Notes:

• You can type special characters with a backslash prefix, as shown by hovering over the character bar above. Documentation links for primitives are here.
• Any modifier with enter will execute (not just shift).
• BQN uses ‿ (backslash-space) for stranding, and not juxtaposition. If you don't know what went wrong, check for this first!
• This implementation is still WIP, and in particular some syntax errors are not checked (which can cause an "Internal compiler error" later). Support for function headers and multiple bodies is missing, and Under (⌾) only works for a small class of structural functions. Performance is bad.