— procedure: find-matching-item list predicate

Returns the first element in list for which predicate is true; returns #f if it doesn't find such an element. (This means that if predicate is true for #f, it may be impossible to distinguish a successful result from an unsuccessful one.) Predicate must be a procedure of one argument.

For compatibility, list-search-positive is an alias for find-matching-item. list-search-negative is similar but the sense of the predicate is reversed.

— procedure: memq object list
— procedure: memv object list
— procedure: member object list

These procedures return the first pair of list whose car is object; the returned pair is always one from which list is composed. If object does not occur in list, #f (n.b.: not the empty list) is returned. memq uses eq? to compare object with the elements of list, while memv uses eqv? and member uses equal?.¹

          (memq 'a '(a b c))                      => (a b c)
          (memq 'b '(a b c))                      => (b c)
          (memq 'a '(b c d))                      => #f
          (memq (list 'a) '(b (a) c))             => #f
          (member (list 'a) '(b (a) c))           => ((a) c)
          (memq 101 '(100 101 102))               => unspecified
          (memv 101 '(100 101 102))               => (101 102)
     

— procedure: member-procedure predicate

Returns a procedure similar to memq, except that predicate, which must be an equivalence predicate, is used instead of eq?. This could be used to define memv as follows:

          (define memv (member-procedure eqv?))