Extensii pentru limbajul LET | EOPL3 Let language extensions - Stack operations

Constructori:
* stack -> list
* push : expVal x list -> list

Observatori:
* top : list -> expVal
* pop : list -> list
* size : list -> num-val


> lang.scm_________________________________________________
;; the grammar
        (expression    ("stack") stack-exp)
        (expression ("push" expression expression) push-exp)
        (expression ("top" expression) top-exp)
        (expression ("pop" expression) pop-exp)
        (expression ("size" "(" expression ")") size-exp)


> interp.scm_______________________________________________ 
 ;; value-of
        (stack-exp ()
            '())
        (push-exp (exp1 exp2)
            (cons (value-of exp1 env) (value-of exp2 env)))
        (top-exp (exp1)
            (car (value-of exp1 env))   
        )
        (pop-exp (exp1)
            (cdr (value-of exp1 env))
        )
        (size-exp (exp1)
            (num-val (length (value-of exp1 env)))
        )


> tests.scm__________________________________________________

        (stack-1 "top stack" error)
        (stack-2 "top push 3 push 4 stack" 3)
        (stack-3 "pop stack" error)
        (stack-4 "pop push 1 push 2 push zero?(0) stack" (2 #t))
        (stack-5 "size(stack)" 0)
        (stack-6 "size(push 100 stack)" 1)
        (stack-7 "let x=1 in let y=2 in top pop push x push -(x,y) push y stack" -1)

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