The Reasoned Schemer by Daniel P. Friedman, William E. Byrd, & Oleg Kiselyov

• The Law of Fresh
• If x is fresh, then (= v x) succeeds and associates x with v.
• The Law of =
• (= v w) is the same as (= w v)
• The Law of cond-e
• To get more values from cond, pretend that the successful cond line has failed, refreshing all variables that got an associated from that line.
• The Law of cond-i
• Cond-i behaves like cond-e, except that its values are interleaved.
• We believe that it is natural to extend functional programming to relational programming.
• In order to do relational programming, we need only two constants: #s and #u, and only three operators: =, fresh, and cond-e.
• #s and #u are reminiscent of the Boolean constants: #t and #f; fresh expressions resemble lambda expressions; and cond-e expressions are syntactically like cond expressions.
• The First Commandment
• To transform a function whose value is a Boolean into a function whose value is a goal, replace cond with cond-e and un-nest each question and answer. Un-nest the answer #t (or #f) by replacing it with #s (or #u).
• The Second Commandment
• To transform a function whose value is not a Boolean into a function whose value is a goal, add an extra argument to hold its value, replace cond with cond-e, and un-nest each question and answer.
• The Third Commandment
• If prior to determining the question of a cond-a (or cond-u) line a variable is fresh, it must remain fresh in the question of that line.