[lispm]

> Note that deciphering type tag info in a Lisp value can be called "parsing".

No. You are still operating on the level of s-expressions, a data format. The type-tag of LET is SYMBOL.

Here we have some Lisp code in the form of an s-expression:

   (let ((let 'let))
     ((lambda (let)
        (let ((let let))
          let))
      let))

All above LET have the same type tag, but in terms of syntax they have a different purpose in the form above: we have special operators, variable declarations, variable usage, data objects. I can't just car/cdr down the lists and call TYPE-OF. This always returns SYMBOL for LET.

On the level of a syntax tree we would want to know what it is in terms of syntactic categories: variable, operator, data object, function, macro, etc. Lisp source code has no representation for that and we need to determine that by parsing the code.

[kazinator]

The type tag is to be "parsed out" in some implementation specific way. For instance, we first look at the LSB of the Lisp value. If we find a 0 there, then it's a pointer; we dereference the pointer into some heap record, where we retrieve a second-level type tag. Except that, perhaps, if the whole thing is 0 bits, then it's nil; we must not chase that as a pointer. And we find a 1 in in the LSB, then perhaps we look at other surrounding bits for the type. All of this examination of bits with irregularities and cases looks like parsing.