Link to message: http://groups.google.com/group/comp.lang.python/msg/b72788cc5569d778

---------- Forwarded message ---------- From: metawilm@gmail.com metawilm@gmail.com Date: Dec 16, 2006 12:54 PM Subject: Re: CLPython (was Re: merits of Lisp vs Python) To:

Paul Rubin wrote:

metawilm@gmail.com writes:

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a = 'hello' a[0] = 'H' # attempt to change first letter to upper case

As CLPython mirrors Python semantics, this results in a TypeError. The internal representation of an immutable Python string is a mutable Lisp string, but there is no way you can actually modify it from within CLPython.

How do you manage that? The compiler can't check. Is there some kind of special dispatch on the assignment statement instead of turning it into a setf?

Indeed, the assignment of subitems is dynamically dispatched, and always goes via __setitem__ methods, as specified in the language. The __setitem__ methods for strings and tuples raise exceptions. The ones for lists and vectors could be translated into a simple (setf (aref ..) ..) and (setf (nth ..) ..), unless the index is actually a slice and actually a subsequence must be replaced.

Now, the above description using __setitem__ methods is how it apparently works, as observed from the outside. Internally there is a generic function (setf py-subs) that does the work itself, skipping the lookup and calling of __setitem__, if the object is a vector or dict. (The user can call x.__setitem__ explicitly, so that method must exist internally.)

Maybe you were thinking that CLPython, given Python code, outputs a big pile of self-contained equivalent Lisp code that can be run independently, and then CLPython is finished. Instead, only a small amount of Lisp code is generated, but that code can only be run when the CLPython environment is loaded. The generated code refers to that environment, e.g. s[0] = 'x' is translated into a call to (setf (py-subs ..) ..), and function (setf py-subs) is part of the CLPython environment that must be loaded at run-time.

If you compile a Python function, the result is a (mostly) normal Lisp function - its origin from the Python world does not really matter. Also, all Python data structures are first-class Lisp data. Thus CLPython objects live happily together with "normal" Lisp objects in the same image.

And now we arrive at the most exciting part: other Lisp libraries can be loaded in the same image, and we can create connections. Like, after loading CLIM and defining how CLPython objects should be presented, we should get a Python interpreter where classes are displayed graphically and where you can inspect and modify them by mouse. Imagine that you have interactively added a method to a class by dragging a function to a class, then being able to right-click and select "write method", which will write the definition of the method in the right place in the class definition source code in your Climacs (CLIM-based Emacs) buffer.

That's the kind of features I have in mind, and the best thing is that conceptually a lot of the work consists of connecting dots that already out there. But as there are so many of them, a few extra pencils would be quite welcome <wink>

- Willem