Hacker News new | ask | show | jobs
by arcticbull 1989 days ago
Without speaking for the OP, it seems like the case being made is not that the result is executed on a real, finite machine approximation (as all programs are) but that there exists an abstract Python machine wherein the memory does not need to be so described. This does not exist for CSS.
2 comments
bawolff 1989 days ago
Why not? I can imagine a modified abstract css machine which applies the styles to an infinitely long html document. That's much more contrived than the python case, but if we are going to be making arbitrary changes from finite machines, im not sure under what basis we would draw the line.
link
maweki 1989 days ago
The html document is potentially infinite, but its finite length is not encoded in the program (css).

One idea is, to have the same program, and if it terminates, it will terminate if you pick a large enough machine. You don't need to "rewrite" the program to use a larger machine.

So in this case the html document is the tape. Potentially infinite, but not in reality. The css program may handle it as infinite.

link
weare138 1988 days ago
>that there exists an abstract Python machine wherein the memory does not need to be so described

This is getting a little esoteric. Python interpreters are created in other languages like C where memory does need to be described. How can this 'abstract Python machine' even be implemented? Python or any higher level interpreted language will always have the same limitations of the language it's implemented in and like anything we do on modern computers, can be reduced to assembly where again memory has to be 'described'. Python may hide it for us but it's there.

link
ivanbakel 1988 days ago
>How can this 'abstract Python machine' even be implemented?

It wouldn't be - the machine is abstract. It exists only in terms of a mathematical model, that describes how Python code behaves in a defined mathematical way (its semantics). The real implementations of Python should have the same behaviour as the abstract machine, but the property of Turing-completeness really only exists for the abstract version - the real versions, being bounded in memory, are limited to certain sizes of programs.

link