Understanding "let" expression in LISP

Question

I am extremely new to lisp, had previous experience with functional programming (Haskell, SML). Why is this code returning 14, and not 10 (i.e. 1 + 2y + 3 + 1)?

(defvar x 1)

(defun g (z)
  (+ x z))

(defun f (y)
  (+ (g 1)
     (let ((x (+ y 3)))
        (g (+ y x)))))

(f 2)

Answer 1

Because you used (DEFVAR X 1), which declares X to be a global special variable. This then causes every other later binding of X to use dynamic binding: here in (LET ((X ....

Style & Convention in Lisp

Convention in Lisp: use *X* instead of X for special variables.

(defvar *x* 1)

Your code then is:

(defvar *x* 1)   ; global special variable *X*

(defun g (z)
  (+ *x* z))     ; use special variable *X*

(defun f (y)
  (+ (g 1)
     (let ((x (+ y 3)))    ; lexical binding of X
        (g (+ y x)))))     ; use lexical binding of X

run:

? (f 2)
10