intro.lisp

;;; General form

;;; CL has two fundamental pieces of syntax: ATOM and S-EXPRESSION.
;;; Typically, grouped S-expressions are called `forms`.

10            ; an atom; it evaluates to itself
:thing        ; another atom; evaluating to the symbol :thing
t             ; another atom, denoting true
(+ 1 2 3 4)   ; an s-expression
'(4 :foo t)   ; another s-expression

;;; Function application are written as (f x y z ...) where f is a function and
;;; x, y, z, ... are the arguments.

(+ 1 2)                ; => 3

;;; If you want to create literal data, use QUOTE to prevent it from being
;;; evaluated

(quote (+ 1 2))        ; => (+ 1 2)
(quote a)              ; => A

;;; The shorthand for QUOTE is '

'(+ 1 2)               ; => (+ 1 2)
'a                     ; => A

;;; Variables aren't used the same way as other languages. Lisp is more about data flow than setting up a bunch of state.
;;; Use 'let to set some local variables

(let ((x 5)
      (y 10))
    (+ x y))    ; => 15

;;; There is no return statement in lisp (there are no statements at all - everything is an expression
;;; S-expressions evaluate to the value of their last form

(progn
  (+ 1 2)
  (+ 2 3)
  10)   ; => 10

(defun return-x (x)
  x)
(return-x 10)    ; => 10