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Crafting your own compiler: from Python logic to high-speed WebAssembly

Notes from the EuroPython 2026 workshop on building a Forth-to-WebAssembly compiler in Python.

What is WebAssembly?

WebAssembly (Wasm) is a binary instruction format for a stack-based virtual machine. There are no registers, every instruction pops its operands off the stack and pushes the result back on. For example, i32.add pops two values, and pushes their sum. Or similarly, i32.le_s (~ less or equal) pops two, pushes a boolean. Control flow (block, loop, br) works the same way: structured, nested regions instead of jumps to arbitrary addresses.

It's designed as a compiler target, not something you write by hand. Wasmtime, the runtime this workshop uses, runs .wasm binaries standalone from the command line (sandboxed).

What is Forth?

Forth is written in Reverse Polish Notation (RPN): operators come after their operands, 2 3 + instead of 2 + 3. Read left to right, every token is either a value to push or an operator to apply to what's already on the stack. The same order a stack machine executes in. That's why compiling RPN to Wasm feels almost natural: reading a forth program and reading its compiled wasm are nearly the same.

Adding two numbers:

forth 
1 2 +
wat 
i32.const 1
i32.const 2
i32.add

A comparison used as a loop condition, from one of the workshop's example programs:

forth 
i n <=
wat 
local.get $i
local.get $n
i32.le_s

Each RPN token becomes one Wasm instruction, in the same order.

The pipeline

The compiler converts a .4th source file to a runnable .wasm binary:

graph TD subgraph FE["frontend"] direction LR A[.4th source] --> B[strip comments, tokenize] --> C[find variables, declare locals] --> D[code generation] end subgraph BE["backend"] direction LR E[.wat text] --> F[wat2wasm] --> G[.wasm binary] --> H[wasmtime runtime] end FE ~~~ BE

The frontend's code generation output is the backend's .wat text input. Tokenizing and code generation are the mechanical step described above. Wasm needs every local declared before the function body can use it - Forth doesn't, you just write x! and x exists. So before generating any instructions, the compiler walks the whole program once to collect every variable name, then turns that list into the declarations that Wasm requires.

The full implementation is on GitHub, the workshop material is at arielortiz.info/europython2026.

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