### 20Sharing and Equality

#### 20.1Re-Examining Equality

Consider the following data definition and example values:
data BinTree:
| leaf
| node(v, l :: BinTree, r :: BinTree)
end

a-tree =
node(5,
node(4, leaf, leaf),
node(4, leaf, leaf))

b-tree =
block:
four-node = node(4, leaf, leaf)
node(5,
four-node,
four-node)
end
In particular, it might seem that the way we’ve written b-tree is morally equivalent to how we’ve written a-tree, but we’ve created a helpful binding to avoid code duplication.

Because both a-tree and b-tree are bound to trees with 5 at the root and a left and right child each containing 4, we can indeed reasonably consider these trees equivalent. Sure enough:
<equal-tests> ::=
 check: a-tree is b-tree a-tree.l is a-tree.l a-tree.l is a-tree.r b-tree.l is b-tree.r end

However, there is another sense in which these trees are not equivalent. concretely, a-tree constructs a distinct node for each child, while b-tree uses the same node for both children. Surely this difference should show up somehow, but we have not yet seen a way to write a program that will tell these apart.

By default, the is operator uses the same equality test as Pyret’s ==. There are, however, other equality tests in Pyret. In particular, the way we can tell apart these data is by using Pyret’s identical function, which implements reference equality. This checks not only whether two values are structurally equivalent but whether they are the result of the very same act of value construction. With this, we can now write additional tests:
check:
identical(a-tree, b-tree) is false
identical(a-tree.l, a-tree.l) is true
identical(a-tree.l, a-tree.r) is false
identical(b-tree.l, b-tree.r) is true
end

There is actually another way to write these tests in Pyret: the is operator can also be parameterized by a different equality predicate than the default ==. Thus, the above block can equivalently be written as:We can use is-not to check for expected failure of equality.
check:
a-tree is-not%(identical) b-tree
a-tree.l is%(identical) a-tree.l
a-tree.l is-not%(identical) a-tree.r
b-tree.l is%(identical) b-tree.r
end
We will use this style of equality testing from now on.

Observe how these are the same values that were compared earlier (<equal-tests>), but the results are now different: some values that were true earlier are now false. In particular,
check:
a-tree is b-tree
a-tree is-not%(identical) b-tree
a-tree.l is a-tree.r
a-tree.l is-not%(identical) a-tree.r
end
Later we will return both to what identical really means [What it Means to be Identical] and to the full range of Pyret’s equality operations [A Family of Equality Predicates].

#### 20.2The Cost of Evaluating References

From a complexity viewpoint, it’s important for us to understand how these references work. As we have hinted, four-node is computed only once, and each use of it refers to the same value: if, instead, it was evaluated each time we referred to four-node, there would be no real difference between a-tree and b-tree, and the above tests would not distinguish between them.

This is especially relevant when understanding the cost of function evaluation. We’ll construct two simple examples that illustrate this. We’ll begin with a contrived data structure:
L = range(0, 100)

Suppose we now define
L1 = link(1, L)
L2 = link(-1, L)
Constructing a list clearly takes time at least proportional to the length; therefore, we expect the time to compute L to be considerably more than that for a single link operation. Therefore, the question is how long it takes to compute L1 and L2 after L has been computed: constant time, or time proportional to the length of L?

The answer, for Pyret, and for most other contemporary languages (including Java, C#, OCaml, Racket, etc.), is that these additional computations take constant time. That is, the value bound to L is computed once and bound to L; subsequent expressions refer to this value (hence “reference”) rather than reconstructing it, as reference equality shows:
check:
L1.rest is%(identical) L
L2.rest is%(identical) L
L1.rest is%(identical) L2.rest
end

Similarly, we can define a function, pass L to it, and see whether the resulting argument is identical to the original:
fun check-for-no-copy(another-l):
identical(another-l, L)
end

check:
check-for-no-copy(L) is true
end
or, equivalently,
check:
L satisfies check-for-no-copy
end
Therefore, neither built-in operations (like .rest) nor user-defined ones (like check-for-no-copy) make copies of their arguments.Strictly speaking, of course, we cannot conclude that no copy was made. Pyret could have made a copy, discarded it, and still passed a reference to the original. Given how perverse this would be, we can assume—and take the language’s creators’ word for it—that this doesn’t actually happen. By creating extremely large lists, we can also use timing information to observe that the time of constructing the list grows proportional to the length of the list while the time of passing it as a parameter remains constant. The important thing to observe here is that, instead of simply relying on authority, we have used operations in the language itself to understand how the language behaves.

