Regulus

Regulus ("Reggie") is an experimental Gleam to WebAssembly compiler written in Rust.

The compiler uses tree-sitter to parse Gleam source, builds compiler-owned data structures, lowers a supported subset of Gleam to core IR, and emits .wasm with optional WAT output.

Gleam source
  -> tree-sitter syntax tree
  -> AST
  -> name resolution
  -> type checking
  -> core IR
  -> WebAssembly module
  -> .wasm / optional WAT

Status

Regulus is not a full Gleam compiler yet. It can compile and run small programs using scalar values, managed values, pattern matching, closures, selected stdlib modules, and host imports. The compiler pipeline is intentionally visible so that each layer can be tested and explained.

At a glance

Area	Status
Single-file Gleam to Wasm	Supported
Type checking	Broad subset
Managed runtime values	Partial
Standard library	Selected modules and intrinsics
Whole-project linked output	Supported subset
Dependency source loading	Selected Hex and path packages
Browser, Node.js, and WASI ABIs	Incomplete

Working today

• Parse Gleam source with tree-sitter
• Build an AST with source spans
• Report diagnostics with source spans
• Resolve imports, modules, locals, types, constructors, and fields
• Type check scalar values, structured values, calls, branches, and patterns
• Infer generic functions, generic constructors, closures, and list types
• Lower the supported subset to core IR
• Emit deterministic .wasm
• Render optional WAT
• Run scalar and managed-value exports in Wasmtime tests
• Load gleam.toml and discover project modules
• Compile supported projects into linked Wasm output
• Load selected Hex and path dependency sources
• Lower supported externals to Wasm imports
• Validate target-aware host imports

Supported Gleam surface

• Public and private functions
• Function annotations and inferred parameters
• Int, Float, Bool, String, and Nil
• Local bindings and structured patterns
• Direct, imported, and external calls
• case expressions, guards, and nested patterns
• Type aliases, custom types, opaque types, generics, and constructors
• Records, custom values, tuples, lists, and bit arrays
• Anonymous functions, captures, closures, pipelines, and use
• Selected stdlib modules and intrinsics

WebAssembly output

• Function definitions, imports, and exports
• Scalar ABI values: i64, f64, and i32
• Managed values as guest-memory pointers
• Linear memory and static data segments
• Runtime objects for strings, records, lists, tuples, closures, and custom values
• Branches, comparisons, equality, pattern checks, and failure paths
• Selected runtime helpers and stdlib intrinsics

Not yet implemented

• Compile every valid Gleam project shape
• Compile broad dependency source modules without subset limits
• Compile the full Gleam standard library from source
• Provide complete browser, bundler, and Node.js host ABIs
• Provide complete WASI adapters
• Add garbage collection, reference counting, or heap growth checks

For more detail, see the case-study book in docs/book and the user/development docs in docs/website.

Usage

Build a Gleam project into one linked Wasm artifact:

cargo run -q -p compiler_cli -- build examples/scalar_project

Compile a single Gleam source file:

cargo run -q -p compiler_cli -- compile fixtures/e2e/public_id.gleam \
  -o .sandbox/public_id.wasm \
  --emit wasm,wat \
  --dump-dir .sandbox/dumps

--emit selects artifacts such as wasm, wat, ast, resolved, typed, and ir. The --dump-dir flag writes debug output for compiler stages. See CLI usage for project builds, targets, artifacts, and run.

Development

Use the Rust workspace from the repository root:

cargo fmt
cargo test
cargo clippy --workspace --all-targets

To run examples & tests:

cargo test -p compiler_core
cargo test -p compiler_cli

cargo run -q -p compiler_cli -- compile fixtures/e2e/public_id.gleam \
  --out-dir .sandbox --emit wasm,wat
cargo run -q -p compiler_cli -- run fixtures/e2e/public_id.gleam
cargo run -q -p compiler_cli -- build examples/scalar_project \
  --target bundler --out-dir .sandbox

Documentation

cd docs/website
pnpm install
pnpm docs:dev