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Fix for unify algorithm in logic.py #1101

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Merged
merged 5 commits into from
Aug 19, 2019
Merged

Fix for unify algorithm in logic.py #1101

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219f0b1
Fix issue #1053
alessandrocuda Aug 16, 2019
0a0a1ae
added two more test in test_logic.py
alessandrocuda Aug 16, 2019
3d5c095
Fixed brackets missing in test_logic.py for the new test
alessandrocuda Aug 16, 2019
9b682e1
Fixed typo error, missing space and double quotes for docstrings
alessandrocuda Aug 16, 2019
01876d5
comments changed to cascade_substitution function in logic.py
alessandrocuda Aug 16, 2019
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34 changes: 31 additions & 3 deletions logic.py
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Original file line number Diff line number Diff line change
Expand Up @@ -193,8 +193,7 @@ def parse_definite_clause(s):


# Useful constant Exprs used in examples and code:
A, B, C, D, E, F, G, P, Q, x, y, z = map(Expr, 'ABCDEFGPQxyz')

A, B, C, D, E, F, G, P, Q, a, x, y, z, u = map(Expr, 'ABCDEFGPQaxyzu')

# ______________________________________________________________________________

Expand Down Expand Up @@ -1370,7 +1369,9 @@ def unify_var(var, x, s):
elif occur_check(var, x, s):
return None
else:
return extend(s, var, x)
new_s = extend(s, var, x)
cascade_substitution(new_s)
return new_s


def occur_check(var, x, s):
Expand Down Expand Up @@ -1415,6 +1416,33 @@ def subst(s, x):
else:
return Expr(x.op, *[subst(s, arg) for arg in x.args])

def cascade_substitution(s):
"""This method allows to return a correct unifier in normal form
and perform a cascade substitution to s.
For every mapping in s perform a cascade substitution on s.get(x)
and if it is replaced with a function ensure that all the function
terms are correct updates by passing over them again.

This issue fix: https://github.com/aimacode/aima-python/issues/1053
unify(expr('P(A, x, F(G(y)))'), expr('P(z, F(z), F(u))'))
must return {z: A, x: F(A), u: G(y)} and not {z: A, x: F(z), u: G(y)}

>>> s = {x: y, y: G(z)}
>>> cascade_substitution(s)
>>> print(s)
{x: G(z), y: G(z)}

Parameters
----------
s : Dictionary
This contain a substution
"""

for x in s:
s[x] = subst(s, s.get(x))
if isinstance(s.get(x), Expr) and not is_variable(s.get(x)):
# Ensure Function Terms are correct updates by passing over them again.
s[x] = subst(s, s.get(x))

def standardize_variables(sentence, dic=None):
"""Replace all the variables in sentence with new variables."""
Expand Down
6 changes: 6 additions & 0 deletions tests/test_logic.py
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Original file line number Diff line number Diff line change
Expand Up @@ -163,7 +163,13 @@ def test_unify():
assert unify(x & 4 & y, 6 & y & 4, {}) == {x: 6, y: 4}
assert unify(expr('A(x)'), expr('A(B)')) == {x: B}
assert unify(expr('American(x) & Weapon(B)'), expr('American(A) & Weapon(y)')) == {x: A, y: B}
assert unify(expr('P(F(x,z), G(u, z))'), expr('P(F(y,a), y)')) == {x: G(u, a), z: a, y: G(u, a)}

# test for https://github.com/aimacode/aima-python/issues/1053
# unify(expr('P(A, x, F(G(y)))'), expr('P(z, F(z), F(u))'))
# must return {z: A, x: F(A), u: G(y)} and not {z: A, x: F(z), u: G(y)}
assert unify(expr('P(A, x, F(G(y)))'), expr('P(z, F(z), F(u))')) == {z: A, x: F(A), u: G(y)}
assert unify(expr('P(x, A, F(G(y)))'), expr('P(F(z), z, F(u))')) == {x: F(A), z: A, u: G(y)}

def test_pl_fc_entails():
assert pl_fc_entails(horn_clauses_KB, expr('Q'))
Expand Down