Lisp - Convert symbol to string only if not already a string

Question

Is there a way to convert symbol to string only if it is not already a string in lisp?

It should work like this:

(only-if-convertion 'ABC) => "ABC"

(only-if-convertion "ABC") => "ABC"

Answer 1

Use the function STRING .

CL-USER > (string "FOO")
"FOO"

CL-USER > (string 'FOO)
"FOO"

Answer 2

CL-USER> (defun symbol-or-string-to-string (x)
       (typecase x
         (symbol (symbol-name x))
         (string x)
         (otherwise (error "Wrong type"))))
SYMBOL-OR-STRING-TO-STRING
CL-USER> (symbol-or-string-to-string "foo")
"foo"
CL-USER> (symbol-or-string-to-string 'foo)
"FOO"
CL-USER> (symbol-or-string-to-string #())
; Evaluation aborted.
CL-USER> 

But the idea of converting it repetitively sounds odd. Can you show why are you needing to do it?

Answer 3

You can use the format function to do the conversion. Granted it's slower than the other options listed, but it can work on other data types, controls upcase/downcase, etc. So for development, or non-inner-loop portions of the code, this could be useful for you:

CL-USER>
(format nil "~a" "str")
"str"
CL-USER>
(format nil "~a" 'str)
"STR"
CL-USER> 
(format nil "~(~a~)" 'str)
"str"
CL-USER>
(format nil "~(~a~)" "str")
"str"
CL-USER> 
~