[dang]

Thanks! Super interesting.

Of course the meaning of (not (consp thing)) for atom is classical, but don't you think it's a bit weird? It always struck me that way. In a language where lists were the only composite data structure (i.e. not being a cons means being a number or a string or a symbol), it made sense, but that meaning degrades once you have vectors and hashtables. Those are no more deserving of the name 'atom' than lists are. Indeed if one indulges in the oxymoron of treating atomicity as a spectrum, vectors feel more atomic to me than lists while hashtables feel less atomic.

[kbp]

Older Lisps didn't tend to have strings as a separate datatype, they just used symbols and there was a function EXPLODE (and similar) for turning a symbol into a list of one-character symbols (or fixnums), and a corresponding IMPLODE (and similar) for combining them again, and that's how string processing was done. Vectors and hashes predate "real" strings in Lisp as far as I'm aware.

[lispm]

The older Lisp (60 and before) talked about 'atomic symbols'. ATOM checked whether something is an 'atomic symbol'. Cons cells were not atomic symbols. They were pairs. But atomic symbols could be symbols, numbers, functions and variables. Arrays were not there in an explict form and the few data types of Lisp were made of atomic symbols. A number was an atomic symbol where the name starts with a digit and the symbol's assoc list stored the numeric value...

See: http://www-formal.stanford.edu/jmc/recursive/node4.html