I like the look of this:
> function add(a,b){
> return a + b;
> }
>
> add.matches(1,2); // => true
> add.matches(1); // => false
> add.matches(1,2,3); => false
I've never had the need to use function.length, I probably will at some point
though.
If you did something like this, using the rest argument:
function add( a, b, ...others ) {
...
}
Then the following should occur:
add.matches(1,2) // => true
add.matches(1) //=> false
add.matches(1,3,4) //=> true
Tom
On 12 Jun 2012, at 22:57, Russell Leggett wrote:
> On Tue, Jun 12, 2012 at 1:06 PM, Brendan Eich <[email protected]> wrote:
> Russell Leggett wrote:
> It does bring up something else, though, that I've avoided mentioning so far,
> which is pattern matching. I haven't mentioned it because there is clearly a
> strawman <http://wiki.ecmascript.org/doku.php?id=strawman:pattern_matching>
> for it, and that never made it to harmony, but in light of this thread I
> wanted to reiterate how useful it would be. It would not really help
> function.length, but would address the real underlying problem that the arity
> dispatcher is trying to tackle. In a language without overloading, the
> ability to do pattern matching would be an excellent solution to a very
> common problem. We already have destructuring, it seems like such a small
> jump to pattern matching.
>
> You are preacher, dherman and I are choir. Maybe patterns will make ES7. We
> shall try again.
>
>
> Hard to tell from looking at the strawman why that never made it. If its a
> matter or feature bloat, I would rate that higher than some other things like
> default args or array comprehensions.
>
> See March 2011 TC39 meeting notes, second day.
>
> Quotes below. Note some fine specimens of TC39's future-proofing
> fetishization on parade. I will say no more, as I think Dave agrees patterns
> were not fully baked.
>
> The way to get refutable matching into ES7 is to work now to address all the
> valid worries, and say why the other worries are false.
>
> This thread gave me an interesting idea on how to possibly attack pattern
> matching in ES6 with no new syntax, and still leave room for more sugar
> later. It actually comes from thinking about the original issue with
> function.length and using it for arity-based dispatch. What if we just gave a
> better method than length? What if we had something like
> function.matches(args)? Where it would return true if all arguments were
> bound, and no parameters resulted in an undefined binding.
> function add(a,b){
> return a + b;
> }
>
> add.matches(1,2); // => true
> add.matches(1); // => false
> add.matches(1,2,3); => false
>
> This still suffers from the same problem as function.length, and when dealing
> with simple arity and no destructuring/rest params, would act exactly the
> same. However, Irakli's dispatcher utility does lay the groundwork for
> something more interesting.
>
> function makePoint(x,y){ return {x:x,y:y}; }
> function drawLine({x:x1,y:y1},{x:x2,y:y2}){...}
>
> let p1 = makePoint(1,2), p2 = makePoint(3,4);
> drawLine.matches(p1,p2); // => true
> drawLine.matches(1,2); // => false
> drawLine.matches({x:1}, {y:2}) // => false
> //only has to be a structural subset
> drawLine.matches({x:1,y:2,z:3}, {x:1, y:2}) // => true
>
> With that simple boolean function, Irakli's dispatcher utility could be
> rewritten to loop through the list and check for a match instead of by arity.
> It would work correctly for rest parameters, but even more interesting, would
> work with all of the destructuring patterns.
>
> let drawLine = dispatcher(
> (x1,y1,x2,y2) => ...,
> ({x:x1,y:y1},{x:x2,y:y2}) => ...
> );
>
> Or a more functional replacement for switch
>
> let result = match(value,
> ({x,y,z}) => "3d"
> ({x,y}) => "2d"
> (...anything) => "not a point"
> );
>
> Its not as nice as full pattern matching with literals and wildcards, but it
> could be pretty clean. If guards were added later, that would be an obvious
> fit. If it became a popular pattern, we could pave the cowpath with some
> sugar, and add the things that are missing now.
>
> One of the benefits of getting started this way is that it could be shimmed
> to just use length.
>
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