Language Levels: Example

Consider a beginning student who is accustomed to infix syntax for function application: f (x). This student may accidentally slip into this notation while programming in Scheme, for example, in a cond clause:

(define (length l)
  (cond
   [empty? (l) 0]
   [else (+ 1 (length (rest l)))]))

This definition is syntactically legal in conventional Scheme, and a traditional implementation would signal an error only when the function is invoked. Because the beginner doesn't know about first-class functions, the error message is incomprehensible.

DrScheme restricts the syntax of Beginning Student Scheme to a simple first-order language. By restricting the language, DrScheme can detect and flag beginner mistakes such as the one above. In this example it also highlights the offending clause, suggesting a natural revision to the beginner.