[keithb-]

I think you're asking the right questions.

I would assume that your class targeted Lisp because SICP uses it. While that is a great resource, it is light on details when it comes to the end game: runtime. If you are developing a language today and you aren't considering runtime, then you are just writing macros.

Lexing, and parsing are not trivial tasks, but powerful tooling already exists for these. Compiling means knowing how to translate your language into something that executes, and I would say "must execute well". I'm in favor of hobbies (fun) and exploration (growth), but if you are "perfecting" steps 1&2 and haven't yet considered step 3, then maybe it is time to support another language community rather than develop something on your own.

[inimino]

I'm not sure what you mean by "runtime" here and I'm having a little trouble seeing what you're getting at.

[greenshackle2]

Runtime means program execution, basically.

With steps 1 and 2, you're just translating between different program representations. You could hand-write your programs as syntax trees directly if you really wanted to, and you wouldn't even need those steps. It would be a bit more work, but not insanely so; Lisp syntax looks a lot like a syntax tree already, and some people are perfectly happy writing Lisp.

Step 3 is where you go from 'code' in one form or another (usually AST), to an actual process that's executing on the machine, either through interpretation or compilation. It's where most of the magic happens. You have to turn your AST into a series of machine instructions that will i) do what the code says ii) in an efficient manner.

Sometimes achieving i) at all is hard; it's not immediately clear how to implement certain high level operations using machine instructions. Sometimes there are obvious ways to do it, but there are also much less obvious ways that are much faster (more efficient). Finding those ways is 'optimization'.

Unlike steps 1 and 2, which can be done by hand relatively easily, trying to execute your program by hand, for any non-trivial program, would be an absolute nightmare.

(The exception is very low level languages, like assembly, where your language maps almost 1-to-1 with machine instructions. In that case step 3 is trivial. But if you're designing a language today it's probably not an assembly language.)