thecrypticace / lazy

A lazy evaluation wrapper around iterators

Lazy Collections

What

Lazy is a Collection-like wrapper that can operate on iterators one item at a time (on-demand).

Some notes:

1. Lazy works well for one-shot data sources (e.g. Generators)
2. Lazy supports algorithms which run with minimal overhead. Functional algorithms are well suited to this.
3. Lazy collections can convert the underlying data source into an array using ->eager(). Useful for reiterative purposes.
4. Has convenience methods for simple lazy data generation (e.g. ranges)
5. Has higher-order collection proxy which will allow code like this: $collection->map->people->map->count()->sum()

Why?

Let's say you have this code:

collect($oneMillionNumbers)->filter(function ($n) {
    return $n % 2 === 0;
})->map(function ($n) {
    return $n / 4;
})->filter(function ($n) {
    return $n >= 100;
})
->first()

For a small array $items the work performed here is trivial. If you take a large array containing 1 million numbers starting at one the work performed is large:

• 1,000,000 iterations in filter
• 500,000 iterations in map
• 500,000 iterations in filter
• one iteration in first

Total: 2,000,001 iterations Total: 2,000,001 function calls

Each one of these iterations also incurs the overhead of a function call.

If you replace that call to collect with lazy:

lazy($oneMillionNumbers)->filter(function ($n) {
    return $n % 2 === 0;
})->map(function ($n) {
    return $n / 4;
})->filter(function ($n) {
    return $n >= 100;
})
->first()

These are the stats:

• 400 iterations in filter
• 200 iterations in map
• 200 iterations in filter
• one iteration in first

Since these iterations happen on demand it's not 801 iterations. It's 400 for the entire set. Lazy Collections perform the minimal amount of work needed to get the result.

There are 801 function calls. One for each of the "virtual" iterations in each of the operations. Still, this number is far less than the 2,000,001 calls from the loop above.

Total: 400 iterations Total: 801 function calls

Lazy is effectively performing the same operation as this:

foreach ($oneMillionNumbers as $n) {
    if ($n % 2 === 0) {
        $n = $n / 4;
        if ($n >= 100) {
            return $n;
        }
    }
}

tip: lazy_range(1, 1000000) will generate a Collection with a range from one to one million.