https://refi64.com/posts/the-magic-of-rpython.html
Note that this was written back when I used "like" and "really" in every
sentence, and when I used to think that Python copied tuples (don't ask).
On Feb 3, 2018 at 10:05 AM, > wrote:
> Are you aware of pypy?
>
Yes, but I do not know it well. It seems to be more focused on JIT
compiling with many of its functions written for special cases and I
did not find a way to make experimental changes easily.
I even tried (briefly) to have the main loop of this interpreter run
in RPython but didn't find enough information on which subset of
Python does RPython handle.
> Can you give an example of how you would do that? I don't mean the
> mechanism used, I mean how would a developer implement a new syntactic
> feature. Suppose I wanted to add a new clause to for...else, let's say:
>
> for ... :
>block
> otherwise:
># runs only if the for-loop was empty
>
> How would do I do that?
>
Here's one way to do that. Sorry for the lack of doc for the moment.
To alter the language's grammar, you'd create a string with the new
grammar, parse it and set the result to pyterp.parser. Then, depending
on how the grammar was changed (which affects what AST is created),
you'd change the corresponding function to handle the new semantics.
I didn't quite understand what you mean by "was empty". However, I
noticed that "else" blocks after for are ignored in the current
implement. Here's an example session adding "else".
If you tell me a bit more about the intended behaviour of "otherwise",
I'd be happy to do an example with that clause.
$ ipython -i test/python_repl.py
p>> simport simple_ast
p>> for i in [1, 2]:
... print i
...
1
2
p>> for i in [1, 2, 3]:
... print i
... else:
... print 100
...
1
2
3
p>> ^D
In the above for loop, the else block is ignored! I press Control-D to
exist the interpreter.
In [1]: grammar = python_grammar.full_definition + python_grammar.extra
In [2]: grammar += r"""
...: for_stmt = "for" {exprlist} "in" {testlist} ":" {suite}
{(SAME_INDENT "else" ":" {suite}) | void=pass_stmt}
...: """
In [3]: pyterp.parser = python.Interpreter(i3.parse("grammar", grammar))
In [4]: pyterp.repl()
Now edit for_stmt in simple_ast.py
except StopIteration:
+evaluate(else_block)
return
(If else_block was not already an (ignored) parameter, we'd have to
change the `def for_stmt...` line too.) And run in the REPL we just
restarted.
p>> simple_ast.reload_module(simple_ast)
p>> for i in [1, 2, 3]:
... print i
... else:
... print 100
...
1
2
3
100
p>> for i in [1, 2, 3]:
... print i
... break
... else:
... print 100
...
1
p>>
Some notes:
1. I'm using the host Python interpreter to change the grammar here
but that's not strictly necessary if we expose pyterp itself in the
global scope.
2. I'm editing simple_ast.py and reloading because its less changes
but redefining a new function and setting simple_ast.for_stmt (in
other words, monkey-patching the change) should also work.
If we wanted an "otherwise" clause on top of an "else" clause, we
probably want to make some provision for the runtime to distinguish
between the two so I'd edit the grammar with something like
for_stmt = "for" {exprlist} "in" {testlist} ":" {suite}
{((SAME_INDENT "else" ":" {suite}) | void=pass_stmt)=else_block
((SAME_INDENT "otherwise" ":" {suite}) |
void=pass_stmt)=otherwise_block}
(In this case, "otherwise" blocks have to come after the "else" block
if both are present.)
asrp
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