[sparkie]

Lisps are expression based languages, but not pure. It's easy to mistake it as "like most other languages", but it's not quite the same - everything is an expression and returns a result. There are no "statements".

They appear procedural because of syntax sugar - ie, the body of a function is basically implicitly wrapped in (progn ...), (begin ...), ($sequence ...), etc - which are all equivalent expression forms which evaluate their sub-expressions in order and return the result of the last one.

   (progn a b c)      ;; CommonLisp
   (begin a b c)      ;; Scheme
   ($sequence a b c)  ;; Kernel

   ;; evaluate a, then b, then c, 
   ;; ignore the results of evaluating a and b 
   ;; return the result of evaluating c.

So when you see:

   (define (foo)
       (expr1)
       (expr2)
       (expr3))

If we desugar, it would be

    (define foo (lambda () (begin (expr1) (expr3) (expr3))))

We get behavior that looks just like other procedural languages (without a "return" keyword) - but everything is still an expression.

A similarity is the comma operator in C. Imagine you didn't write statements but the body of your C functions was entirely chains of comma operators.

CommonLisp has a couple of other useful related forms - prog1 and prog2. They still evaluate their sub-expressions in order, but prog1 returns the result of evaluating the first expression, and prog2 returns the result of the second expression.

    (define (foo) (prog1 (expr1) (expr2) (expr3))
    (foo)
 
    ;; evaluates expr1, then expr2, then expr3
    ;; returns the result of evaluating expr1
    ;; ignores the results of evaluating expr2 and expr3

[ux266478]

I'm very familiar with Lisp, you don't have to explain it to me. I think you are mistaken as to the meaning of what is meant by "procedural language" here. It's simply the mode of computation where a program is directly conceived as a hierarchical sequence of steps. I think you got caught up in the idea of a grand disjunction of "expressions" and "statements", with a distinguishing feature involving return values, and so on. But no, that's not particularly relevant here (nor is it universally true, or applicable)

To simplify it, you can consider cons, car, cdr the beating heart of Lisp. These special forms directly encode the execution semantics as the traversal of a head over cells of a tape. Lisp belongs to the same family as the Turing machine, and that's a very big family. SML also belongs to this family. The overwhelming majority of programming languages belong to this family.