My post didn’t indicate this, but the most common source of scope is functions; calling a function starts a new scope that ends when the function returns. Especially in this case, it does often make sense to use the same variable name:
Will have prior=prior, evidence=evidence, and is a good naming scheme. But in most languages, modifying ‘evidence’ in the function won’t affect the value of ‘evidence’ outside the scope of the function. This sometimes becomes confusing to students when the function above gets called like so:
Because their previous model relied on the names being the same, rather than the coincidence of naming being merely helpful.
Overall, I would say that this is still a fertile source of errors, but in some situations the alternative is to have less readable code, which is also a fertile source of errors and makes fixing them more difficult.
Your confusion is due to using “scope”, which is actually a lexical concept. What you’re dealing with here is variable substitution: in order to evaluate a function call such as posterior = ApplyBayes(prior, evidence1), the actual function arguments need to be plugged into the definition of ApplyBayes(·, ·). This is always true, regardless of what variable names are used in the code for ApplyBayes.
I certainly hope that I’m not confused about my word choice. I write compilers for a living, so I might be in trouble if I don’t understand elementary terms.
In all seriousness, my use of the word “scope” was imprecise, because the phenomenon I’m describing is more general than that. I don’t know of a better term though, so I don’t regret my choice. Perhaps you can help? Students that I’ve seen have difficulty with variable substitution seem to have difficulty with static scoping as well, and vice versa. To me they feel like different parts of the same confusion.
In a related note, I once took some of my students aside who where having great difficulty getting static scoping, and tried to teach them a bit of a dynamically-scoped LISP. I had miserable results, which is to say that I don’t think the idea of dynamic scope resonated with them any more than static scope; I was hoping maybe it would, that there were “dynamic scoping people” and “static scoping people”. Maybe there are; my experiment is far from conclusive.
EDIT: Hilariously, right after I wrote this comment the newest story on Hacker News was http://news.ycombinator.com/item?id=4534408, “Actually, YOU don’t understand lexical scope!”. To be honest, the coincidence of the headline gave me a bit of a start.
Is there a reason to use the same variable name within and outside a scope? It seems like a fertile source of errors.
I can see that someone would need to understand that reusing names like that is possible as a way of identifying bugs.
My post didn’t indicate this, but the most common source of scope is functions; calling a function starts a new scope that ends when the function returns. Especially in this case, it does often make sense to use the same variable name:
Will have prior=prior, evidence=evidence, and is a good naming scheme. But in most languages, modifying ‘evidence’ in the function won’t affect the value of ‘evidence’ outside the scope of the function. This sometimes becomes confusing to students when the function above gets called like so:
Because their previous model relied on the names being the same, rather than the coincidence of naming being merely helpful.
Overall, I would say that this is still a fertile source of errors, but in some situations the alternative is to have less readable code, which is also a fertile source of errors and makes fixing them more difficult.
Your confusion is due to using “scope”, which is actually a lexical concept. What you’re dealing with here is variable substitution: in order to evaluate a function call such as
posterior = ApplyBayes(prior, evidence1), the actual function arguments need to be plugged into the definition ofApplyBayes(·, ·). This is always true, regardless of what variable names are used in the code forApplyBayes.I certainly hope that I’m not confused about my word choice. I write compilers for a living, so I might be in trouble if I don’t understand elementary terms.
In all seriousness, my use of the word “scope” was imprecise, because the phenomenon I’m describing is more general than that. I don’t know of a better term though, so I don’t regret my choice. Perhaps you can help? Students that I’ve seen have difficulty with variable substitution seem to have difficulty with static scoping as well, and vice versa. To me they feel like different parts of the same confusion.
In a related note, I once took some of my students aside who where having great difficulty getting static scoping, and tried to teach them a bit of a dynamically-scoped LISP. I had miserable results, which is to say that I don’t think the idea of dynamic scope resonated with them any more than static scope; I was hoping maybe it would, that there were “dynamic scoping people” and “static scoping people”. Maybe there are; my experiment is far from conclusive.
EDIT: Hilariously, right after I wrote this comment the newest story on Hacker News was http://news.ycombinator.com/item?id=4534408, “Actually, YOU don’t understand lexical scope!”. To be honest, the coincidence of the headline gave me a bit of a start.