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What is the benefit of having the assignment operator return a value?

I'm developing a language which I intend to replace both Javascript and PHP. (I can't see any problem with this. It's not like either of these languages have a large install base.)

One of the things I wanted to change was to turn the assignment operator into an assignment command, removing the ability to make use of the returned value.

x=1; /* Assignment. */
if (x==1) {} /* Comparison. */
x==1; /* Error or warning, I've not decided yet. */
if (x=1) {} /* Error. */

I know that this would mean that those one-line functions that C people love so much would no longer work. I figured (with little evidence beyond my personal experience) that the vast majority of times this happened, it was really intended to be comparison operation.

Or is it? Are there any practical uses of the assignment operator's return value that could not be trivially rewritten? (For any language that has such a concept.)

Answer*

> Are there any practical uses of the assignment operator's return value that could not be trivially rewritten?

Generally speaking, no. The idea of having the value of an assignment expression be the value that was assigned means that we have an expression which may be used for both its *side effect* and its *value*, and that is considered by many to be confusing.

Common usages are typically to make expressions compact:

 x = y = z;

has the semantics in C# of "convert z to the type of y, assign the converted value to y, the converted value is the value of the expression, convert that to the type of x, assign to x". 

But we are already in the realm of impertative side effects in a statement context, so there's really very little compelling benefit to that over

 y = z;
 x = y;

Similarly with

 M(x = 123);

being a shorthand for

 x = 123;
 M(x);

Again, in the original code we are using an expression both for its side effects and its value, and we are making a statement that has two side effects instead of one. Both are smelly; try to have one side effect per statement, and use expressions for their values, not for their side effects.

> I'm developing a language which I intend to replace both Javascript and PHP.

If you really want to be bold and emphasize that assignment is a statement and not an equality, then my advice is: **make it clearly an assignment statement**.

 let x be 1;

There, done. Or

 x <-- 1;

or even better:

 1 --> x;

Or even better still

 1 → x;

There's absolutely no way that any of those are going to be confused with `x == 1`.

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1

Is the world ready for non-ASCII Unicode symbols in programming languages?

billpg
– billpg

2014年02月14日 11:25:13 +00:00
Commented Feb 14, 2014 at 11:25
As much as I would love what you suggest, one of my goals is that most "well written" JavaScript can be ported over with little or no modification.

billpg
– billpg

2014年02月14日 11:44:24 +00:00
Commented Feb 14, 2014 at 11:44
2

@billpg: Is the world ready? I don't know -- was the world ready for APL in 1964, decades before the invention of Unicode? Here's a program in APL that picks a random permutation of six numbers out of the first 40: x[⍋x←6?40] APL required its own special keyboard, but it was a pretty successful language.

Eric Lippert
– Eric Lippert

2014年02月14日 15:17:08 +00:00
Commented Feb 14, 2014 at 15:17
@billpg: Macintosh Programmer's Workshop used non-ASCII symbols for things like regex tags or redirection of stderr. On the other hand, MPW had the advantage that the Macintosh made it easy to type non-ASCII characters. I must confess some puzzlement as to why the US keyboard driver doesn't provide any decent means of typing any non-ASCII characters. Not only does Alt-number entry require looking up character codes--in many applications it doesn't even work.

supercat
– supercat

2014年02月14日 18:20:16 +00:00
Commented Feb 14, 2014 at 18:20
Hm, why would one prefer to assign "to the right" like a+b*c --> x? This looks strange to me.

Ruslan
– Ruslan

2015年08月21日 10:49:40 +00:00
Commented Aug 21, 2015 at 10:49

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