31. Practical: An HTML Generation Library, the Compiler

Now you’re ready to look at how the FOO compiler works. The main difference between a compiler and an interpreter is that an interpreter processes a program and directly generates some behavior—generating HTML in the case of a FOO interpreter—but a compiler processes the same program and generates code in some other language that will exhibit the same behavior. In FOO, the compiler is a Common Lisp macro that translates FOO into Common Lisp so it can be embedded in a Common Lisp program. Compilers, in general, have the advantage over interpreters that, because compilation happens in advance, they can spend a bit of time optimizing the code they generate to make it more efficient. The FOO compiler does that, merging literal text as much as possible in order to emit the same HTML with a smaller number of writes than the interpreter uses. When the compiler is a Common Lisp macro, you also have the advantage that it’s easy for the language understood by the compiler to contain embedded Common Lisp—the compiler just has to recognize it and embed it in the right place in the generated code. The FOO compiler will take advantage of this capability.