Introduce a proper sequence type

Author: brandonwillard

cons/core.py

@@ -172,6 +172,24 @@ def __init__(self):
         raise NotImplementedError()
 
 
+class ProperSequenceType(ABCMeta):
+    """An abstract type that can be used as an instance check for proper sequences.
+
+    A proper sequence is a sequence according to `cons` but not a `ConsPair`.
+    """
+
+    def __instancecheck__(self, o):
+        return issubclass(type(o), MaybeCons)
+
+
+class ProperSequence(metaclass=ProperSequenceType):
+    """A sequence according to `cons` but not a `ConsPair`."""
+
+    @abstractmethod
+    def __init__(self):
+        raise NotImplementedError()
+
+
 class NonCons(ABC):
     """A class (and its subclasses) that is *not* considered a cons.
 

tests/test_cons.py

@@ -7,7 +7,14 @@
 from unification import unify, reify, var
 
 from cons import cons, car, cdr
-from cons.core import ConsPair, MaybeCons, ConsNull, ConsError, NonCons
+from cons.core import (
+    ConsPair,
+    MaybeCons,
+    ConsNull,
+    ConsError,
+    NonCons,
+    ProperSequence,
+)
 
 
 def test_noncons_type():
@@ -68,6 +75,19 @@ def test_cons_null():
     assert not isinstance(cycle([5]), ConsNull)
 
 
+def test_proper_sequence_type():
+
+    with pytest.raises(TypeError):
+        ProperSequence()
+
+    assert not isinstance(cons(1, 2), ProperSequence)
+    assert not isinstance(cons(1, 2, 3), ProperSequence)
+    assert not isinstance("hi", ProperSequence)
+    assert not isinstance(1, ProperSequence)
+    assert isinstance([], ProperSequence)
+    assert isinstance((1, 2, 3), ProperSequence)
+
+
 def test_cons_join():
 
     with pytest.raises(ValueError):