6 Conjuctions and Disjunctions

Goals may be combined using the forms %and and %or to form compound goals. (For %not, see Negation as Failure.) Eg,

> (%which (x)       (%and (%member x '(1 2 3))             (%< x 3)))

'((x . 1))

gives solutions for x that satisfy both the argument goals of the %and. Ie, x should both be a member of '(1 2 3) and be less than 3. Typing (%more) gives another solution:

> (%more)

'((x . 2))

> (%more)

#f

There are no more solutions, because [x 3] satisfies the first but not the second goal.

Similarly, the query

> (%which (x)       (%or (%member x '(1 2 3))            (%member x '(3 4 5))))

'((x . 1))

lists all x that are members of either list.

> (%more)

'((x . 2))

> (%more)

'((x . 3))

> (%more)

'((x . 3))

> (%more)

'((x . 4))

> (%more)

'((x . 5))

(Yes, ([x 3]) is listed twice.)

We can rewrite the predicate %computer-literate from Predicates with Rules using %and and %or:

(define %computer-literate

(%rel (person)

[(person)

(%or

(%and (%knows person

'TeX)

(%knows person

'Racket))

(%and (%knows person

'TeX)

(%knows person

'Prolog)))]))

Or, more succinctly:

(define %computer-literate

(%rel (person)

[(person)

(%and (%knows person

'TeX)

(%or (%knows person

'Racket)

(%knows person

'Prolog)))]))

We can even dispense with the %rel altogether:

(define %computer-literate

(lambda (person)

(%and (%knows person

'TeX)

(%or (%knows person

'Racket)

(%knows person

'Prolog)))))

This last looks like a conventional Racket predicate definition, and is arguably the most readable format for a Racket programmer.