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The goal of the MABWA project is to produce an implementation
of PyFL that is much faster than the current lazy interpreter,
that repeatedly decodes the expression tree.
The current interpreter produces Basic PyFL, with all the advanced
features (like while loops) implemented by translation.
Our strategy is to translate Basic PyFL
into the machine language for a FORTH-like stack-based
abstract machine.
The MABWA machine has a conventional FORTH like design,
with a reverse polish instruction language.
To add 2 and 3 you push 2 then 3 on the implicit value stack,
then execute the add instruction that pops the top two elements of
the stack, adds them, then pushes the result on the stack.
The 'commands' begin and end
delimit code blocks, sequences of instructions
that will be manipulated but not immediately executed.
The MABWA machine pushes these two blocks onto the stack
when it encounters them. They become the top two elements of
the stack.
The next three commands test whether x
is less than y
and leave the boolean result on the stack.
The if command pops the top of the stack (it expects this to be a boolean) and examines it. If this test is true, it pops and discards the top of the stack then pops the element underneath (which should be a code block). It begins executing this block. If the boolean is false it again pops both code blocks but begins executing the second, which was on top of the stack.
The next stage is to assemble the MABWA code into machine language.
The result is still text, but at a lower level and more concise.
For example, addition is denoted by +,
and load becomes three instructions.
The pseudo-instruction
" causes the
name of the variable (a string) to be pushed on the stack
without being examined
(as an instruction). Then the instruction $
looks up the definition of the variable - a code block
(not shown) which is then executed.
Environments come into play during evaluation of function calls.
There are two environments, the environment in which the function was
called and that in which it was defined. The body of the
function is evaluated in the defining
environment but the actuals are evaluated in the calling environment.
We plan to write a test interpreter in Python to check out the
logic. But for speed, the ultimate goal, we'll use Apple's Swift.