That's the problem though: I think the thing that makes functions functions it that they operate on values alone (whatever “value” means in the context of the language — but it almost certainly doesn't include code or AST or anything like that), and don't care about, or even get to see, the particular expression that produced a given value. The thing that makes statements (or macros, or various other compile-time stuff) not-functions is that they don't just have access to values, but to the code as well, which lets them do stuff like print the expression that produced a value, which a function could never do (in most languages; I'm sure there are some out there, maybe a LISP or something, that attach the AST to expressions as metadata).