guess is a procedure, and not a number. The error message is telling you that it expects a number and not a procedure as the argument to sub1.
If you evaluate guess in the REPL, you can see the problem: > guess #<procedure:guess> What you want is the following: > (guess) 50 The problem was that you were passing in the procedure guess to sub1, when what you wanted to pass in was the result of applying the procedure. When you have 0 argument functions, this distinction can sometimes be confusing, since the difference between guess and (guess) can seem purely stylistic. But the former is a procedure, while the latter is the resulting of evaluating that procedure with no arguments. In Racket, the use of parens around an identifier means that the identifier is a procedure that is being applied with 0 arguments. I hope that helps. -Spencer Gordon On Sunday, October 20, 2013 at 11:42 AM, Joe Python wrote: > #lang racket > (define lower 1) > (define upper 100) > > (define (guess) > (quotient (+ lower upper) 2)) > > (define (smaller) > (set! upper (max lower (guess (sub1 guess)))) > (guess))
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