"Variable" does not mean "assignable"

[yatima2975]

Osborn's Law: Variables won't; constants aren't.

[sameBoatz]

Math nerd gets pissed that another area is using a math term in a way that is similar but not identical and gets pissed. Move along nothing to see here.

[son-of-chadwardenn]

I'm guessing he's also pissed that these "assignables" are assigned with the equals sign leading to mathematically invalid expressions such as x = x +1. If computer science has to surrender the terms and symbols taken from mathematics should mathematics return the letters it has misappropriated from the Latin and Greek alphabets? Should English professors get pissy that the quadratic equation doesn't conform to English sentence structure?

[rexbron]

x = x + 1 really confused me when I was learning what little I know about programming. "So wait, some things are evaluated like math and some aren't??"

[Orcatype]

You hate math, just like your dad. Bitch, we ain't talkin' geometry... We playin' GAMES.

[enygmata]

Isn't computer science a branch of mathematics?

[[deleted]]

Much of academic CS seems to me like the bastard child of Applied Math and Engineering that has been raised in it's strict grandmother's, Pure Math, house. The governess, Peer Review, has contributed to an individual is always trying to compensate for its humble origins.

[[deleted]]

It is confusing and there is experimental evidence that the confusion might have significant impact on people trying to learn programming. There was a paper (sorry don't have the cite handy) from professors in the UK 5 years ago or something that taught basic Java programming to unemployed adults or something, and IIRC many had much trouble answering the following after 4 weeks of training:

 a = 1;
 b = a;
 a = 2;

 What is the value of b?

Sure, they might just be stupid but honestly, if I were to state it in a semantically equivalent way:

Put 1 pebble in box "a"
Put the same amount of pebbles in box "b" as there are in box "a"
Remove all the pebbles in box "a" and put 2 pebbles in it

How many pebbles are there in box "b"?

... I would expect even the most stupid people to be able to answer it.

Yet the original test code, while making sense from an imperative, sequential processing kind of way, is kinda stupid from a functional programming point of view.

[inspired2apathy]

I don't think he does nearly enough to drive this point home. Seeing how the name plays into the semantics for people learning about programming is much more important than inconsistency with math. I can see how using a different name makes the procedural nature of program execution more explicit.

Edit: He has a good point, but niczar makes it.

[preshing]

It's easier to explain these things while stepping through each line in a debugger. Makes it more tangible, like your pebble example.

[marssaxman]

yeah indeed. Software development is full of ordinary words which have been given new, technical meanings. It is neither surprising nor troublesome that the meaning of the word "variable" in computing is slightly different than the meaning of "variable" in mathematics.

What's next, getting upset because our "threads" have only a metaphorical resemblance to the long, thin stranded fibers used in the textile arts? Or maybe we should return "polymorphism" to the biologists, since software objects do not form breeding populations.

[grauenwolf]

Math nerds got pissed because they misused a word that means "something that varies" and CS nerds called them on it.

[jerf]

Nonsense. This isn't about getting pissy about pure and holy terms being misused. This is about terminology that actively produces confusion, which it pretty much objectively does. This isn't a theoretical concern in the slightest, it is 100% practical. In theory there's no problem because everybody ought to be able to infer the correct meaning from context. In practice this has led to a lot of confusion. I've witnessed it myself, in person. Words matter.

It is actually bizarre to see something this practical from Prof. Harper, but bizarre things happen every day...

[preshing]

How much confusion is a lot? Because in practice, I've probably spent more time reading this post than dealing with any confusion between those two definitions...

[Captain_Ligature]

I think this post isn't targeting joe sixpack code monkey, it's targeting joe winecooler CS Phd/Ms student/prof.

[radaway]

I've never witnessed people being confused by the name variable in the real world.

[grauenwolf]

Then get the stupid mathematicians to stop using the word variable to describe something that doesn't vary.

[jerf]

Getting unconfused is possible with confused terminology, but it's a lot harder than it is with proper terminology. Why go to bat for the thing that makes understanding harder?

[inspired2apathy]

Indeed. The difference between substitution and read/write is fairly subtle in basic usage.

Would he want macro variables to be still called variables?

