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Want some logic programming without leaving the comfort of your irb session?

I've taken the interpreter I found in a couple articles in Japanese, added syntactic sugar and some new predicates. It's quite powerful now; you can find below some logical programs, including symbolic differentiation and simplification.

If you want to play along, you're going to need my modified tiny_prolog.rb and a few new predicates defined in tiny_prolog_ext.rb.

Syntax

Logical code looks like this with the tiny Prolog-ish interpreter in Ruby:

require 'tiny_prolog_ext'

# rules
# read as "X and Y are siblings if Z is the parent of both"
sibling[:X,:Y] <<= [ parent[:Z,:X], parent[:Z,:Y], noteq[:X,:Y] ]
parent[:X,:Y] <<= father[:X,:Y]
parent[:X,:Y] <<= mother[:X,:Y]

# facts: rules with "no preconditions"
father["matz", "Ruby"].fact
mother["Trude", "Sally"].fact
father["Tom", "Sally"].fact
father["Tom", "Erica"].fact
father["Mike", "Tom"].fact

query sibling[:X, "Sally"]
# >> 1 sibling["Erica", "Sally"]

I've added some sugar to the original interpreter, so there's no need to predeclare the predicates, and rules/facts can be specified more naturally. So you basically define facts with

predicate[1,:X].fact  # predicate[1,:X] holds for any X
predicate[2, 1].fact  # predicate[2,1] holds

and rules with

predicate[:VAR1, :VAR2] <<= [ other[:VAR1], predicates[:VAR2] ]

meaning that predicate[:VAR1,:VAR2] holds if both other[:VAR1] and predicates[:VAR2] do.