Two question about scheme syntax in EOPL3

[ziqiang bao]

My first question：

3.2.1 Specifying the Syntax

(scan&parse "-(55, -(x,11))")

#(struct:a-program

#(struct:diff-exp

#(struct:const-exp 55)

#(struct:diff-exp

#(struct:var-exp x)

#(struct:const-exp 11))))

Here what does '#' do? I do not know how to use it.

My 2ed question：

3.2.4 Specifying the Behavior of Expressions 

(value-of (diff-expexp1 exp2) ρ)

= (num-val

    (-(expval->num (value-ofexp1 ρ))

      (expval->num (value-ofexp2 ρ))))

Here what does '-' stand for? I have not seen such usage for '-'.

[Mitchell Wand]

3.2.1  The #(struct:a-program ...) is how Racket prints out structures.  Try the following in the interaction window:

(define-struct foo (field1))

(define a-foo (make-foo 3))

a-foo

3.2.2.  The - is subtraction, as in (- 3 2).

--Prof. Wand

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[ziqiang bao]

Thank you very much, Prof.Wand.

These days, I meet two other questions.

1.  In page 60 of EOPL3,  figure 3.2,  one can see

Expression ::= -(Expression , Expression)

                    diff-exp (exp1 exp2)

here, what does "-" mean?  it seems do not like substraction.

2. In the page 60, 

(scan&parse "-(55, -(x,11))")

 #(struct:a-program

     #(struct:diff-exp

        #(struct:const-exp 55)

            #(struct:diff-exp

              #(struct:var-exp x)

              #(struct:const-exp 11))))

Here, what does "scan&parse" mean ? In racket, I do not find its defintion.

ps: My english is not good.~ Thanks again. 

2013/12/11 Mitchell Wand <wa...@ccs.neu.edu>

[Mitchell Wand]

On Mon, Dec 16, 2013 at 9:37 AM, ziqiang bao <vsfl...@gmail.com> wrote:

Thank you very much, Prof.Wand.

These days, I meet two other questions.

1.  In page 60 of EOPL3,  figure 3.2,  one can see

Expression ::= -(Expression , Expression)

                    diff-exp (exp1 exp2)

here, what does "-" mean?  it seems do not like substraction.

Here the minus sign is the symbol in the program that is intended to denote subtraction.  For example, the string 

"-(30,5)"  is an expression that will evaluate to 25.

2. In the page 60, 

(scan&parse "-(55, -(x,11))")

 #(struct:a-program

     #(struct:diff-exp

        #(struct:const-exp 55)

            #(struct:diff-exp

              #(struct:var-exp x)

              #(struct:const-exp 11))))

Here, what does "scan&parse" mean ? In racket, I do not find its defintion.

scan&parse is the function that converts a string to its syntax tree.  See Appendix A, and look in the code repository https://github.com/mwand/eopl3 .  scan&parse is usually defined in the lang.scm of each language.

--Prof. Wand