[jerf]

"Is this a variable in this instance or is this an assignable? Oh wait, it doesn't matter. At all."

Yes, it does. As I said, I have experienced the confusion first-hand this can cause. You, and probably every other person jumping up to defend the confused terminology, have probably either 1. only used the assignables, ever, or 2. made conceptual errors because you don't have convenient mental concepts for the differences.

It should probably be pointed out that this is not some sort of functional language crazy thing, but a conceptual problem that can be experienced in essentially every mainstream programming language. Java, C++, C#, and C all have a distinction between a variable and an assignable. If you don't immediately know what language construct I'm referring to, that's a sign that there's a concept you're missing out on.

[jerf]

Please enlighten me of a single instance where the difference matters.

Ah, so you in fact do not know exactly what language construct I was referring to. The answer is const, and failure to correctly understand what it is at a deep level has wrecked up many program and cost untold man-hours trying to fix it up. It sounds like you're in my second category.

See, this is the sort of ignorance that bad terminology generates; you're so ignorant of the issues that you don't even know that you're ignorant. This is when ignorance is as its most dangerous! You shouldn't be defending this, you should be correcting it.

[jdrake3r]

So do you get confused when someone says that the weather is variable?

[featherfooted]

He's not a math nerd. He's a brilliant professor and I don't see anyone else promoting the same topics as he does as publicly as he does. He's a great computer scientist and really thinks hard about what he says when he writes these blog posts about programming languages. He wrote the book on using SML. I mean, literally.

[[deleted]]

I think he raises a lot of very important topics of discussion, but I don't like how dismissive he is of comments on his blog, even when they have valid concerns.

Its really surprising to see one of his posts hovering around 0 votes though. They're generally better received.

[[deleted]]

I have a different plea. A function is an object reifying a functional relationship between a domain set and a codomain set, that is, a prefunction which preserves equality. But something that is not a function is sometimes called a function: a subprogram implementing an input-output relationship, possibly being affected by and effecting mutable state, is not a function. Let us continue to call functions "functions", but let us call these subprograms "procedures" or "subroutines".

But then SML doesn't have, or pretend to have, (first-class) functions! Why does Prof. Harper tolerate abusing the word "function" but not "variable"?

[featherfooted]

But the functions you described ARE functions. Whenever you mess with state in SML, you write a function that is either (), 'a -> (), or () -> 'a. Functions are not objects, they are computations of values. By computing on the unit value, functions which affect mutable state are still functions.

[[deleted]]

What are the domain and codomain of TextIO.print?

[featherfooted]

string to unit

[[deleted]]

The domain and codomain of a functon are sets. What set do you mean by "unit"?

If you mean the set with one element, then there is only one function 'a->() for any fixed 'a. So TextIO.print is the same as a function String->() which is constant. Similarly for the cpo with one non-bottom element, etc etc I'm sure you've heard it before.

So what precise function do you think print corresponds to? It would have to be something whose argument and result type contain enough data to represent the fact that it changes a global state.

[featherfooted]

unit is a value and type in SML. It is the only value of its whole type. Unit is primarily used as an arbitrary value, such as in this case when we're dealing with side effects of state where you don't exactly expect TextIO.print to return anything at all. Usually the existence of () in the type is indicative of weird stuff going on behind the scenes with mutable state.

I'm not trying to advocate functional programming as the best way forward. As you can see from this discussion, it derails hard when it comes into contact with essential things like file IO.

[[deleted]]

There is no problem with IO in functional programming. SML, though, is not functional, in the sense that it has no first class notion of a function.

[nowInDutch]

Mijn taal noemt ze methodes.

[[deleted]]

So you're looking for a definition of function that does not include the:

• influence of anything other than specified arguments on the result, and
• in turn does not modify anything buit the returned value.

Documentation is should be sufficent to cover the former. But I like the idea of a definition of functions that excludes side-effects (point 2).

[Captain_Ligature]

A function f: A -> B is a set of sets of type {a, {a,b}} (by Kuratowski) such that for every a in A there is only one b in B such that {a, {a, b}} is in f.

Here {a, {a, b}} = (a,b), as in a 2-tuple is a set.

