scheme - Unexpected persistence of data

Question

This question already has answers here:

Answer 1

Are you doing something like this:

CL-USER> (defun foo ()
           (let ((value '(1)))     ; '(1) is literal data
             (incf (car value))))
FOO
CL-USER> (foo)
2
CL-USER> (foo)
3
CL-USER> (foo)
4
CL-USER> (foo)
5

Quoted data is literal data; there's only one copy of it, and the consequences of modifying it are undefined. The behavior above is common, but you can't depend on it. Some compilers will give a warning when you do this. E.g., in SBCL:

CL-USER> (defun foo ()
           (let ((value '(1)))
             (incf (car value))))
; in: DEFUN FOO
;     (INCF (CAR VALUE))
; --> LET* 
; ==>
;   (SB-KERNEL:%RPLACA #:TMP1 #:NEW0)
; 
; caught WARNING:
;   Destructive function SB-KERNEL:%RPLACA called on constant data.
;   See also:
;     The ANSI Standard, Special Operator QUOTE
;     The ANSI Standard, Section 3.2.2.3
; 
; compilation unit finished
;   caught 1 WARNING condition
FOO

The relevant text from the HyperSpec on quote is:

The consequences are undefined if literal objects (including quoted objects) are destructively modified.

Create modifiable lists with, e.g., (list 1), not '(1). This is a common pitfall until you've encountered it. There are some other questions on StackOverflow that mention this issue. A very specific one is

• Why does this function return a different value every time?

but there are a bunch, too:

• LISP: Why doesn't mapcan accept my list give as parameters? (an interesting case with a literal list inside a lambda function passed to mapcan)
• Unexpected List Duplication using Sort with Common Lisp
• LISP - Global variable keep their old value after reinitialization
• this answer to Sudoku table generator failure, lisp
• How does Lisp function remember state in this code?
• Do property lists in Common Lisp refer to some global state?
• Why does this function return a different value every time?
• Strange behavior invoking destructive Common LISP function receiving as argument a list created with quote
• Local variable keeps data from previous execution
• What is happening with this Common Lisp code?
• Unexpected persistence of data (this question)
• setf in a function does not work
• duplicating and modifying the head of a list of list, in Lisp

The same thing happens in Scheme, though the citations to the documentation are obviously different. For R⁵RS, the documentation is the following:

4.1.2 Literal expressions

… As noted in section 3.4, it is an error to alter a constant (i.e. the value of a literal expression) using a mutation procedure like set-car! or string-set!.

3.4 Storage model

… In many systems it is desirable for constants (i.e. the values of literal expressions) to reside in read-only-memory. To express this, it is convenient to imagine that every object that denotes locations is associated with a flag telling whether that object is mutable or immutable. In such systems literal constants and the strings returned by symbol->string are immutable objects, while all objects created by the other procedures listed in this report are mutable. It is an error to attempt to store a new value into a location that is denoted by an immutable object.

There are questions about this as well:

• How could I explain this unexpected behavior of my Scheme code?
• Difference between '(()) and (cons null null)