Commit 5e4eb801 authored by POTTIER Francois's avatar POTTIER Francois

Add example of counting EAdd nodes.

parent 0c772441
......@@ -100,6 +100,26 @@ Every method is parameterized with an environment \oc|env|, which is carried
down into every recursive call and is otherwise unused.
% The type of this environment is therefore a priori undetermined.
\caption{Counting the number of addition nodes in an expression}
Naturally, traversing a data structure without actually computing anything is
not a very sensible thing to do. Things become interesting when at least one
visitor method is overridden so as to obtain a nontrivial behavior. Suppose,
for instance, that we wish to count the number of addition nodes in an
expression. This can be done as shown in \fref{fig:expr04}. We create an
object~\oc|v| which is both a counter (that is, it has a mutable
field~\oc|count|) and a visitor, and we override its method
\dataconvisitor{EAdd} so that the counter is incremented every time this
method is invoked. There remains to run the visitor, by invoking its
\tyconvisitor{expr} method, and to return the final value of the counter. The
environment, in this example, is unused; we let it have type \unit.
% ------------------------------------------------------------------------------
open Expr00
let count (e : expr) : int =
let v = object
val mutable count = 0
method count = count
inherit [_] iter as super
method! visit_EAdd env e0 e1 =
count <- count + 1;
super#visit_EAdd env e0 e1
end in
v#visit_expr () e;
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