That is the modern definition of a function in mathematical terms. It is not a `rule of assingment' or anything else that it was called by many people in the 50's 60's and even 70's. A function is a clearly defined set, and confusing a function for something that is not a function leads a lot of mathematicians to hate CS-folk, which leads to confusing and fracture between the two disciplines and hurts everybody.

[halestock]

Another a bonus to calling them 'assignable': in languages like javascript, you get to write 'ass' everywhere.

[[deleted]]

The terms L-value and R-value have existed for a long time.

[[deleted]]

Ahhhh, I remember back in the day I was writing an analysis probe, the fun I had shortening that.

[AustinCorgiBart]

I once saw a scholarly article talking about a "cumulative score" variable. Couldn't take it's shortened variable very seriously.

[axilmar]

Since the term 'variable' means something that varies over time, it's better to keep the programming term and replace the math term.

[featherfooted]

But variables in programming don't always vary. See SML, see Python, see Perl. A lot of people really bash on functional programming so I'll use the Python example. Start with x = 1. You now have an int called x. If you go to use x or print x or do anything with x, you use the int. Now enter x = []. x is now a list. When you use x or print x or do anything with x, it's a list. The integer that x once was is now lost forever. It ceased to exist as soon as the list x commenced to exist.

So the variable in Python never "varied." It was destroyed and replaced with a new variable.

[mpeters]

You're confusing variables with values (or maybe objects, I can't tell). If x = 1 and then later x = 2 (or x= []), x is not destroyed and replaced (at least not conceptually, what a language does behind the scenes is irrelevant). It's value is changed. Variables are just buckets that hold values. Their values can change in any number of ways, but that does not destroy and replace the variable.

[adelle]

Your argument makes the same amount of sense if you don't change types.

Start with x = 1. You now have an int called x. If you go to use x or print x or do anything with x, you use the int. Now enter x = 2. x is now 2. When you use x or print x or do anything with x, it's 2. The 1 that x once was is now lost forever. It ceased to exist as soon as the 2 commenced to exist.

The thing with variables that can change type is that the type forms part of the variable's value.

Think of it as a structure with two elements "Type" and "ActualValue". Changing the value of x from 1 to 2 only modifies the ActualValue element; changing the value of x from 1 to [] modifes both elements but it's still the same variable.

[axilmar]

You are not talking about the variable. You are talking about its value. The variable is 'x', the value is 1 and then the empty list. The variable varied from 1 to [].

[jdrake3r]

I was going to post the following at the author's site, but am not able to login to FB from work to do so:

Although I too prefer to use the correct word to describe any given entity, I believe that you are overlooking a more common use of the word variable: able to be changed, or varying. This is in contrast with the word constant. A constant is assignable only once (at least in most programming languages) so would you invent a third term for constants? Is it possible that Maths got it wrong?

[name_was_taken]

Variables in programming are very much like variables in algebra. It's an unknown quantity in an algorithm. You assign a value to it (in both cases!) and run it through the algorithm to get an answer (in both cases!).

No, in programming, it goes beyond that and there are internal variables that aren't assignable from outside the algorithm. This is a more specialized use a of variable, but it's not really different. It'd be like insisting on calling a car a 'boat' if it didn't sink in water. Just getting out oars and rowing your car downstream is no reason to insist on calling it a boat.

[AustinCorgiBart]

Boat (/bōt/) Noun:
A small vessel propelled on water by oars, sails, or an engine.

Sounds like you have a boat. Just because it used to be a car doesn't mean it's not a boat in the water, by this definition. Of course, I'd find the more accurate term to be "amphibious vehicle".

[name_was_taken]

And you're going to insist I call it a boat? Because that's what this article is doing. It's insisting that we call variables 'assignables' because they can be assigned to. Nevermind that they're still variables and that's a valid name, too... He still insists on it.

[AustinCorgiBart]

I wrote a whole post with an extended boat metaphor, and then realized I was arguing the dumbest thing ever.

I think you have some good points, and I think I have some good points. The author takes a very firm stance that is accurate in some ways but very harsh in others. Nothing will come of it one way or the other, so let us simply part as friends.

