Unbound variable in Common Lisp

Question

I'm new to Lisp and I was reading about an text-generator example in ANSI Common Lisp, Chapter 8. I follow the example and defined a function "see" in the scope of a LET variable "prec",

(let ((prec '|.|))
  (defun see (symb)
    (let ((pair (assoc symb (gethash prev *words*))))
      (if (null pair)
        (push (cons symb 1) (gethash prev *words*))
      (incf (cdr pair))))
    (setf prev symb)))

and saved it into a lisp file.

Then when I returned to REPL and tried to invoke see after loading the compiled version of the file, an error occurred:

The variable PREV is unbound. [Condition of type UNBOUND-VARIABLE]

How do I invoke see properly? And what's a lexical closure for? I'm all confused.

Thanks for helping.

jwmc jwmc · Accepted Answer · 2012-09-13T10:56:28

Looks like you've typed prec instead of prev in the enclosing let form.

Lexical closures are functions that 'close over' a part of the lexical environment (hence the name). There are many good introductions to closures in lisp that I will not attempt to repeat here but, essentially, let is the most common way to manipulate the lexical environment; in this case, you want to add the binding for prev, which will then be available to code within the body of the form. Your function see will 'close over' this binding, and so each call to see has access to it, even though when you make these calls, you will no longer be in the lexical environment established by the let form. You could say the function definition takes the binding with it, in a sense.

As you appear to have mis-typed the name of the prev, your function is trying to refer to a binding that has not been established at that point in the code.

Unbound variable in Common Lisp

1 Answers