feat(builders): support coercion from Builder and Deferred instances

koerce/builders.py

@@ -152,13 +152,12 @@ def __rxor__(self, other: Any) -> Deferred:
 
 @cython.cclass
 class Builder:
-    # TODO(kszucs): cover with tests
     @staticmethod
-    def __coerce__(value):
+    def __coerce__(value) -> Builder:
         if isinstance(value, Builder):
             return value
         elif isinstance(value, Deferred):
-            return value._builder
+            return cython.cast(Deferred, value)._builder
         else:
             raise ValueError(f"Cannot coerce {type(value).__name__!r} to Builder")
 

koerce/tests/test_builders.py

@@ -18,6 +18,7 @@
     Item,
     Just,
     Map,
+    Builder,
     Seq,
     Unop,
     Var,
@@ -510,3 +511,10 @@ def test_deferred_is_not_truthy(obj):
         TypeError, match="The truth value of Deferred objects is not defined"
     ):
         bool(obj)
+
+
+def test_builder_coercion():
+    assert Builder.__coerce__(Deferred(Var("a"))) == Var("a")
+    assert Builder.__coerce__(Var("a")) == Var("a")
+    with pytest.raises(ValueError):
+        Builder.__coerce__(1)