Pairs are a basic building block in all Lisp languages. It is a simple two-part value with one called the head and the other called the tail. You can access the head of a pair with ph (pair head) and the tail of a pair with pt (pair tail).

p := pair 1 2
printf "p is %s\n" p
printf "ph of p is %s\n" (ph p)
printf "pt of p is %s\n" (pt p)
$ idio simple-pairs
p is (1 & 2)
ph of p is 1
pt of p is 2

Either of the head or tail can be any other value including other pairs. This leads to some very rich data structures.

It also leads to some tersely named accessors such as phtt which is shorthand for ph (pt (pt x)).


If the tail of a pair is another pair we can sense a linked list forming where the “data” part is stored in the head.

This can be rather verbose to create manually so there is a convenience function, list.

There is also a slightly different printing format which does away with some of the extra parentheses and “pair separators”, &. The result is the rather more traditional “list” printed form: (e1 e2 e3 ...).

l1 := pair 1 (pair 2 (pair 3 #n))
printf "l1 is %s\n" l1
printf "ph of l1 is %s\n" (ph l1)
printf "pt of l1 is %s\n" (pt l1)

l2 := list 1 2 3
printf "l2 is %s\n" l2
printf "ph of l2 is %s\n" (ph l2)
printf "pt of l2 is %s\n" (pt l2)
$ idio simple-lists
l1 is (1 2 3)
ph of l1 is 1
pt of l1 is (2 3)
l2 is (1 2 3)
ph of l2 is 1
pt of l2 is (2 3)

The #n in the find pair of l1, above, marks a proper list. The vast majority of lists are proper lists cleanly ending with a tail of #n (nil or null). list always creates proper lists.

Improper lists appear from time to time where that final tail is not #n.

List Functions

With a list we can test for membership of that list with variations on the equality functions:

  1. memq tests for membership using eq?

  2. memv tests for membership using eqv?

  3. member tests for membership using equal?

These membership functions return #f if they fail but the remainder of the list (inclusive of the matching element) if they succeed:

p1 := pair 1 2
p2 := pair 1 2
things := list 1 "hello" p1 'symbol
printf "things is %s\n" things
printf "memq 3 in things is %s\n" (memq 3 things)
printf "memq 1 in things is %s\n" (memq 1 things)
printf "memq p1 in things is %s\n" (memq p1 things)
printf "memv \"hello\" in things is %s\n" (memv "hello" things)
printf "member p2 in things is %s\n" (member p2 things)
$ idio list-membership
things is (1 "hello" (1 & 2) symbol)
memq 3 in things is #f
memq 1 in things is (1 "hello" (1 & 2) symbol)
memq p1 in things is ((1 & 2) symbol)
memv "hello" in things is ("hello" (1 & 2) symbol)
member p2 in things is ((1 & 2) symbol)

Association Lists

If we make the “data” part more complicated, in particular, a list itself, we can treat this list of lists as a lookup table where the key is the first element in each list.

These association lists are very common and have equivalent key lookup functions:

  1. assq tests the key using eq?

  2. assv tests the key using eqv?

  3. assoc tests the key using equal?

;; by quoting the list of lists we prevent the evaluator trying to
;; run functions called a, b and c
data := '((a "apple" fruit)
          (b "banana" fruit)
          (c "carrot" vegetable))

;; symbols are *always* eq?

sym := 'b
printf "assq %s is %s\n" sym (assq sym data)
$ idio alists
assq b is (b "banana" fruit)

Last built at 2023-11-09T07:11:44Z+0000 from 77077af (dev) for Idio 0.3