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Concatenation

Join

Concatenate a list of strings with some separator between.

Signature
(join x separator) → joined
Arguments: x — Guard (string-listp x).
Returns: joined — Type (stringp joined).

(join x separator) joins together the list of strings x, inserting the string separator between the members. For example:

(join '("a" "b" "c") ".") = "a.b.c"
(join '("a" "b" "c") "->") = "a->b->c"

We always return a string; an empty x results in the empty string, and any empty strings within x just implicitly don't contribute to the result.

Any sort of string concatenation is slow, but join is reasonably efficient: it creates a single character list for the result (in reverse order) without any use of coerce, then uses rchars-to-string to build and reverse the result string.

Definitions and Theorems

Function: join-aux

(defun join-aux (x separator acc)
  (declare (xargs :guard (string-listp x))
           (type string separator))
  (cond ((atom x) acc)
        ((atom (cdr x))
         (revappend-chars (car x) acc))
        (t (let* ((acc (revappend-chars (car x) acc))
                  (acc (revappend-chars separator acc)))
             (join-aux (cdr x) separator acc)))))

Function: join$inline

(defun join$inline (x separator)
  (declare (type string separator))
  (declare (xargs :guard (string-listp x)))
  (let ((acl2::__function__ 'join))
    (declare (ignorable acl2::__function__))
    (mbe :logic (cond ((atom x) "")
                      ((atom (cdr x)) (str-fix (car x)))
                      (t (cat (car x)
                              separator (join (cdr x) separator))))
         :exec (rchars-to-string (join-aux x separator nil)))))

Theorem: stringp-of-join

(defthm stringp-of-join
  (b* ((joined (join$inline x separator)))
    (stringp joined))
  :rule-classes :type-prescription)

Theorem: join-aux-removal

(defthm join-aux-removal
  (implies (and (string-listp x)
                (stringp separator))
           (equal (join-aux x separator acc)
                  (revappend (coerce (join x separator) 'list)
                             acc))))

Theorem: list-equiv-implies-equal-join-1

(defthm list-equiv-implies-equal-join-1
  (implies (list-equiv x x-equiv)
           (equal (join x separator)
                  (join x-equiv separator)))
  :rule-classes (:congruence))

Theorem: streqv-implies-equal-join-2

(defthm streqv-implies-equal-join-2
  (implies (streqv separator separator-equiv)
           (equal (join x separator)
                  (join x separator-equiv)))
  :rule-classes (:congruence))

Theorem: istreqv-implies-istreqv-join-2

(defthm istreqv-implies-istreqv-join-2
  (implies (istreqv separator separator-equiv)
           (istreqv (join x separator)
                    (join x separator-equiv)))
  :rule-classes (:congruence))