getting 'undefined variable' warning, eventhough its defined

Question

When executing the below code in sbcl repl, i'm getting the 'undefined variable' warning for variables test-cases, n. I've gone through similar questions on the forum and the warning appears to occur if a variable is setf/setq'ed without defining first. But i have defined those variables using defparameter but still getting the warning.

One thing i noticed is, if i don't reference the variables in the format statement, warning is not occuring. I've tried using defvar as well. but it is still throwing the warning. can someone help me understand why the warning is thrown when used in a statment even if the variable is defined?

    (defun main ()
           (defvar test-cases 10)
           (defvar l 12)
           (defvar n 13)
           (format t "~a ~a ~a" test-cases l n))

; in: DEFUN MAIN
;     (FORMAT T "~a ~a ~a" TEST-CASES L N)
; 
; caught WARNING:
;   undefined variable: N
; 
; caught WARNING:
;   undefined variable: TEST-CASES
; 
; compilation unit finished
;   Undefined variables:
;     N TEST-CASES
;   caught 2 WARNING conditions
WARNING: redefining COMMON-LISP-USER::MAIN in DEFUN

DEFVAR is for global variables defined on the top-level. Use LET to define local variables. See Practical Common Lisp chapter 6. Variables — jkiiski

coredump coredump · Accepted Answer · 2019-04-19T07:36:04

The following explains why you have an error, but note that it is rarely necessary to define global variables from inside functions, most of the time you will use a local binding with LET.

Here is a reduced test case:

(defun foo () (defvar bar nil) bar)

At the time foo is being compiled, bar is undefined (assuming a fresh CL environment). This is why the compiler complains. If, however, you call foo, the variable will be declared (globally) and then its value will be returned.

The call to DEFVAR has an effect on the global environment only when it is executed, or at compile-time when it is a top-level form:

However, the compile-time side effects described below only take place when they appear as top level forms.

...

Side-effects: If a defvar or defparameter form appears as a top level form, the compiler must recognize that the name has been proclaimed special. However, it must neither evaluate the initial-value form nor assign the dynamic variable named name at compile time.

Thus, compiling a call to defvar inside a function body does not declare the symbol as a special variable.

getting 'undefined variable' warning, eventhough its defined

1 Answers