Cache sequence results (#102)

* Cache sequence results

* Fix typechecking errors

* Silence Pylint slots error

Author: chrisrink10

basilisp/lang/runtime.py

@@ -353,7 +353,10 @@ def first(o):
         return None
     if isinstance(o, lseq.Seq):
         return o.first
-    return to_seq(o).first
+    s = to_seq(o)
+    if s is None:
+        return None
+    return s.first
 
 
 def rest(o):
@@ -364,12 +367,12 @@ def rest(o):
     if isinstance(o, lseq.Seq):
         s = o.rest
         if s is None:
-            return llist.List.empty()
+            return lseq.empty()
         return s
-    s = to_seq(o).rest
+    s = to_seq(o)
     if s is None:
-        return llist.List.empty()
-    return s
+        return lseq.empty()
+    return s.rest
 
 
 def next(o):  # pylint:disable=redefined-builtin

basilisp/lang/seq.py

@@ -84,23 +84,29 @@ class _Sequence(Seq[T]):
 
     Do not directly instantiate a Sequence. Instead use the `sequence` function
     below."""
-    __slots__ = ('_first', '_seq')
+    __slots__ = ('_first', '_seq', '_rest')
 
     def __init__(self, s: Iterator, first: T) -> None:
         self._seq = s  # pylint:disable=assigning-non-slot
         self._first = first  # pylint:disable=assigning-non-slot
+        self._rest: Optional[Seq] = None  # pylint:disable=assigning-non-slot
 
     @property
     def first(self) -> T:
         return self._first
 
     @property
     def rest(self) -> Optional["Seq[T]"]:
+        if self._rest:
+            return self._rest
+
         try:
             n = next(self._seq)
-            return _Sequence(self._seq, n)
+            self._rest = _Sequence(self._seq, n)  # pylint:disable=assigning-non-slot
         except StopIteration:
-            return _EmptySequence()
+            self._rest = _EmptySequence()  # pylint:disable=assigning-non-slot
+
+        return self._rest
 
     def cons(self, elem):
         return Cons(elem, self)

tests/seq_test.py

@@ -24,4 +24,9 @@ def test_sequence():
     assert [1, 2, 3] == [e for e in lseq.sequence([1, 2, 3])]
     assert llist.l(1, 2, 3) == lseq.sequence([1, 2, 3])
     assert llist.l(1, 2, 3) == lseq.sequence(llist.l(1, 2, 3))
-    assert llist.l(1, 2, 3) == llist.list(lseq.sequence([1, 2, 3]))
+    assert llist.l(1, 2, 3) == llist.list(lseq.sequence([1, 2, 3]))
+
+    s = lseq.sequence([1, 2, 3])
+    assert 2 == s.rest.first
+    assert 3 == s.rest.rest.first
+    assert None is s.rest.rest.rest.first