Scheme - Lambda as a Bad Function Object

Question

I'm working on a metacircular evaluator in Scheme for homework, and I need to allow the user to install special forms by adding them to a table. The idea is that, when the user enters something like (square 5), the evaluator will look up forms named square. If it's found, it will return a lambda statement, something like (lambda (x) (* x x)).

I'm having a problem when the code returns the lambda statement. I get the following error message:

Error: Bad function object:(lambda (x) (* x x))

The really strange part is that I can pass parameters into functions retrieved from my table, it's just that I have to previously define the procedure body as a lambda statement, not as a list that starts with lambda

For reference, here's the code that doesn't work. exp will be something like (install-special-form 'square (lambda (x) (* x x))), so in this case, name evaluates to square and func evaluates to (lambda (x) (* x x)):

(define (install-eval exp)
  (define name (cadadr exp))
  (define func (caddr exp))
    (if (special-form-lookup (list name func))
        #f
       (begin
         (append! special-forms-table (list name func))
         name)))

And here's some code that does work:

(define (install exp-list)
  (append! special-forms-table exp-list))
(install (list 'square (lambda (x) (* x x))))

I'm guessing my problem is that, when using the code that doesn't work, lambda is evaluated as a quote, rather than an actual lambda? How can I get my user input to store an actual lambda statement that can be retrieved and used?

Óscar López Óscar López · Accepted Answer · 2017-03-13T21:11:17

You're probably storing the lambda as a list of symbols, not as an actual procedure. That's why this won't work:

(define f '(lambda (x) (* x x)))
(f 10)
=> Error: Bad function object: (lambda (x) (* x x))

Try evaluating it first:

((eval f) 10)
=> 100

Scheme - Lambda as a Bad Function Object

2 Answers