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Comment by sevenproxies | Hacker News Reader

In PHP

   array_sum([1, 2, 3, 4, 5]);

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The GP's point is that Ruby doesn't require a built-in method that specifically sums an array to still get a clean, terse operation. You can use #inject to apply an arbitrary operation to (operator, last result, current element) and arrive at a result.

For example, given a list of numbers, you can generate a bitwise OR mask easily:

    [1,2,3,4,5].inject(:|)

What this does is iterate over the list and apply ($last_result | $current_element) and return the result, which is passed on as $last_result to the next iteration. $last_result is 0 by default. This is equivalent to (as of PHP 5.4):

    array_reduce([1,2,3,4,5], function($v, $e) { return $v | $e; }, 0);

Or prior to PHP 5.3:

    function or_mask($v, $e) { return $v | $e; }
    array_reduce(array(1,2,3,4,5), "or_mask", 0);

It's doable in both languages, but Ruby's functional language lineage and its object-oriented nature results in an exceptionally clean approach.

Prior to PHP 5.3 this could work as well:

    array_reduce(array(1,2,3,4,5), create_function('$v, $e', 'return $v | $e;'), 0);

I'm really glad that 5.3 got proper anonymous functions. Specifying function bodies as strings to be eval'd makes me twitch.

In case this was a serious entry... which I honestly can't tell.

The problem with array_sum is not that it's too long but instead that it's combined two rather specific bits of functionality into a fixed (if common) form, while the Ruby code separates the ideas of "folding" and "addition" allowing for many orthogonal creations.

To pick another favorite, in Haskell you could write

    Foldable.foldl1 (+)

which sums any foldable thing—anything which has elements which can be combined together one-by-one. So it'll apply to Trees or Sets or Dictionaries (well, Maps) just as easily while also allowing things like (+) to be replaced by other binary operators.

PHP does have array_reduce, however operators like '+' can't be used as callables so you'd have to make your own wrapper functions.

Edit: Screw it, haven't self promoted on a while. In Pharen (https://github.com/scriptor/pharen), which compiles to PHP, you could do:

    (reduce (+) 0 [1 2 3 4 5])

That's really cool, but the syntax is strange to me. How do you distinguish between operator sectioning (+) and a side-effecting function call (f)?

It works (superficially) similarly to how Haskell does things and treats (+) as a partial function call by recognizing that + isn't getting at least 2 arguments. You could also do stuff like:

    ((+ 1) 2) ; => 3

    (map (* 2) [1 2 3]) ; => [2 4 6]

If a function f takes any arguments and Pharen knows this, it'll convert (f) into a partial function call as well. Unfortunately I haven't figured out a way to check if a function has any side effects. Otherwise in this case at least operators are treated like functions.

So it's tracking the arity of functions and turning anything which has been applied to fewer than ARITY arguments to a partial application?

I suppose that comes at the cost of (+ 1 2 3 4)?

Why would it?

My hypothesis was that the compiler was tracking the arities of each function in order to open up opportunities for partial application. That means there's some ambiguity when you write something like (+ 1) which, in Scheme is a complete application equal to 1 but may also be interpreted as a partial application equal to (lambda (x) (+ 1 x)).

There are many ways to mitigate that ambiguity statically and dynamically, but I feel there's always going to be a tradeoff between favoring partial application, favoring variadic functions, and the complexity/sophistication of your static checking or runtime environment.

I just made a stab at where the solution might lie in that design space. Turns out I was wrong.

Surprisingly, not really.

    {-# OPTIONS -fglasgow-exts #-}
    
    class BuildList a r  | r-> a where
      build' :: [a] -> a -> r
    
    instance BuildList a [a] where
      build' l x = reverse$ x:l
    
    instance BuildList a r => BuildList a (a->r) where
      build' l x y = build'(x:l) y
    
    varargs x = build' [] x
    
    main = print $ ( sum $ varargs 1 2 3 4 5 6 100)


    $ ./Test
    121

http://okmij.org/ftp/Haskell/vararg-fn.lhs

I love that example, but it does depend upon some pretty arcane features in Haskell's type inference engine.

It does track the arity of functions, but remember that this happens at compile time. Regular addition in Pharen is still just regular addition in PHP:

    (def a (+ 1 2 3 4)) ; => $a = (1 + 2 + 3 + 4);

Of course, there are situations where partial application won't always happen because the compiler is unable to detect it:

    (fn add (x y)
      (+ x y))

    (map #add [1 2 3])

Pharen won't be able to realize that `add` here is being partially applied inside `map`. It's not smart enough for that yet. If you try to run this you'll end up with 'Missing argument 2' and 'Undefined variable: y' all over the place.

However, I'm not putting a whole lot of emphasis on partials. They're there as a convenience, but they're not really a core part of the language.

But yes, back to the original point, all the tracking is done at compile time so that for addition with two or more arguments the resulting PHP will look like regular addition in PHP.

Gotcha, very interesting!

Well how about this then.

    foreach(array(1,2,3,4,5) as $x) $i=$i+$x; echo $i;

The array_reduce answers get closer to what I was looking for. This might be a "one line" answer, but it requires two special forms, assignment, sequencing operators (;), and two fresh variables.

This works, is fast enough and readable. Perfectly fine solution to me.

1. Separate folding and addition.

2. ?

3. Profit.

Orthogonality and composability dramatically increase abstraction, code reuse, testability, and likelihood of writing correct code, both for library maintainers and library users.

Step 2 is actually extremely well known.

array_sum_of_1_2_3_4_5();

(Added in PHP 6)

Made me laugh :)

the funny thing is, with proper abuse of __call you can almost get that to work: https://gist.github.com/kennethrapp/328b6bd1bda8a8f94092

That's not funny. Horrifying, maybe, but not funny.

Edit: Strike 'maybe'. I looked at it again and it's definitely horrifying. I saved it anyway, though; Halloween's only a few weeks off, and I think I shall print it out and stick it on the wall of my cube next to the PHP hammer and the "periodic" table of Perl 6 operators.

kudos. almost as cool as the cpp template hack to compute area based on part of the source code 'ascii art' size. (which I can't find anymore. googlefu--)

    irb(main):001:0> x = [1,2,3,4,5]
    => [1, 2, 3, 4, 5]
    irb(main):002:0> x.inject &:*
    => 120

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