BEGIN;
CREATE EXTENSION pair;
SELECT pair('foo', 'bar')
     , pair('HEY'::text, 'bar')
     , pair('foo'::text, 'woah'::text)
     , pair('ick', 'foo'::text)
     , pair('foo'::text, 1)
     , pair(12.3, 'foo'::text)
     , pair(1, 12)
;
   pair    |   pair    |    pair    |   pair    |  pair   |    pair    |  pair  
-----------+-----------+------------+-----------+---------+------------+--------
 (foo,bar) | (HEY,bar) | (foo,woah) | (ick,foo) | (foo,1) | (12.3,foo) | (1,12)
(1 row)

SELECT 'foo' ~> 'bar' AS arrowop
     , 'HEY'::text ~> 'bar'
     , 'foo'::text ~> 'woah'::text
     , 'ick' ~> 'foo'::text
     , 'foo'::text ~> 1
     , 12.3 ~> 'foo'::text
     , 1 ~> 12
;
  arrowop  | ?column?  |  ?column?  | ?column?  | ?column? |  ?column?  | ?column? 
-----------+-----------+------------+-----------+----------+------------+----------
 (foo,bar) | (HEY,bar) | (foo,woah) | (ick,foo) | (foo,1)  | (12.3,foo) | (1,12)
(1 row)

CREATE TABLE kv (
    pair pair
);
INSERT INTO kv VALUES('foo' ~> 'bar'), (1 ~> 2);
SELECT (pair).k, (pair).v FROM kv ORDER BY (pair).k;
  k  |  v  
-----+-----
 1   | 2
 foo | bar
(2 rows)

SELECT (pair('foo', 'bar')).k;
  k  
-----
 foo
(1 row)

SELECT (pair('foo', 'bar')).v;
  v  
-----
 bar
(1 row)

SELECT ('foo' ~> 'bar').k;
  k  
-----
 foo
(1 row)

SELECT ('foo' ~> 'bar').v;
  v  
-----
 bar
(1 row)

ROLLBACK;