logic: add gensym() for fresh unique symbols (#230)

Per Norvig's suggestion on #230, add a Lisp-style gensym(): successive calls
return distinct logic symbols s_0, s_1, s_2, ... (Expr objects), which makes it
easy to build an array/dict of symbols or to standardize apart, without having to
invent names by hand. Test added.

Author: dmeoli

aima/logic.py

@@ -159,6 +159,20 @@ def is_prop_symbol(s):
     return is_symbol(s) and s[0].isupper()
 
 
+def gensym(prefix='s_'):
+    """Return a fresh logic symbol with a unique name, like Lisp's ``gensym`` [#230].
+
+    Successive calls return distinct symbols ``s_0``, ``s_1``, ``s_2``, ... -- handy
+    for building an array or dict of symbols, or for standardizing apart. Pass a
+    ``prefix`` to control the name stem.
+    """
+    gensym.counter += 1
+    return Expr(prefix + str(gensym.counter))
+
+
+gensym.counter = -1
+
+
 def variables(s):
     """Return a set of the variables in expression s.
     >>> variables(expr('F(x, x) & G(x, y) & H(y, z) & R(A, z, 2)')) == {x, y, z}

tests/test_logic.py

@@ -441,5 +441,13 @@ def test_hybrid_wumpus_agent_first_action():
     assert agent.current_position.get_location() == (1, 1)
 
 
+def test_gensym():
+    s1, s2, s3 = gensym(), gensym(), gensym()
+    assert len({s1, s2, s3}) == 3                         # all distinct
+    assert all(is_symbol(s.op) for s in (s1, s2, s3))     # genuine symbols
+    assert gensym('v_').op.startswith('v_')               # custom prefix
+    assert variables(s1 | s2) == {s1, s2}                 # usable in expressions
+
+
 if __name__ == '__main__':
     pytest.main()