssembly instruction set. It exists to make writing WebAssembly by hand less tedious, not to be a general-purpose language. It is basic, low-level, and small on purpose — this is what it is, not what it's becoming.

Cheat sheetGuideLimitationsSpec
version0.1.0 — no tags, no changelog
targetcore wasm + reftypes + sign-ext + GC
testsblack-box only, no rust unit tests
stabilitynone — syntax can change
counter.wastinput
global Counter: mut i32 = 0;func increment() -> i32 {Counter = Counter + 1;return Counter;}
compiles to WAT, 1:1see below

Read this before you write anything real: this is a from-scratch, single-maintainer compiler with no unit tests, no release process, and documented cases of control-flow bugs that produced valid-but-wrong WebAssembly. See limitations and rough edges.

In the language

  • Typed source close to Wasm: i32/i64/f32/f64, no implicit conversion
  • GC structs (nominal, inheriting), arrays, i31, casts, flow-sensitive narrowing
  • Structured control flow: labeled block/loop, for, is-tests
  • Packed i8/i16 fields, data segments, typed function references
  • A wasm.<opcode> escape hatch for the instruction catalog
  • Readable, comment-annotated WAT output
  • An LSP: hover, completion, rename, references, go-to-def, semantic tokens

Not in the language

  • Exception handling, SIMD, threads, memory64, stringref
  • Source-level tail calls (only a --tail-calls output flag on your own return)
  • A raw call_ref, or any branch instruction in the wasm.* escape hatch
  • Module linking — one file compiles to exactly one module
  • Unicode identifiers, block comments
  • Any stability guarantee whatsoever

GC types

References carry nullability in the spelling.

&Type is non-null, ?Type is nullable. Structs inherit nominally; arrays are one element type, explicitly allocated.

GC structs and arrays →
struct Shape {area: f64,}struct Circle : Shape {radius: f64,}let circle: &Circle =new Circle { area: 3.14, radius: 1.0 };

Look it up

Reference, not a tutorial.

WAT

Output, not a black box

Every instruction traces back to a source line.

Output is WAT text with a comment above each generated instruction sequence naming the declaration or statement that produced it. Binary output and any optimization are separate, explicit, opt-in steps — never on by default.

Tooling and CLI