# Pattern Variables

To take this to the next level, you might want to ask "Who are the sons of Thomas and Norma?". In this case, you are passing `Thomas` and `Norma` into Pyke, and you'd like Pyke to pass something back out to you as part of the answer (in addition to the thumbs up).

Pattern variables serve as output parameters. They start with a `$`:

    family.son_of($son, Thomas, Norma)

You can use whatever name you like after the `$`. Pyke will answer with a thumbs up binding `$son` to `Bruce`. If you don't like that answer, you can reject that answer and ask Pyke for another answer ("nope, try again!"). Each time Pyke finds another son for Thomas and Norma, it answers with another thumbs up and `$son` bound to a different value.

If you reject the last son of `Thomas` and `Norma` (or if `Thomas` and `Norma` have no sons in the first place), Pyke will answer with a thumbs down.

We say that Pyke binds `Bruce` to the pattern variable `$son` when it comes back with its first thumbs up. When we tell Pyke "nope, try again!", Pyke must first unbind `$son` before it can go on and bind it to the next value. The "nope" part does the unbinding, and the "try again" part does the binding again to a new value.

So at any point in time, a pattern variable is either bound to a value or unbound. If we follow a particular pattern variable through time, we might see that it is alternately bound and unbound many times as Pyke tries to find a suitable answer to your question. Specifically, when Pyke comes back with the final thumbs down, `$son` is unbound.

## Anonymous Pattern Variables

Suppose we want to know who Norma's sons are? In this case we don't care about the father. We use anonymous variables as "don't care" placeholders.

An anonymous variable is any pattern variable who's name starts with an underscore (`_`). The rest of the name doesn't matter and just serves as documentation (and so `$_` is all that's strictly needed).

So "Who are Norma's sons?" looks like:

    family.son_of($son, $_father, Norma)

We're giving Norma as input to Pyke, and asking for the `$son` as output and telling Pyke that we don't care about the `$_father`.

Anonymous variables are never bound to values. (So you could say that they are always unbound).

## Pattern Variable Identity

Now this is very important, so pay attention! The same pattern variable name means the same pattern variable. Thus, if you say `$son` twice, it's the same pattern variable. And, a pattern variable can only be bound to one value (at a time), so you mean the same data value must appear in both places.

Note

This does not apply to anonymous pattern variables. Since they are never bound to a value, each use of the same anonymous variable can match different data values.

So if you wanted to see all of the sons with the same name as their fathers, you would ask:

    family.son_of($father, $father, $_mother)

Note

The Pyke family_relations example never uses the same name for both father and son because that would cause confusion about which of them was the father to both of their sons and daughters.

In these cases, it modifies the name of the son to make it unique.

Thus, this question would always fail within the family_relations example...

And so here is the complete explanation of how pattern variables are matched to a data value.

First, the pattern variable is checked to see if it is already bound to a value.

If it is bound to a value, then this bound value has to match the data for the match to succeed.

If it is unbound, then the pattern variable is bound to the data value as a by-product of doing the match, and the match always succeeds.

And so pattern variables only "match any value" when they are unbound. And in matching that value, they become bound to it as a by-product of doing the match. Once bound to a value, a pattern variable will only match that value (much like a literal pattern).

Copyright © 2007-2009 Bruce Frederiksen

Page last modified Mon, Oct 27 2008.