Based on Dwayne Reeves' blog post introducing HSL, it seems that the main advantage is the fact that arrays are native values, not objects. This has two important consequences:

1. For users, the semantics are different when the values cross through arguments. Objects are passed as references, and mutations affect the original object. On the other hand, values are copied on write after passing through arguments, so without references (which are finally to be completely banned in Hack) the callee can't mutate the value of the caller, with the exception of the much stricter inout parameters.

   The article cites the invariance of the mutable containers (Vector, Set, etc.) and generally how shared mutable state couples functions closer together. The soundness issues as discussed in the article are somewhat moot because there were also immutable object containers (ImmVector, ImmSet, etc.), although since these interfaces were written in userland, variance boxed the function type signature into tight constraints. There are tangible differences from this: ImmMap<Tk, +Tv> is invariant in Tk solely because of the (function(Tk): Tv) getter. Meanwhile, dict<+Tk, +Tv> is covariant in both type parameters thanks to the inherent mutation protection from copy-on-write.

2. For the compiler, static values can be allocated quickly and persist over the lifetime of the server. Objects on the other hand have arbitrarily complicated construction routines in general, and the collection objects weren't going to be special-cased it seems.

I will also mention that for most use cases, there is minimal difference even in code style: e.g. the -> reference chains can be directly replaced with the |> pipe operator. There is also no longer a boundary between the privileged "standard functions" and custom user functions on collection types. Finally, the collection types were final of course, so their objective nature didn't offer any actual hierarchical or polymorphic advantages to the end user anyways.

Arrays in php are meant to be both dictionaries and vectors. That's very confusing. Also in arrays, there is key coercion to integer. Which is more confusing.

Hack dicts are meant to remove the ambiguity and fix the problem with the keys.

The recommendation from the HHVM/HackLang guys is to use C\contains() as you can read here but you need to install the HSL package (hhvm/hsl) using composer. This library contains loads of functions to deal with the new array-type structures: vec, dict, keyset. Those functions in the HSL Library are prefixed with C, Vec, Dict, Keyset and you'll find also Math, Str, etc., very useful for other needs.

After installing that package it becomes available but typically it is more handy to add use namespace to avoid the long prefix:

use namespace HH\Lib\C;

This is how you can do it:

$exists = C\contains($occupiedNumbers, $newItemNumber);