Define struct with one field in scheme

Question

I am working on a homework assignment for a class. The problem statement says to use the data definition:

(define-struct diff-exp exprs)
(define-struct mult-exp exprs)
;; An Expr is one of
;; -- Number
;; -- (make-diff-exp (cons Expr LOExpr))
;; -- (make-mult-exp (cons Expr LOExpr))
;; Interpretation: a diff-exp represents a difference,
;; and a mult-exp represents a multiplication.
;; A List of Exprs (LOExpr) is one of
;; -- empty
;; -- (cons Expr LOExpr)

However, when I have just that in the source, Dr. Scheme (Intermediate Student Language) says:

define-struct: expected a sequence of field names after the structure type name in `define-struct', but found something else

Is there something I am missing here or did my teacher give me an invalid data definition?

Answer 1

Like Anon suggested in the comment above, define-struct takes a list of fields; if you need only one, then use a list of one element. Example code:

(define-struct diff-exp (exprs))

(let ((myexpr (make-diff-exp (list foo bar))))
  (diff-exp-exprs myexpr))

You can rifle through the many sundry features of define-struct in the PLT Scheme documentation.