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Why Does “=” Mean Assignment?

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Re: Why Does “=” Mean Assignment?

#301

Post by rocqua »

jcranberry wrote: post #232
vlasev wrote: post #173

Earlier quoted context omitted.

You have something similar in mathematics. Recurrence relations. Since we are dealing with a sort of time difference inside a computer, something like "x = x + 1" can be interpreted as "the value of x at the next time unit is the value of x at the previous time unit plus one". That is "x[n+1] = x[n] + 1" and this is a recurrence relation. My guess is that early programmers were deeply aware of this time difference, s…

What you have there is a sequence. Recursively defined or not, taking the indices out of a sequence takes away the only thing that makes it a sequence (the mapping from the natural numbers) and would be a horrible abuse of notation. I think language designers were certainly consciously aware that they were designing something well defined, and chose to use this kind of syntax because its simpler, rather than an impli…

There is a really simple mapping between recurrent sequences and functions. Simply, given a function $f: A -> A$ that is, a function that outputs from the set it gets input from.

We then have the sequence $x_n+1 = f(x_n)$. Often, when dealing with incremental algorithms, the notation x' = f(x) is used. Here x' (pronounced x prime) stands for "the next value of x". It's a nice balance between the correctness of using indices and the conciseness of leaving them out.

Going to a higher level, the sequence x' = f(x) is essentially trying to find a fixed point of the function f. To look at this in an actual for or while loop, you need to consider the stopping condition of the loop as part of the function.

Re: Why Does “=” Mean Assignment?

#302

Post by rocqua »

Izkata wrote: post #231
rocqua wrote: post #199

Earlier quoted context omitted.

The way these recurrence relations are taught at university is to distinguish the new value from the old value by using a ' pronounced prime. Here, you would write x' = x + 1 to give the recurrence relation x[n+1] = x[n] + 1. Or, more generally x' = f(x) for x[n+1] = f(x[n]). The reason for this is because in mathematics x = x + 1 is absurd (ignoring modulo arithmetic).

In highschool, we used numeric subscripts for that. Ticks(apostrophes) were kept for derivatives.

If only there were enough notation that every notation could be unique.

Re: Why Does “=” Mean Assignment?

#303

Post by lmm »

PrimHelios wrote: post #147

>A common FP critique of imperative programming goes like this: “How can a = a + 1? That’s like saying 1 = 2. Mutable assignment makes no sense.” This is a notation mismatch: “equals” should mean “equality”, when it really means “assign”. I sort of disagree with this. Many functional languages pull heavily from lambda calculus and other forms of mathematics. In math, "a = a + 1" isn't the same as "1 = 2". The issue i…

> rebinding a bound variable is not the same as "1 = 2".

Therefore using "=" to mean "bind variable" is a notation mismatch, which is the whole point of what you quoted, no?

Re: Why Does “=” Mean Assignment?

#304

Post by bogomipz »

rdtsc wrote: post #13

Another interesting way to think about it is as "match". That is try to match the stuff on the right with the stuff on the left. Take Erlang for example: 1> X = 1. 1 2> X = 2. ** exception error: no match of right hand side value 2 Notice variables are immutable (not just values themselves). Once X becomes 1, it can only match with 1 after that. You might think this is silly or annoying, why not just allow reassignme…

>"In other languages we might say we have assignment and destructuring ..."

I'm not familiar with this term "destructuring." Can you elaborate on the concept? Might you have an example of a destructure operation and a language where its's used?

Re: Why Does “=” Mean Assignment?

#305

Post by lmm »

mavhc wrote: post #23

Is it really worth allowing assignment in places so weird that you also need ==? Wouldn't it be easier to use = for everything and get rid of those weird edge cases?

Using = for everything still has the problem that "x is always and forever this value" looks exactly the same as "y is this value for now but will be a different value in the future" or "give z, which previously had a different value, this new value". These are three different things and should have different syntaxes; = is appropriate for the first (and using it for that is not incompatible with using it for equality comparison) but not for the second or third.

Re: Why Does “=” Mean Assignment?

#306

Post by bogomipz »

njitbew wrote: post #106
rcharles wrote: post #48

Earlier quoted context omitted.

Some(all?) compilers transform your code into something like this internally. Its called static single assignment form. https://en.wikipedia.org/wiki/Static_single_assignment_form

Though it's true that compilers usually turn assignments into SSA, that's not really the same as what parent was referring to. Static single assignment has the same semantics as normal C-style assignments. The parent was referring to assignments that have (Erlang's) pattern match semantics. Also, there's a simple counterexample. You can have static single assignment that has dynamic multiple assignment (e.g. within a…

>"Though it's true that compilers usually turn assignments into SSA"

What is SSA here?

Re: Why Does “=” Mean Assignment?

#307

Post by mazesc »

bogomipz wrote: post #304
rdtsc wrote: post #13

Another interesting way to think about it is as "match". That is try to match the stuff on the right with the stuff on the left. Take Erlang for example: 1> X = 1. 1 2> X = 2. ** exception error: no match of right hand side value 2 Notice variables are immutable (not just values themselves). Once X becomes 1, it can only match with 1 after that. You might think this is silly or annoying, why not just allow reassignme…

>"In other languages we might say we have assignment and destructuring ..." I'm not familiar with this term "destructuring." Can you elaborate on the concept? Might you have an example of a destructure operation and a language where its's used?

I could imagine by destructuring he means e.g. unapply in Scala: https://docs.scala-lang.org/tour/extractor-objects.html

Re: Why Does “=” Mean Assignment?

#308

Post by lmm »

dkrich wrote: post #108

I always liked DHH's take on these sorts of arguments (paraphrasing): who the hell cares? Once you know the purpose of the '=' how often do you make mistakes reading or writing code? Whereas Java is all about protecting developers from themselves, Ruby (for example) let's you get away without variable type declaration because at the end of the day, how often do you not know whether a particular variable is a string o…

> Once you know the purpose of the '=' how often do you make mistakes reading or writing code?

A nontrivial proportion of people who start trying to learn to program never get past that point, so it's worth taking them into account.

> at the end of the day, how often do you not know whether a particular variable is a string or an integer?

Strawman. Types are not about the difference between a string and an integer, they're about the difference between a user id and an order id, or a non-empty list and a possibly-empty list, or...

Re: Why Does “=” Mean Assignment?

#309

Post by bogomipz »

kbp wrote: post #154
jghn wrote: post #10

Earlier quoted context omitted.

There’s also <-, most commonly seen in R but draws it’s heritage from the APL keyboard

In the original Smalltalk implementation, the character corresponding to _ in ASCII was a left facing arrow, which Smalltalk used for assignment at the time.

I'm not following. The corresponding ASCII code for _ is 96 no?

http://www.theasciicode.com.ar/extended-ascii-code/underline...

Re: Why Does “=” Mean Assignment?

#310

Post by wiz21c »

80386 wrote: post #261
cwzwarich wrote: post #28

Earlier quoted context omitted.

The use of = for assignment goes back to Heinz Rutishauser's Suplerplan from 1951, and it was adopted by Fortran.

Rutishauser might have gotten it from Konrad Zuse's Plankalkül, which used the right double arrow ⇒, which looks a little like =. Plankalkül had the order and the assignment reversed relative to C, so to increment a variable Z1, you'd write: | Z + 1 ⇒ Z V | 1 1 S | 1·n 1·n 1·n (The first line is the 'main line'; Z stands for Zwischenwert, i.e. "intermediate value". The second line is the 'value line', which contains…

The discussion here show how much computer languages are human's work, with all humans' little approximations, taste, creativity, etc. Great read :-)
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