Fix pregroup swap rotation (#258)

discopy · Jan 12, 2024 · ec3c570 · ec3c570
2 parents bf9ded1 + 6d098e6
commit ec3c570
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diff --git a/discopy/grammar/pregroup.py b/discopy/grammar/pregroup.py
@@ -178,6 +178,9 @@ class Swap(frobenius.Swap, Box):
     """
     A pregroup swap is a frobenius swap in a pregroup diagram.
     """
+    def rotate(self, left=False):
+        return (type(self)(self.left.l, self.right.l) if left else
+                type(self)(self.left.r, self.right.r))
 
 
 class Spider(frobenius.Spider, Box):

diff --git a/test/grammar/pregroup.py b/test/grammar/pregroup.py
@@ -61,3 +61,11 @@ def test_from_tree():
     loves = Word('loves', n.r @ s @ n.l)
     sentence = Alice @ loves @ Bob >> Cup(n, n.r) @ Id(s) @ Cup(n.l, n)
     assert sentence == from_tree(sentence.to_tree())
+
+
+def test_pregroup_swap_rotation():
+    s, n = Ty('s'), Ty('n')
+    assert Swap(n, s).r.dom == Swap(n, s).cod.r
+    assert Swap(n, s).l.dom == Swap(n, s).cod.l
+    assert Swap(n, s).l.cod == Swap(n, s).dom.l
+    assert Swap(n, s).r.cod == Swap(n, s).dom.r