ch7/

Using the ideas from `ByteCounter`, implement counters for words and for lines.

The `sort.Interface` type can be adapted to other uses.

Add additional handlers so that clients can create, read, update, and delete database entries.

Change the handler for `/list` to print its output as an HTML table, not text.

Add a `String` method to `Expr` to pretty-print the syntax tree.

Write a program that reads a single expression from the standard input, prompts the user to provide values for any variables, then evaluates the expression in the resulting environment.

Write a web-based calculator program.

Extend `xmlselect` so that elements may be selected not just by name, but by their attributes too, in the manner of CSS, so that, for instance, an element like `<div id="page" class="wide">` could be selected by matching `id` or `class` as well as its name.

Write a function `CountingWriter` with the signature below that, given an `io.Writer`, returns a new `Writer` that wraps the original, and a pointer to an `int64` variable that any moment contains the number of bytes written to the new `Writer`.

Write a `String` method for the `*tree` type in `gopl.io/ch4/treesort` (§4.4) that reveals the sequence of values in the tree.

The `strings.NewReader` function returns a value that satisfies the `io.Reader` interface (and others) by reading from its argument, a string.

The `LimitReader` function in the `io` package accepts an `io.Reader r` and a number of bytes `n`, and returns another `Reader` that reads from `r` but reports an end-of-file condition after `n` bytes.

Add support for Kelvin temperatures to `tempFlag`.

Many GUIs provide a table widget with a stateful multi-tier sort: the primary sort key is the most recently clicked column head, the secondary sort key is the second most clicked column head, and so on.

Use the `html/template` package (§4.6) to replace `printTracks` with a function that displays the tracks as an HTML table.