WAFER

WebAssembly Forth Engine in Rust

An optimizing Forth 2012 compiler targeting WebAssembly. WAFER JIT-compiles each word definition to a separate WASM module and executes it via wasmtime (CLI) or the browser's WebAssembly API (web REPL).

Highlights

• 200+ words across 12 Forth 2012 word sets, all at 100% compliance
• Optimizing compiler with 6 IR passes + stack-to-local promotion (loops + IF) + consolidation
• Faster than gforth on all benchmarks in release mode (2-10x faster)
• JIT compilation — each : definition compiles to its own WASM module
• Self-recursive direct calls — RECURSE compiles to native call instead of call_indirect
• Consolidation mode — recompile all words into a single optimized WASM module
• Interactive REPL with line editing (rustyline)
• Browser REPL — runs entirely in the browser via wasm-pack + js-sys
• Runtime abstraction — ForthVM<R: Runtime> is generic over execution backend (wasmtime or browser)

Installation

Requires Rust 1.85+ (edition 2024).

cargo install --git https://github.com/ok2/wafer.git wafer

This installs the wafer binary to ~/.cargo/bin/.

To install from a local checkout:

cargo install --path crates/cli

Usage

# Interactive REPL (type BYE to exit)
wafer

# Run a Forth file
wafer program.fth

# Pipe input
echo ': SQUARE DUP * ; 7 SQUARE .' | wafer

# Consolidation: recompile all words into a single optimized WASM module
wafer --consolidate program.fth

# Consolidation with WASM output
wafer --consolidate -o output.wasm program.fth

Example REPL session:

: FIB DUP 2 < IF DROP 1 ELSE DUP 1 - RECURSE SWAP 2 - RECURSE + THEN ;
: FIBS 0 DO I FIB . LOOP ;
12 FIBS CR    \ prints: 1 1 2 3 5 8 13 21 34 55 89 144

VARIABLE COUNTER  0 COUNTER !
: BUMP COUNTER @ 1 + COUNTER ! ;
BUMP BUMP BUMP COUNTER @ .  \ prints: 3

Building from source

git clone --recurse-submodules https://github.com/ok2/wafer.git
cd wafer
cargo build --workspace --release

If you already cloned without --recurse-submodules, fetch the Forth 2012 test suite with:

git submodule update --init

Performance

WAFER beats gforth (the GNU Forth reference implementation) on all benchmarks in release mode:

Benchmark                   WAFER     CONSOL     gforth      WAFER/gf
Fibonacci(25)                1629       1535       3422        0.45x
Factorial(12)x10K             340        339        638        0.53x
GCD-bench(500)                 18         15         30        0.50x
NestedLoops(50)                84         73        720        0.10x
Collatz(2K)                  1212       1202       3914        0.31x

Times in microseconds. WAFER/gf < 1.0 means WAFER is faster. CONSOL = after CONSOLIDATE.

Testing

# All tests (~450 currently passing)
cargo test --workspace

# Forth 2012 compliance suite
cargo test -p wafer-core --test compliance

# Cross-engine comparison (WAFER vs gforth, requires gforth)
cargo test -p wafer-core --test comparison -- --nocapture --ignored

# Optimization benchmark report (WAFER-internal)
cargo test -p wafer-core --test benchmark_report -- --nocapture --ignored

# Lints
cargo clippy --workspace

Architecture

Forth Source -> Outer Interpreter -> IR -> [Optimize] -> WASM Codegen (wasm-encoder)
                                                              |
                                                    Runtime trait instantiation
                                                    (shared memory + table)
                                                         /           \
                                              NativeRuntime      WebRuntime
                                              (wasmtime)         (js-sys)

