MoMolog » Prototype

Prototype Hash was killed

aljoscha — Sat, 09 Feb 2008 14:43:53 +0000

In what they call a “backwards compatibility change” the prototype core developers have introduced some changes to the prototype Hash class.
A little history:
From version 1.5 the Hash class started behaving a bit differently because now the Enumerable methods where copied into the Hash prototype instead of into the instances, as before. That helped minimize the memory footprint but introduced incompatibilities and removed the possibility to patch Enumerables methods because after patching Enumerable one would have to manually copy its methods into all “inheriting” classes again.
To my understanding that is in conflict with the dynamic inheritance in prototype-based languages like javascript.
See this thread for more info.

Now with 1.6. things get worse. The Hash object is not an JS object with added functionality anymore, its a wrapper around one.
The following code will work in 1.5.0.2 but not in 1.6:
test = $H({a: 1, b:2}); alert(test.a); // -> 1 in version 1.5
Instead, in prototype 1.6 you have to write test.get('a') to get to the property “a” of the underlying object. In case you need the whole object, there is always test.toObject() to the rescue. Bye bye, consise and short notation.

I feel the developers forgot what made prototype so wildly successfull: Its beautiful concept of enhancing the built in objects, not wrapping them.

I consider leaving the prototype world alltogether and will switch to mootools.

Unlike prototype, mootools returns a hash on filtering a hash.

Testcase I

aljoscha — Mon, 16 Oct 2006 09:42:53 +0000

These tests check that, with the prototype fix in place

$H({fruit:"apple", vegetable:"cucumber"}).reject(
  function(el) { return (el[1] == 'cucumber');}
) == $H({fruit:"apple"})

and

$H({fruit:"apple", vegetable:"cucumber"}).findAll(
  function(el) { return (el[1] == 'apple'); }
) == $H({fruit:"apple"})

Fixing the Prototype Enumeration Mixin

aljoscha — Sat, 14 Oct 2006 11:42:56 +0000

The following article describes ticket #3592 in the RoR Trac and explains a proposed solution.

Prototypes Enumerable mixin class does not properly respect the mixees internal format. For example the reject and findAll methods, operating on Hashes, return Arrays instead of Hashes.

var a = $H({a:1, b:2, c:1, d:3});
document.writeln(a.inspect().escapeHTML());

=>#

var b = a.reject(function(val){ return (val[1]==1) });
document.writeln(b.inspect().escapeHTML());

=> [['b', 2], ['d', 3]]   // !!! should be hash

var c = b.findAll(function(val){ return (val[0]=='d') });
document.writeln(c.inspect().escapeHTML());

=> [['b', 2], ['d', 3]]   // !!! should be hash

In Ruby (from which the inspiration for the Enumerable Mixin stems), the reject method does return a hash, not an array, when operating on an hash:

irb(main):001:0> a={:a => 1, :b => 2, :c => 1, :d => 3}  

=> {:b=>2, :c=>1, :a=>1, :d=>3}  

irb(main):002:0> b=a.reject{|k,v| v==1}

=> {:b=>2, :d=>3}

The patch requires all classes that want to mixin Enumerable to define two more methods similar to _each:
_new, which returns an empty object of that class and
_add, that adds an element. Enumerable mixin thus needs to make no more assumptions about the internal structure of its mixee class.

After the fix the methods return correctly:

=> #