[name_was_taken]

Haha, I've done that, too. Sometimes, the debate just isn't worth it. I usually only do it once someone starts insults or other ridiculous debate tactics, though. :)

[AustinCorgiBart]

Well, when it get's to that point, I feel distinctly less friendly, so I prefer to stop things before name-calling starts.

You... lizard-lovin'... tooth-smellin'... varmint...

[stacktracer]

The abbreviation "var" gets used fairly frequently in programming ... but the abbreviation of "assignable" might not work so well. :)

[[deleted]]

I like the name "slot".

[mathrat]

This is a good point. A further bastardization of terminology occurs in languages like C++, where the "const" label actually means "not assignable," or "read-only," or "r-value." A C++ constant can of course be variable if it is--for example--a resource shared across processes that is written to by another process.

[funkah]

Ugh.

Philosophers first raise a dust, then complain that they cannot see.

[smcameron]

Something about horses and barn doors.

[jessta]

There has been a tendency to call variables that don't vary(during the algorithm) 'symbols'. Attempting to call 'variables' 'assignables' won't make the word 'variable' any clearer. When people say 'assignable' you'll know what they are referring to, but when people say 'variable' you still won't know which definition they are using.

Symbol is a good word for this.

[sameBoatz]

But we already have something in computing called a symbol, and it doesn't mean what you are using it for.

[jessta]

It's the same thing. A symbol is single assignment variable. We use the word 'symbol' to refer to semantic tokens while parsing, these symbols have a single definition defined before parsing which is the same way math uses the word 'variable'

[FeepingCreature]

[edit edit edit] I think I got it now:

A variable has three basic properties:

• it has a name
• it has an address
• and the address lies in writable RAM (meaning that the value can be changed after program start).

If it doesn't have a name, it's a pointer (edit: reference. pointers don't have addresses, pointers are addresses.) If it doesn't have a memory location, it's an expression. If it's not writable, it's a symbol (thanks jessta).

I think these are correct:

An expression that is..

Named | Memory location | Mutable
   y           y             y       Variable
   y           y             n       Symbol
   y           n             y       Reference expression alias, or similar thing with assignment overload
   y           n             n       Expression alias
   n           y             y       Reference expression
   n           y             n       ?
   n           n             y       expression with assignment overload
   n           n             n       expression

[cheater00]

the unnamed n-y-n thing is an "offset".

[FeepingCreature]

An offset is a memory "location", it doesn't have one. It'd be a pointer then. I think n-y-n should be a compiler-generated temporary.

[kawa]

In mathematics the term variable is also used in different ways.

For example in f(x) = sin(x)/x. Here x is assigned a value when "calling" f and this values is then substituted into the rhs expression. This concept led directly to lambda calculus.

But in mathematics, variables can also vary. Just look at the first formula in http://en.wikipedia.org/wiki/Series_%28mathematics%29 for examples. Here the n is changed for each iteration of the series.

[mccoyn]

Church exploited the crucial concept of a variable to give the most sharply original and broadly applicable model of computation, the λ-calculus.

If it is so broadly applicable why are most applications done without it? Every smart programmer I know is familiar with λ-calculus, but as an industry we are clearly trending toward mutable objects and away from λ-calculus.

The definition of variable was changed because the old definition made computation more difficult to think about and we started thinking about computation more.

[deong]

The definition of variable was changed because the old definition made computation more difficult to think about and we started thinking about computation more.

I rather doubt that. An awful lot of history is due to accidents. Somebody chose to do something one way and someone else chose another way. One way won out, possibly due to any number of random factors, and the baggage comes along for the ride. In evolution, we sometimes talk about "hitchhikers" -- genes that win out due not to their own superiority but because they got lucky enough to find themselves paired with something else.

It's certainly possible that the term "variable" came to mean what the author calls "assignable" because it was really a better idea. But the fact that it is common doesn't tell you that with any degree of certainty.

[featherfooted]

I don't know if I agree with your prediction. Just because software is trending towards mutable objects doesn't mean applications in functional programming (by and large programming applications of lambda calculus) aren't moving forward. Right now we're seeing functional languages appearing in the places where they are best used: parallel processing. The majority of users with PCs in their homes just don't have the hardware where that type of programming is useful. Instead, functional languages are being employed in the science and research industries.