add predefined variables

Inspired by `sympy.abc`

docs/api/terms.rst

@@ -4,6 +4,7 @@ Package terms
 .. toctree::
    :maxdepth: 2
 
+   terms/abc
    terms/logic
    terms/arithmetic
    terms/pairs

docs/api/terms/abc.rst

@@ -0,0 +1,8 @@
+Module abc
+==========
+
+.. toctree::
+   :maxdepth: 2
+
+.. automodule:: lambda_calculus.terms.abc
+    :members:

src/lambda_calculus/terms/__init__.py

@@ -17,6 +17,7 @@
     "Variable",
     "Abstraction",
     "Application",
+    "abc",
     "arithmetic",
     "logic",
     "pairs",

src/lambda_calculus/terms/abc.py

@@ -0,0 +1,17 @@
+#!/usr/bin/python3
+
+"""Predefined Variables for all ASCII letters"""
+
+from string import ascii_letters
+from . import Variable
+
+__all__ = tuple(ascii_letters)
+
+# better IDE autocompletion
+a, b, c, d, e, f, g, h, i, j = map(Variable, "abcdefghij")
+k, l, m, n, o, p, q, r, s, t = map(Variable, "klmnopqrst")
+u, v, w, x, y, z = map(Variable, "uvwxyz")
+
+A, B, C, D, E, F, G, H, I, J = map(Variable, "ABCDEFGHIJ")
+K, L, M, N, O, P, Q, R, S, T = map(Variable, "KLMNOPQRST")
+U, V, W, X, Y, Z = map(Variable, "UVWXYZ")

tests/terms/test_abc.py

@@ -0,0 +1,24 @@
+#!/usr/bin/python3
+
+"""Tests for the predefined variables"""
+
+from unittest import TestCase
+from string import ascii_letters
+from lambda_calculus.terms import Variable, abc
+
+
+class PredefinedVariablesTest(TestCase):
+    """Tests for the predefined variables"""
+
+    def test_complete(self) -> None:
+        """Check whether all letters exist"""
+        self.assertEqual(tuple(ascii_letters), abc.__all__)
+        for letter in ascii_letters:
+            self.assertTrue(hasattr(abc, letter))
+
+    def test_variables(self) -> None:
+        """Test the predefined variables"""
+        for letter in ascii_letters:
+            variable = getattr(abc, letter)
+            self.assertIsInstance(variable, Variable)
+            self.assertEqual(variable.name, letter)