Lisp does not use a tokenizer or a parser. It uses a reader, which reads a textual representation of data into data objects. A reader is similar to a tokenizer

Lisp does not use a tokenizer or a parser. It uses a reader, which reads a textual representation of data into data objects. A reader is similar to a tokenizer, but the reader just creates an internal representation of some data from an external representation. Basically what a primitive JSON reader would do. A parser would know the programming language syntax and create an internal representation of the program. The Lisp reader does not know anything about the programming language syntax. It just knows about numbers, strings, lists, arrays. But the reader does not know about conditionals, variable declarations, iteration statements, assignment statements, ... var answer = 6 * 7; (defparameter answer (* 6 7)) If we quote it, it is just Lisp data: CL-USER 8 > '(defparameter answer (* 6 7)) (DEFPARAMETER ANSWER (* 6 7)) As you can see the evaluator just returns the data, though the symbols are upcased by default. The data is not enriched as a tokenizer or parser might do. Ideally the data structure prints as it is read. There are no type or syntax annotations necessary/used. Thus it is neither a tokenizer output nor a program parse tree. In your first link the tokenizer breaks up the input in strings and categorizes it. The 42 is read into a string '42' and it is annotated as being a numeric. The Lisp reader does not do that. It reads the two characters 42 and returns an integer number object with the value 42. We can describe this data, it is a list with three elements: CL-USER 9 > (describe *) ; describe the last value (DEFPARAMETER ANSWER (* 6 7)) is a LIST 0 DEFPARAMETER ; a symbol 1 ANSWER ; another symbol 2 (* 6 7) ; a list On this level of s-expressions we know nothing about Lisp as a programming language. It is only known that DEFPARAMETER is a symbol, but not known that it is a language construct and which. It's just a symbol in the first position of a list. Since this list is also a valid Lisp program (because we wrote it as such), we can compute its value and some side-effects. This process is called evaluation. CL-USER 10 > (eval **) ; eval the second last value ANSWER ; it just returns the name of the defined variable We can also input it directly, since the READ-EVAL-PRINT-LOOP already evaluates: CL-USER 11 > (defparameter answer (* 6 7)) ANSWER As you can see, the input to the evaluator is not a tokenizer output and not a parse tree. It's just an expression tree, where the leaves are data objects. It's not (token 3 :type number) or (literal-data :type number :value 3) or something similar. It is just the 3 as a data object. The evaluator itself could be an interpreter or a compiler. The interpreter runs walks the expression tree and computes the values from the expressions, which it sees in data format. A compiler-based evaluator would take the whole expression tree, compile it and then run it.