Fwd: Fix Left-Associative Compound Conditional Statements!!

Joseph Groseclose jgrosecl49 at gmail.com
Thu Sep 24 14:10:07 UTC 2015


## Left-Associative Compound Conditional Statements
I would like the ability to compare many values in compound. As I do not
see any real reason to avoid this, here is my proposal:

Following [this document](
http://yehudakatz.com/2011/09/13/a-proposal-for-es-next-proposals/) (thanks
@wykatz)

### What problem are we solving:
Often, it makes it easier and more convenient to compare values in a range
as opposed to comparing one to another and that value to even another. In
order to avoid this, JavaScript could add the ability to "chain" many
comparators, failing after the first inequality that is falsy.

### Provide Non-Trivial Example Code Showing the Problem:
For the remainder of this issue/request, I will be using (almost
exclusively) the "<" comparator, but any normal operators should work with
this proposal, including but not limited to:
* ==
* ===
* >
* <
* >=
* <=
* yada yada yada (not an operator)

Two examples:
```javascript
let foo = 1,
    bar = 2,
    baz = 3;
if (foo < bar && bar < baz) {
    // ...do something!
}
```
While vaguely trivial, this snippet demonstrates the easiest way to
accurately compare these three values in a basic conditional. We have to
compare either the first two values and the second to values serially or in
parallel.

Likewise (and slightly less trivial):
```javascript
let arr = [ 'foo', 'bar', 'baz' ];
for (var i = 0; i < arr.length; ++i) {
    // do something...
    if (i > 1) {
         break;
    }
}
```
The above illustrates an example where we want to iterate in a range. There
are several solutions to this (including a compound condition in the `for`
statement, cloning and slicing, or reversing the array and setting `i`
greater than zero), but this is even less convenient than the original
example!

### Show How the Example Code is Improved With the Proposal:
Now, we only have two left-associative conditions to evaluate as opposed to
three
```javascript
let foo = 1,
    bar = 2,
    baz = 3;
if (foo < bar < baz) {
    // ...do something!
}
```

We have saved ourselves the trouble of including a good deal of logic in
our simple `for` loop as well:
```javascript
let arr = [ 'foo', 'bar', 'baz' ];
for (let i = 0, y = 2; i < arr.length < y; ++i) {
    // do something...
}
```

What are Alternate ES3 (or ES5) Solutions to the Same Problem?:
I've included some solutions to how this can be avoided, it is worth noting
in this section that this is not so much about this situation being
unavoidable so much as it is a ~~matter of convenience~~.

Actually, scratch that, its not even JUST a matter of convenience. A simple
evaluation in your developer console will tell you that this currently
evaluates inconveniently (I am avoiding the term "incorrectly" here because
it is not theoretically incorrect if we consider these evaluations left
associative):
```javascript
1 < 2 < 3 // true where...
true < 3 // true...because
+true // 1
1 < 3 // true
```
Bear in mind that this is not a catch-all definition of this behavior:
```javascript
false === false == false // false (wut?)
```

### If the Proposal is For New Syntax, What if Anything Does it Desugar To?
The desugaring is shown above, but the concatenation of conditionals can be
converted into their non-concatenated equivalents:
```javascript
(1 < 2 < 3) === (1< 2 & 2 < 3) // Not a typo
```

### If New Syntax, Can it Be Compiled to ES3 (or ES5)?
Absolutely, in the same way it has been outlined above

### Does the Proposal Change any Existing Semantics?
I'm not entirely sure this is semantic (because the comparators of any two
values should evaluate in the same way they always have), but it is worth
noting in this section that any two values should work with compound
conditionals, not just numbers. For example,
```javascript
[] == [] === []
```
should fail comparison.

I realize this effectively makes `===` a silver bullet in compound
conditional statements, but I can't see any other way this would work.

Please let me know if there is any more context or detail I can provide, or
whether there are considerations I have not made.
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