CLHS: Function SUBLIS, NSUBLIS

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Function SUBLIS, NSUBLIS

Syntax:

sublis alist tree `&key` key test test-not => new-tree

nsublis alist tree `&key` key test test-not => new-tree

Arguments and Values:

alist---an association list.

tree---a tree.

test---a designator for a function of two arguments that returns a generalized boolean.

test-not---a designator for a function of two arguments that returns a generalized boolean.

key---a designator for a function of one argument, or nil.

new-tree---a tree.

Description:

sublis makes substitutions for objects in tree (a structure of conses). nsublis is like sublis but destructively modifies the relevant parts of the tree.

sublis looks at all subtrees and leaves of tree; if a subtree or leaf appears as a key in alist (that is, the key and the subtree or leaf satisfy the test), it is replaced by the object with which that key is associated. This operation is non-destructive. In effect, sublis can perform several subst operations simultaneously.

If sublis succeeds, a new copy of tree is returned in which each occurrence of such a subtree or leaf is replaced by the object with which it is associated. If no changes are made, the original tree is returned. The original tree is left unchanged, but the result tree may share cells with it.

nsublis is permitted to modify tree but otherwise returns the same values as sublis.

Examples:

     (sublis '((x . 100) (z . zprime))
             '(plus x (minus g z x p) 4 . x))
    =>  (PLUS 100 (MINUS G ZPRIME 100 P) 4 . 100)
     (sublis '(((+ x y) . (- x y)) ((- x y) . (+ x y)))
             '(* (/ (+ x y) (+ x p)) (- x y))
             :test #'equal)
    =>  (* (/ (- X Y) (+ X P)) (+ X Y))
     (setq tree1 '(1 (1 2) ((1 2 3)) (((1 2 3 4)))))
    =>  (1 (1 2) ((1 2 3)) (((1 2 3 4))))
     (sublis '((3 . "three")) tree1)
    =>  (1 (1 2) ((1 2 "three")) (((1 2 "three" 4))))
     (sublis '((t . "string"))
              (sublis '((1 . "") (4 . 44)) tree1)
              :key #'stringp)
    =>  ("string" ("string" 2) (("string" 2 3)) ((("string" 2 3 44))))
     tree1 =>  (1 (1 2) ((1 2 3)) (((1 2 3 4))))
     (setq tree2 '("one" ("one" "two") (("one" "Two" "three"))))
    =>  ("one" ("one" "two") (("one" "Two" "three")))
     (sublis '(("two" . 2)) tree2)
    =>  ("one" ("one" "two") (("one" "Two" "three")))
     tree2 =>  ("one" ("one" "two") (("one" "Two" "three")))
     (sublis '(("two" . 2)) tree2 :test 'equal)
    =>  ("one" ("one" 2) (("one" "Two" "three")))

     (nsublis '((t . 'temp))
               tree1
               :key #'(lambda (x) (or (atom x) (< (list-length x) 3))))
    =>  ((QUOTE TEMP) (QUOTE TEMP) QUOTE TEMP)

Side Effects:

nsublis modifies tree.

Affected By: None.

Exceptional Situations: None.

See Also:

subst, Section 3.2.1 (Compiler Terminology), Section 3.6 (Traversal Rules and Side Effects)

Notes:

The `:test-not` parameter is deprecated.

Because the side-effecting variants (e.g., nsublis) potentially change the path that is being traversed, their effects in the presence of shared or circular structure structure may vary in surprising ways when compared to their non-side-effecting alternatives. To see this, consider the following side-effect behavior, which might be exhibited by some implementations:

     (defun test-it (fn)
       (let* ((shared-piece (list 'a 'b))
              (data (list shared-piece shared-piece)))
         (funcall fn '((a . b) (b . a)) data)))
     (test-it #'sublis) =>  ((B A) (B A))
     (test-it #'nsublis) =>  ((A B) (A B))

The following X3J13 cleanup issues, not part of the specification, apply to this section:

- TEST-NOT-IF-NOT:FLUSH-ALL
- MAPPING-DESTRUCTIVE-INTERACTION:EXPLICITLY-VAGUE
- CONSTANT-MODIFICATION:DISALLOW
- DOTTED-LIST-ARGUMENTS:CLARIFY

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