Add implies to truth table

predicate/formatter/format_dot.py

@@ -16,6 +16,7 @@
 from predicate.formatter.helpers import set_to_str
 from predicate.ge_predicate import GePredicate
 from predicate.gt_predicate import GtPredicate
+from predicate.implies import Implies
 from predicate.is_falsy_predicate import IsFalsyPredicate
 from predicate.is_instance_predicate import IsInstancePredicate
 from predicate.is_none_predicate import IsNonePredicate
@@ -142,6 +143,8 @@ def to_value(predicate: Predicate):
                     dot.edge(kv, to_value(key), label="key")
                     dot.edge(kv, to_value(value), label="value")
                 return node
+            case Implies(left, right):
+                return add_node_left_right("implies", label="=>", left=left, right=right)
             case IsInstancePredicate(klass):
                 name = klass[0].__name__  # type: ignore
                 return add_node("instance", label=f"is_{name}_p")

predicate/implies.py

@@ -1,4 +1,6 @@
+from dataclasses import dataclass
 from functools import singledispatch
+from typing import override
 
 from predicate.always_false_predicate import AlwaysFalsePredicate
 from predicate.always_true_predicate import AlwaysTruePredicate
@@ -107,3 +109,25 @@ def _(predicate: InPredicate, other: Predicate) -> bool:
             return predicate.v.issubset(v)
         case _:
             return False
+
+
+@dataclass
+class Implies[T](Predicate[T]):
+    """A predicate class that models the 'implies' (=>) predicate."""
+
+    left: Predicate
+    right: Predicate
+
+    def __call__(self, x) -> bool:
+        return not self.left(x) or self.right(x)
+
+    def __repr__(self) -> str:
+        return f"{self.left} => {self.right}"
+
+    @override
+    def explain_failure(self, x) -> dict:
+        return {"reason": f"{self.left} doesn't imply {self.right}"}
+
+
+def implies_p_p(left: Predicate, right: Predicate) -> Predicate:
+    return Implies(left=left, right=right)

predicate/parser.py

@@ -1,6 +1,7 @@
 from lark import Lark, Transformer, UnexpectedEOF  # type: ignore
 
 from predicate import always_false_p, always_true_p
+from predicate.implies import Implies
 from predicate.named_predicate import NamedPredicate
 from predicate.predicate import Predicate
 
@@ -9,7 +10,8 @@
     predicate: expression | variable
 
     variable: WORD
-    ?expression: grouped_expression | or_expression | and_expression | xor_expression | not_expression | false | true
+    ?expression: grouped_expression | or_expression | and_expression | xor_expression | not_expression
+                | implies_expression | false | true
 
     false: "false"
     true: "true"
@@ -18,6 +20,7 @@
     and_expression: predicate "&" predicate
     xor_expression: predicate "^" predicate
     not_expression: "~" predicate
+    implies_expression: predicate "=>" predicate
 
     %import common.WORD   // imports from terminal library
     %ignore " "           // Disregard spaces in text
@@ -40,6 +43,10 @@ def false(self, _item) -> Predicate:
     def grouped_expression(self, item):
         return item[0]
 
+    def implies_expression(self, items: tuple[Predicate, Predicate]):
+        left, right = items
+        return Implies(left=left, right=right)
+
     def not_expression(self, item: tuple[Predicate]) -> Predicate:
         predicate = item[0]
         return ~predicate

predicate/truth_table.py

@@ -5,6 +5,7 @@
 
 from predicate.always_false_predicate import AlwaysFalsePredicate
 from predicate.always_true_predicate import AlwaysTruePredicate
+from predicate.implies import Implies
 from predicate.named_predicate import NamedPredicate
 from predicate.predicate import (
     AndPredicate,
@@ -33,7 +34,9 @@ def get_named_predicates(predicate: Predicate) -> list[str]:
 
     def get_names() -> Iterator[str]:
         match predicate:
-            case AndPredicate(left, right) | OrPredicate(left, right) | XorPredicate(left, right):
+            case (
+                AndPredicate(left, right) | OrPredicate(left, right) | XorPredicate(left, right) | Implies(left, right)
+            ):
                 yield from get_named_predicates(left)
                 yield from get_named_predicates(right)
             case NotPredicate(not_predicate):
@@ -58,7 +61,7 @@ def execute_predicate(predicate: Predicate, values: dict) -> bool:
 def set_named_values(predicate: Predicate, values: dict) -> None:
     """Set the named predicate values."""
     match predicate:
-        case AndPredicate(left, right) | OrPredicate(left, right) | XorPredicate(left, right):
+        case AndPredicate(left, right) | OrPredicate(left, right) | XorPredicate(left, right) | Implies(left, right):
             set_named_values(left, values)
             set_named_values(right, values)
         case NotPredicate(not_predicate):

test/formatter/test_format_dot.py

@@ -23,6 +23,7 @@
     not_in_p,
     to_dot,
 )
+from predicate.implies import Implies
 from predicate.named_predicate import NamedPredicate
 from predicate.set_predicates import is_real_subset_p, is_real_superset_p, is_subset_p, is_superset_p
 from predicate.standard_predicates import (
@@ -99,6 +100,14 @@ def test_format_dot_any():
     assert dot
 
 
+def test_format_dot_implies(p, q):
+    predicate = Implies(p, q)
+
+    dot = to_dot(predicate)
+
+    assert dot
+
+
 def test_format_dot_not():
     predicate = ~always_true_p
 

test/test_parser.py

@@ -1,6 +1,7 @@
 import pytest
 
 from predicate import always_false_p, always_true_p
+from predicate.implies import Implies
 from predicate.named_predicate import NamedPredicate
 from predicate.parser import parse_expression
 
@@ -108,6 +109,14 @@ def test_parser_grouped(p, q, r):
     assert predicate == p & (q | r)
 
 
+def test_parser_implies(p, q):
+    expression = "p => q"
+
+    predicate = parse_expression(expression)
+
+    assert predicate == Implies(p, q)
+
+
 def test_parser_failure(p, q):
     expression = "p ^"
 

test/test_truth_table.py

@@ -1,6 +1,7 @@
 import pytest
 
 from predicate import always_false_p, always_true_p, le_p
+from predicate.implies import implies_p_p
 from predicate.named_predicate import NamedPredicate
 from predicate.truth_table import get_named_predicates, set_named_values, truth_table
 
@@ -77,6 +78,14 @@ def test_truth_table_false(p):
     assert result == [False, True]
 
 
+def test_truth_table_implies(p, q):
+    predicate = implies_p_p(p, q)
+
+    result = [row[1] for row in truth_table(predicate)]
+
+    assert result == [True, True, False, True]
+
+
 def test_truth_table_true(p):
     predicate = p | always_true_p