• Runtime abstraction: ForthVM<R: Runtime> separates the compiler from the execution engine
  • NativeRuntime — wasmtime-based, for CLI, tests, and AOT compilation
  • WebRuntime — browser WebAssembly API via js-sys, for the browser REPL
• Subroutine threading via WASM function tables (call_indirect for cross-word, direct call for self-recursion)
• JIT mode: each new word compiles to a separate WASM module linked to shared memory/globals/table
• IR-based pipeline with 6 optimization passes (peephole, constant folding, strength reduction, DCE, tail call detection, inlining) plus stack-to-local promotion (with loop and IF/ELSE support), DO/LOOP index locals, and consolidation
• Dictionary: linked-list word headers in simulated linear memory

Project Structure

crates/
  core/       wafer-core: dictionary, IR, codegen, optimizer, outer interpreter, Runtime trait
  cli/        wafer: CLI REPL, file execution, consolidation
  web/        wafer-web: browser REPL (wasm-bindgen + WebRuntime + HTML/CSS/JS frontend)
tests/        Forth 2012 compliance suite (git submodule)

Forth 2012 Compliance

Tested against Gerry Jackson's Forth 2012 test suite. 12 of 14 word sets pass at 100%.

Word Set	Status
Core	100% (0 errors)
Core Plus	100% (0 errors)
Core Extensions	100% (0 errors)
Double-Number	100% (0 errors)
Exception	100% (0 errors)
Facility	100% (0 errors)
Floating-Point	100% (0 errors)
Locals	100% (0 errors)
Memory-Allocation	100% (0 errors)
Programming-Tools	100% (0 errors)
Search-Order	100% (0 errors)
String	100% (0 errors)
File-Access	Not started (requires WASI integration)
Extended-Character	Not started

Implemented Words

Over 200 words are implemented across the following categories:

Category	Words
Stack	DUP DROP SWAP OVER ROT NIP TUCK 2DUP 2DROP 2SWAP 2OVER ?DUP PICK DEPTH
Arithmetic	+ - * / MOD /MOD NEGATE ABS MIN MAX 1+ 1- 2* 2/ */ */MOD M* UM* UM/MOD FM/MOD SM/REM S>D <# # #S #> HOLD SIGN
Comparison	= <> < > U< 0= 0< 0<> 0> WITHIN
Logic	AND OR XOR INVERT LSHIFT RSHIFT
Memory	@ ! C@ C! +! 2@ 2! HERE ALLOT , C, CELLS CELL+ CHARS CHAR+ ALIGNED ALIGN MOVE FILL CMOVE CMOVE>
Control	IF ELSE THEN DO LOOP +LOOP I J UNLOOP LEAVE BEGIN UNTIL WHILE REPEAT RECURSE EXIT
Defining	: ; VARIABLE CONSTANT VALUE CREATE DOES> IMMEDIATE DEFER
I/O	. U. .S CR EMIT SPACE SPACES TYPE ." S" ACCEPT
Return stack	>R R> R@
System	EXECUTE ' CHAR [CHAR] ['] DECIMAL HEX BASE STATE >IN >BODY ENVIRONMENT? SOURCE ABORT TRUE FALSE BL
Compiler	LITERAL POSTPONE [ ] EVALUATE ABORT"
Parsing	WORD FIND COUNT >NUMBER
Exceptions	CATCH THROW
Double-cell	D+ D- D. D.R DNEGATE DABS D= D< D0= D0< D>S 2CONSTANT 2VARIABLE 2LITERAL M+ M*/
Strings	COMPARE SEARCH SLITERAL REPLACES SUBSTITUTE UNESCAPE
Floating-Pt	F+ F- F* F/ FABS FNEGATE FSQRT FSIN FCOS FTAN FEXP FLOG FMIN FMAX and 55+ more
Case	CASE OF ENDOF ENDCASE

Web REPL

Build and run the browser-based REPL:

cd crates/web
wasm-pack build --target web --out-dir www/pkg
python3 -m http.server -d www 8080
# Open http://localhost:8080/

Roadmap

• File-Access word set — requires WASI integration for file I/O
• Extended-Character word set — Unicode support
• Self-hosting — minimal Rust kernel (~35 primitives), everything else in Forth

License

MIT OR Apache-2.0