add lambda terms for pairs

Author: Deric-W

lambda_calculus/__init__.py

@@ -4,7 +4,7 @@
 
 from .terms import Variable, Abstraction, Application
 
-__version__ = "1.9.0"
+__version__ = "1.10.0"
 __author__  = "Eric Niklas Wolf"
 __email__   = "[email protected]"
 __all__ = (

lambda_calculus/terms/__init__.py

@@ -17,7 +17,8 @@
     "Abstraction",
     "Application",
     "arithmetic",
-    "logic"
+    "logic",
+    "pairs"
 )
 
 T = TypeVar("T")

lambda_calculus/terms/pairs.py

@@ -0,0 +1,39 @@
+#!/usr/bin/python3
+
+"""Implementation of pairs"""
+
+from . import Variable, Abstraction, Application
+from .logic import TRUE, FALSE
+
+__all__ = (
+    "PAIR",
+    "FIRST",
+    "SECOND",
+    "NIL",
+    "NULL"
+)
+
+PAIR = Abstraction.curried(
+    ("x", "y", "f"),
+    Application.with_arguments(
+        Variable("f"),
+        (Variable("x"), Variable("y"))
+    )
+)
+
+FIRST = Abstraction("p", Application(Variable("p"), TRUE))
+
+SECOND = Abstraction("p", Application(Variable("p"), FALSE))
+
+NIL = Abstraction("x", TRUE)
+
+NULL = Abstraction(
+    "p",
+    Application(
+        Variable("p"),
+        Abstraction.curried(
+            ("x", "y"),
+            FALSE
+        )
+    )
+)

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "lambda_calculus"
-version = "1.9.0"
+version = "1.10.0"
 description = "Implementation of the Lambda calculus"
 requires-python = ">=3.10"
 keywords = []

tests/terms/test_pairs.py

@@ -0,0 +1,64 @@
+#!/usr/bin/python3
+
+"""Tests for pair terms"""
+
+from unittest import TestCase
+from lambda_calculus.visitors.normalisation import BetaNormalisingVisitor
+from lambda_calculus.terms import Term, Variable, Application, pairs
+from lambda_calculus.terms.logic import TRUE, FALSE
+
+
+class PairTest(TestCase):
+    """Test for pair terms"""
+
+    visitor: BetaNormalisingVisitor
+
+    def setUp(self) -> None:
+        """create a visitor"""
+        self.visitor = BetaNormalisingVisitor()
+
+    def make_pair(self, a: Term[str], b: Term[str]) -> Term[str]:
+        """make a pair (a, b)"""
+        return Application.with_arguments(pairs.PAIR, (a, b))
+
+    def test_first(self) -> None:
+        """test accessing the first element"""
+        self.assertEqual(
+            self.visitor.skip_intermediate(
+                Application(
+                    pairs.FIRST,
+                    self.make_pair(Variable("a"), Variable("b"))
+                )
+            ),
+            Variable("a")
+        )
+
+    def test_second(self) -> None:
+        """test accessing rhe second element"""
+        self.assertEqual(
+            self.visitor.skip_intermediate(
+                Application(
+                    pairs.SECOND,
+                    self.make_pair(Variable("a"), Variable("b"))
+                )
+            ),
+            Variable("b")
+        )
+
+    def test_null(self) -> None:
+        """test check for NIL pair"""
+        self.assertEqual(
+            self.visitor.skip_intermediate(
+                Application(pairs.NULL, pairs.NIL)
+            ),
+            TRUE
+        )
+        self.assertEqual(
+            self.visitor.skip_intermediate(
+                Application(
+                    pairs.NULL,
+                    self.make_pair(Variable("a"), Variable("b"))
+                )
+            ),
+            FALSE
+        )