[DmitrySoshnikov]

Yes, in the "Essentials of Interpretation" class (aka "Building an Interpreter from scratch" we focus exactly on runtime semantics, and evaluating the language. The S-expression allows greatly simplifying, focus on runtime specifics themselves, skipping parsing stage altogether.

In "Essentials of Parsing" class (aka "Parsing Algorithms") we shift exactly to the syntax, and understanding the parsing process from within -- this in general may have nothing to do with runtime -- for the same exact syntax you may have different interpreters or VMs (even with different semantics).

[chubot]

Yeah I think that makes sense, and that's how I learned... The s-expressions let you concentrate on the meaning without getting bogged down in details.

And then parsing has some nice algorithms, but it's full of details ... I actually find the parsing part harder in many respects. At least it's more code to write, and test.