In one of my programs I already use my module. This program creates a new test-file in the ’t’ folder from a template. From all files in that directory I want to automatically pick the next highest number. I don’t want to search for the number myself.

So here is how i read all files from disk and only pick files matching the pattern “%d-$title.t”. Then i want the maximum number.

This example shows how you can “connect” to other modules to use them. In this example its Path::Tiny.

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# get the maximum id from test-files so far
my $maximum_id =
    Seq
    ->new(    path('t')->children )
    ->map(    sub($x) { $x->basename })
    ->choose( sub($x) { $x =~ m/\A(\d+) .* \.t\z/xms ? $1 : undef } )
    ->max->or(0);

Here is how it works.

It begins with max.
max wants to know the highest numerical value. So he ask choose for the next value.
choose also don’t know abbout any value, he ask map to give him one.
map also don’t know about any value, so he ask wrap about one.
wrap finally knows about a value. He gives him the next one, and asks Do you want this?
map looks at it. He says, yes i take all. Then he asks choose is this the basename you wanted?
choose looks at it. He runs a regex on it to proof the correct format, and extract the number from the basename.
1. When it is the correct format, he gives max the requested file. (goto #8)
2. When it not the correct format, he throws it away, and asks map again for the next value. (goto #4)
max stores the first value as the maximum value. Then he ask again for the next value (goto #2). He continues as long choose returns an item itself. And choose continues as long he gets one from map and so on …
When choose does not return a value anymore. max returns the maximum value he stored so far. When choose does not give him a single value. He choose 0 as the default to return.

So max starts the evaluation. It bubbles back to wrap. So execution is basically written in reverse. In the case of max all items must be scanned.

There could be multiple different ways to optimze its performance. But, you probably always will start with replacing/optimizing wrap.

When you give wrap all items at once it internally creates an array and serves all request from it. This is the operation that blocks. But only once, and at the very beginning.

Theoretically there is only one way to fix it. You just give it a function that works like a sequence. You can pass it any function that returns another function.

This is how the internals of sequence works. But there are multiple different helpers (Constructors) to help you create such an construct.

Throughout the system, undef is used like an optional. When you return it, its considered the end of the iterator. Or in some functions returning undef means = skip this item. So undef values basically disappear as long you use Seq.

Here a funny part. Seq can return undef. It’s like talking with others. Inside Seq there is no undef. But it returns it, just like it keeps getting one from the outside itself.

Like max that can return an undef when the sequence not only contains a single value. Luckily max can be given a default value that in that case is returned instead of an undef.

So I am basically fixing “The multi-millard dollar-bug”. Everything as a Perl module to load.

Or at least, an attempt.

References

Sq on Github