ch5ex8

= Exercise 5.8
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Modify `forEachNode`
so that the `pre` and `post` functions return a boolean result
indicating whether to continue the traversal.
Use it to write a function `ElementByID`
with the following signature
that finds the first HTML element with the specified `id` attribute.
The function should stop the traversal as soon as a match is found.

[source,go]
----
func ElementByID(doc *html.Node, id string) *html.Node
----

== Test

The test will have a few HTML elements
with the `id` attribute in several combinations
to validate the implementation is robust.

== Implementation

We had to modify `forEachNode` to return a boolean
to indicate whether to continue traversing the tree
as a result of a `pre` or `post` function
returning `false`.

`elementWithID` uses package level variables
`idToFind` and `nodeFound`
which are set and returned by `ElementByID`.
It returns `false` to tell `forEachNode` to stop traversing the tree.

== Why not use anonymous functions

Because at this point in the book
anonymous functions have not been introduced yet.

Most solutions I have seen
do not correctly stop traversing sibling nodes.
One of my tests covers this scenario.