I recently asked What is the name of the functional argument in fold which is now marked off-topic because it might be opinion based. I do not understand what makes this opinion based. This question looks similar to me: Is there a name for web applications that do most of the navigation on the client side? It does not appear to be held as opinion based.
Which differences make the first question “opinion based” and the second not?
For what it's worth, I voted to reopen. I don't think questions should be evaluated on the asker knowing if there's a single canonical answer a priori. If most standard libraries you're aware of call it either f or op, say so. If the answer is, "there isn't any convention so choose whatever you want," then let that be the answer. Don't close the question. The possibility was even left open by saying "if any."
I suppose the convention being different in different libraries made people say it is a matter of opinion, but the difference is, the asker wasn't asking for an opinion. He had no way of knowing there wasn't some official term for it in lambda calculus. Asking for an opinion is off topic. Asking for a fact and finding out the answer is a matter of opinion is not off topic.
Also, I see questions all the time where someone answers "there is no such thing" and someone else comes right behind them with the correct answer. No one has an exhaustive knowledge of all things computer science, so just because you personally don't know the term doesn't necessarily mean the term doesn't exist. Unless you happen to be someone like Don Syme or Martin Odersky, in which case whatever name you make up off the top of your head immediately becomes official.
Terminology questions are not generally off topic, as evidenced by a tag terminology of its own. To find out whether a question is a good fit for the stackexchange model, we have to ask ourselves whether this question has a single, objective answer.
With your question, this is not the case (but it isn't your fault). The fold has been known by many names: fold, reduce. inject, aggregate, …. An answer could provide a name for the functional argument, but this name might not be known in other traditions that don't call it fold. Therefore, an answer (that doesn't provide an answer for each of the different naming traditions) cannot be universally correct, and two answers that provide the correct answer for one of these traditions each would be equally correct, so there wouldn't be a clear candidate to accept. In short, this question is opinion-based.
There is also the small issue that (even within one such naming tradition) no universal name of the functional argument exists (which itself is a pretty clear name). When mathematically describing or implementing a reduce function, I would simply call the argument f, which is not satisfactory here.
The other question was answerable. It knew that a term had to exist. The asker tried to research this, and listed examples and possible terms, along with explanations why those terms do not sufficiently describe his concept. It showed that prior research on the asker's part had taken place (unlike your question where it was only clarified in the comments that this isn't quoted from an exam you are taking).
fold and its ilk do have a specific name: higher-order functions. Just like fractions have numerators, it seems that higher-order functions should have foo as (one of) their arguments.
Unfortunately, it's hard to tell ahead of time whether a "what is the name of that thing" question is going to be definitively answerable. Without a specific term that clearly applies, any answer is indeed going to be opinion-based.
Since I keep losing this golden bit of text, I'm just going to put it here:
This question appears to be off-topic because it is a "name that thing" question. "Name that thing" questions are bad questions for the same reasons that "identify this obscure TV show, film or book by its characters or story" are bad questions: you can't Google them, they aren't practical in any way, they don't help anyone else, and allowing them opens the door for the asking of other types of marginal questions. See http://blog.stackoverflow.com/2012/02/lets-play-the-guessing-game
It should be reopened.
fold, map, filter, and their ilk are known as higher-order functions:
In mathematics and computer science, a higher-order function (also functional form, functional or functor) is a function that does at least one of the following:
* takes one or more functions as an input * outputs a functionAll other functions are first-order functions. In mathematics higher-order functions are also known as operators or functionals. The derivative in calculus is a common example, since it maps a function to another function.
The functional argument to these is known as a procedural parameter:
In computing, a procedural parameter is a parameter of a procedure that is itself a procedure.
This concept is an extremely powerful and versatile programming tool, because it allows programmers to modify certain steps of a library procedure in arbitrarily complicated ways, without having to understand or modify the code of that procedure.