#### 20.3On the Internet, Nobody Knows You’re a DAG

Despite the name we’ve given it, b-tree is not actually a tree. In a tree, by definition, there are no shared nodes, whereas in b-tree the node named by four-node is shared by two parts of the tree. Despite this, traversing b-tree will still terminate, because there are no cyclic references in it: if you start from any node and visit its “children”, you cannot end up back at that node. There is a special name for a value with such a shape: directed acyclic graph (DAG).

Many important data structures are actually a DAG underneath. For instance, consider Web sites. It is common to think of a site as a tree of pages: the top-level refers to several sections, each of which refers to sub-sections, and so on. However, sometimes an entry needs to be cataloged under multiple sections. For instance, an academic department might organize pages by people, teaching, and research. In the first of these pages it lists the people who work there; in the second, the list of courses; and in the third, the list of research groups. In turn, the courses might have references to the people teaching them, and the research groups are populated by these same people. Since we want only one page per person (for both maintenance and search indexing purposes), all these personnel links refer back to the same page for people.

Let’s construct a simple form of this. First a datatype to represent a site’s content:
data Content:
| page(s :: String)
| section(title :: String, sub :: List<Content>)
end
Let’s now define a few people:
people-pages :: Content =
section("People",
[list: page("Church"),
page("Dijkstra"),
page("Haberman") ])
and a way to extract a particular person’s page:
fun get-person(n): index(people-pages.sub, n) end
Now we can define theory and systems sections:
theory-pages :: Content =
section("Theory",
[list: get-person(0), get-person(1)])
systems-pages :: Content =
section("Systems",
[list: get-person(1), get-person(2)])
which are integrated into a site as a whole:
site :: Content =
section("Computing Sciences",
[list: theory-pages, systems-pages])
Now we can confirm that each of these luminaries needs to keep only one Web page current; for instance:
check:
theory = index(site.sub, 0)
systems = index(site.sub, 1)
theory-dijkstra = index(theory.sub, 1)
systems-dijkstra = index(systems.sub, 0)
theory-dijkstra is systems-dijkstra
theory-dijkstra is%(identical) systems-dijkstra
end

#### 20.4From Acyclicity to Cycles

Here’s another example that arises on the Web. Suppose we are constructing a table of output in a Web page. We would like the rows of the table to alternate between white and grey. If the table had only two rows, we could map the row-generating function over a list of these two colors. Since we do not know how many rows it will have, however, we would like the list to be as long as necessary. In effect, we would like to write:
web-colors = link("white", link("grey", web-colors))
to obtain an indefinitely long list, so that we could eventually write
map2(color-table-row, table-row-content, web-colors)
which applies a color-table-row function to two arguments: the current row from table-row-content, and the current color from web-colors, proceeding in lockstep over the two lists.

Unfortunately, there are many things wrong with this attempted definition.

Do Now!

Do you see what they are?

Here are some problems in turn:
• This will not even parse. The identifier web-colors is not bound on the right of the =.

• Earlier, we saw a solution to such a problem: use rec [Streams From Functions]. What happens if we write
rec web-colors = link("white", link("grey", web-colors))

Exercise

Why does rec work in the definition of ones but not above?

• Assuming we have fixed the above problem, one of two things will happen. It depends on what the initial value of web-colors is. Because it is a dummy value, we do not get an arbitrarily long list of colors but rather a list of two colors followed by the dummy value. Indeed, this program will not even type-check.

Suppose, however, that web-colors were written instead as a function definition to delay its creation:
fun web-colors(): link("white", link("grey", web-colors())) end
On its own this just defines a function. If, however, we use it—web-colors()it goes into an infinite loop constructing links.

• Even if all that were to work, map2 would either (a) not terminate because its second argument is indefinitely long, or (b) report an error because the two arguments aren’t the same length.

All these problems are symptoms of a bigger issue. What we are trying to do here is not merely create a shared datum (like a DAG) but something much richer: a cyclic datum, i.e., one that refers back to itself:

When you get to cycles, even defining the datum becomes difficult because its definition depends on itself so it (seemingly) needs to already be defined in the process of being defined. We will return to cyclic data later: Recursion and Cycles from Mutation.