34 — Self-host: NME running NME

Guide 29 compiled a tiny language called BML. This guide compiles a subset whose words are already NME words — say, set, while, end — the seed of NME running NME.

#Steps

1. The mini-language program — one instruction per line, in NME words:

   set count 0
   while count 3
     say hello
     add count 1
   end
   say done
   

   copy

   while count 3 means "repeat while count < 3"; end closes the block.

2. The whole compiler. Save selfhost.nme:

   # selfhost.nme — NME compiling a tiny NME-like language.
   # The mini language uses NME words: say <text>, set <name> <int>,
   # add <name> <int>, while <name> <int>, end.
   # Run: nme r selfhost
   
   program = [
       "set count 0",
       "while count 3",
       "  say hello",
       "  add count 1",
       "end",
       "say done",
   ]
   
   lines = []
   indent = 0
   for raw in program:
       parts = raw.split()
       verb = parts[0]
       if verb == "say":
           lines.append(" " * indent + f'print("{parts[1]}")')
       elif verb == "set":
           lines.append(" " * indent + f"{parts[1]} = {parts[2]}")
       elif verb == "add":
           lines.append(" " * indent + f"{parts[1]} += {parts[2]}")
       elif verb == "while":
           lines.append(" " * indent + f"while {parts[1]} < {parts[2]}:")
           indent = indent + 4
       elif verb == "end":
           indent = indent - 4
       else:
           lines.append(" " * indent + "# unknown instruction: " + raw)
   
   source = "\n".join(lines)
   show "generated Python:"
   show source
   show ""
   show "running it:"
   exec(source)
   

   copyrun it →

   split() turns each line into words; indent tracks block depth, and exec(source) runs the generated Python — the trick from guide [29].

3. Run it — no server and no input needed:

   nme r selfhost
   

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   generated Python:
   count = 0
   while count < 3:
       print("hello")
       count += 1
   print("done")
   
   running it:
   hello
   hello
   hello
   done
   

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   A compiler written in NME read NME words and ran the result on CPython — the seed of NME compiling NME itself.

4. Korean writes the same compiler; only the comments and report lines change. The full program is in the Korean guide.

#Try it yourself

Add a say hi 3 form that prints hi three times. Hint: translate it to for _ in range(3): print("hi").

#What you learned

• A mini language whose words are NME words is closer to NME itself.
• say, set, add, while, and end map to Python one line each.
• A compiler that reads NME-like source is the seed of NME running NME.