I am taking a class at Wright State University and we are working on
problem 3.27 from the EOPL 2nd Edition. This problem has us modifying
the closure procedure to only bind the free variables in our environment
instead of storing all the variables. I am trying to write some code
that will create a list of all the variables in my expression. This will
then be passed to a difference procedure (that has been provided) to
determine which variables are free. Unfortunately, every time I run my
procedure, I receive the error: "procedure application: expected
procedure, given: x (no arguments)" What I should be returning is the
list '(x) as "x" is the only variable in my simple expression. The
string I am running is as follows:
(define str2 "let x = 15 y = 27
in let f = proc () x
in let x = 8
in (f)")
I then run the command:
(run str2)
After it works through its beginning work, it finally hits the proc-exp
where it calls closure. My closure function calls me free-vars procedure
but it apparently does not return a list like I think it should. I have
tried numerous ways and always get this same error. Any ideas on where I
am going wrong would be greatly appreciated. I'm sure there are errors
inside my logic, but if anybody can help me figure out why I am not
returning a list out of (free-vars) that would be great! My code looks like:
(define free-vars
(lambda (body)
(list (free-vars-list body))
)
)
(define free-vars-list
(lambda (body)
(
(if (list? body) (append (free-vars-list (car body))
(free-vars-list (cdr body)))
(cases expression body
;; When we have a var-exp, return the variable
(var-exp (id) id)
(primapp-exp (prim rands)
(append(free-vars-list (car rands)) (free-vars-list (cdr rands))))
(if-exp (test-exp true-exp false-exp)
(append (free-vars-list test-exp)
(free-vars-list true-exp) (free-vars-list false-exp)))
(begin-exp (exp1 exps) (append
(free-vars-list exp1) (free-vars-list exps)))
(let-exp (ids rands body) (append
(free-vars-list body)))
(proc-exp (ids body) (append (free-vars-list
body)))
(app-exp (rator rands) (append
(free-vars-list rator) (free-vars-list rands)))
(else '())
)
)
)
)
)
(define closure
(lambda (ids body env)
(let ((todd (free-vars body)))
(let ((freevars (difference todd ids)))
(let ((saved-env
(extend-env
freevars
(map (lambda (v) (apply-env env v)) freevars)
(empty-env))
))
(lambda (args)
(eval-expression body (extend-env ids args env)))
)
)
))
)
My call stack looks like:
procedure application: expected procedure, given: x (no arguments)
C:\Users\Todd\Documents\CS784\Assign2\3-5-mod.scm: 255:4
(if (list? body) (append (free-vars-list (car body))
(free-vars-list (cdr body)))
(cases expression body
;; When we have a var-exp, return the variable
(var-exp (id) id)
(primapp-exp (prim rands)
(append(free-vars-list (car rands)) (free-vars-list (cdr rands))))
(if-exp (test-exp true-exp false-exp)
(append (free-vars-list test-exp)
(free-vars-list true-exp) (free-vars-list false-exp)))
(begin-exp (exp1 exps) (append
(free-vars-list exp1) (free-vars-list exps)))
(let-exp (ids rands body) (append
(free-vars-list body)))
(proc-exp (ids body) (append (free-vars-list
body)))
(app-exp (rator rands) (append
(free-vars-list rator) (free-vars-list rands)))
(else '())
)
)
)
)
)
(define closure
(lambda (ids body env)
C:\Users\Todd\Documents\CS784\Assign2\3-5-mod.scm: 247:4
(list (free-vars-list body))
)
)
(define free-vars-list
(lambda (body)
C:\Users\Todd\Documents\CS784\Assign2\3-5-mod.scm: 276:4
(let ((freevars (difference todd ids)))
(let ((saved-env
(extend-env
freevars
(map (lambda (v) (apply-env env v)) freevars)
(empty-env))
))
(lambda (args)
(eval-expression body (extend-env ids args env)))
)
)
))
)
(run str2)
C:\Users\Todd\Documents\CS784\Assign2\3-5-mod.scm: 138:4
(map (lambda (x) (eval-rand x env)) rands)))
C:\Users\Todd\Documents\CS784\Assign2\3-5-mod.scm: 120:8
(let ((args (eval-rands rands env)))
(eval-expression body (extend-env ids args env))))
Thanks for all your help!
Todd
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