[plt-scheme] Very basic macro question

Hi!

I am trying to learn syntax-rules macros but have some very basic questions regarding manipulation of s-expressions. 

Below is some simple code to illustrate my questions:

#lang scheme

(define-syntax foo
  (syntax-rules ()
    ((_ (x y))
     #t)))


(define sexpr '(a b))

;; one
(foo sexpr)

;; two
(foo '(a b))

;; three
(foo (a b))

;; four
(eval `(foo ,sexpr))


So I understand why the first example fails. I am not sure why example two matches as I would only assume example three would. I understand why example four works but is there anyway I can do this without eval?

TIA.




 		 	   		  
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