29 — Bootstrap: NME compiling a tiny language

Guide 28 interpreted lines directly. The next step is the seed of a real compiler: a program that translates a tiny language into another language and runs the result. Writing a compiler in NME is called bootstrapping, and it is how every real compiler grows. The example examples/bootstrap.nme does exactly this.

#Steps

1. Define a tiny language with five instructions. BML (beginner mini language) keeps one instruction per line:

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

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2. Read it line by line and translate each instruction into Python. The show instruction becomes a print, set becomes an assignment, and while opens a block. Indentation tracks the block depth:

   # part of examples/bootstrap.nme
   lines = []
   indent = 0
   for raw in program:
       parts = raw.split()
       verb = parts[0]
       if verb == "set":
           lines.append(" " * indent + f"{parts[1]} = {parts[2]}")
       elif verb == "show":
           lines.append(" " * indent + f'print("{parts[1]}")')
       elif verb == "while":
           lines.append(" " * indent + f"while {parts[1]} < {parts[2]}:")
           indent = indent + 4
       elif verb == "end":
           indent = indent - 4
   

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3. Join the translated lines and run them. Because NME itself runs on CPython, the compiler can execute what it produced:

   # part of examples/bootstrap.nme
   source = "\n".join(lines)
   exec(source)
   

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4. Run the whole program:

   nme r examples/bootstrap
   

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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 translated BML to Python, and Python ran it.

#Try it yourself

Add a sub <name> <int> instruction to the translator (-= in Python), then a BML program that counts down from 5 to 1.

#What you learned

• A compiler translates source text into another language, then runs it.
• split() turns a line into words; f"..." builds the output line.
• Indentation depth tracks nested blocks.
• Writing a compiler inside NME is bootstrapping — the seed of self-hosting.