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Write Wahoo on the left; on the right, look at what each phase of the compiler makes of it, and run it. Everything happens in your browser: the compiler is a WebAssembly module, and so is every program it produces.
New to the language? The language page fits on one screen. Each tab corresponds to a chapter: tokens, syntax tree, WebAssembly (switch the optimizer off to compare) and bytes. Errors and warnings appear under the editor as you type, and Share puts the program in the address bar, so a link reproduces it exactly. Programs that loop forever are stopped after three seconds.