One last element of the public API, before I get to html, is another macro, in-html-style. This macro controls whether FOO generates XHTML or regular HTML by setting the *xhtml* variable. The reason this needs to be a macro is because you’ll want to wrap the code that sets *xhtml* in an **EVAL-WHEN** so you can set it in a file and have it affect uses of the html macro later in that same file.

If the outer html expands into an **IF** expression with two versions of the code, one for when *pretty* is true and one for when it’s false, it’s silly for nested html forms to expand into two versions too. In fact, it’ll lead to an exponential explosion of code since the nested html is already going to be expanded twice—once in the *pretty*-is-true branch and once in the *pretty*-is-false branch. If each expansion generates two versions, then you’ll have four total versions. And if the nested html form contained another nested html form, you’d end up with eight versions of that code. If the compiler is smart, it’ll eventually realize that most of that generated code is dead and will eliminate it, but even figuring that out can take quite a bit of time, slowing down compilation of any function that uses nested calls to html.

With that function, you can then define html like this: