On Sat, 10 Jan 2009 15:46:35 +0000, Mark Wooding wrote: > [Another tome. I hope this contains sufficient new material to continue > to be of interest to other readers.]
I found it interesting. I don't know about others. However, at 756 lines (including quoting) it was nearly double the size of my previous tome, so I think this discussion is clearly not going anyway. I think this conversation is reaching it's natural end. Frustration levels are rising. I could -- and in fact intended to -- write a point-by-point argument, but that would only send the size of this skyrocketing and probably nobody would read it. So I'm going to take a different tack in an attempt to reduce frustration levels: if I can't convince the other side they're wrong, can I at least get them to understand where I'm coming from a little better? As I see it, this conversation is floundering on two radically different ideas about what it means to say a language uses pass-by-foo. On the one hand, some people (me and possibly rurpy) consider "this is pass-by-foo" to be a statement about behaviour directly visible to the programmer. We have a set of behavioral traits in mind, and if a language exhibits those behaviours, then it is clearly and obviously pass-by-foo no matter how that behaviour is implemented. I'll call these the behaviorists. On the other hand, others (I'm going to put Joe and Mark into this group) consider "pass-by-foo" to be a statement about mechanism, or if you prefer, implementation. If a compiler does X, then the language is pass- by-foo, regardless of behaviour. I'll call these the mechanists. To a behaviorist, Python simply can't be pass-by-value, because it doesn't behave like pass-by-value in other languages (particularly C and Pascal). The Python community has a tradition of duck-typing: if it quacks like a duck and swims like a duck, then it is a duck in every way that is important. If it can't swim and doesn't quack, it isn't a duck, no matter how much duck DNA is in it. If it flies, that's irrelevant to the question, because some ducks fly and some don't. According to the behaviorist view, what makes an evaluation strategy call- by-reference is whether or not it exhibits the following three behaviours: (1) passing a value to a function does not copy the value; (2) modifications to the value inside the function are visible to the caller; (3) assignments to the value inside the function are visible to the caller. where "value" means the thing the programmer manipulates symbolically in source code. Values are ints, strings, lists and so forth. Pointers or references are *only* values if the language allows you to write the equivalent of: ptr = SomeReferenceTo(x) # like Pascal's ptr := ^x; x = Dereference(ptr) # like Pascal's x := ptr^; According to this viewpoint, Python clearly cannot be pass-by-reference because (3) is not true. Similarly, what makes pass-by-value is: (1) passing a value to a function makes a copy of the value, where value is an entity the programmer can symbolically manipulate in source code (lists, ints, floats etc.); (2) modifications to the value inside the function are not visible to the caller; (3) assignments to the value inside the function are not visible to the caller. According to this viewpoint, Python clearly cannot be pass-by-value either because neither (1) nor (2) are true. The underlying mechanism is irrelevant. Mechanists take a different view. A typical argument paraphrased from Joe in previous threads is: "Of course Python makes a copy of the value you pass to a function. The difference is that the value you pass is a reference." Mechanists reject the restriction that values only include entities manipulated by the programmer. Consequently they're prepared to say that values in Python aren't the things which Python programmers symbolically manipulate (strings, ints etc.) but are references. Needless to say this claim strikes behaviorists as nonsensical and rather incoherent. If the value of 2 isn't 2, then value has no meaning. If mechanists see the behaviorists as "willfully ignorant", the behaviorists see the mechanists as being blatantly obfuscatory, introducing irrelevant details and ignoring clear examples of the duck quacking. And these two viewpoints are why this debate never ends, merely goes quite for a few weeks or months. -- Steven -- http://mail.python.org/mailman/listinfo/python-list