test: check that empty structs are disallowed

See #9310 for a discussion of whether
empty structs should be allowed or disallowed.
We settled on disallowing them for now,
since pyarrow is like the only backend to support them,
and it is definitely easier to change our mind and allow them later rather than
disallow them later.

Perhaps if you stay in ibis-land, emptry structs are useful ie for doing type manipulations, or maybe you
only use them for intermediate calculations?
But until we see this concrete use case, don't worry about it.

ibis/expr/datatypes/core.py

@@ -831,13 +831,23 @@ def _pretty_piece(self) -> str:
 
 @public
 class Struct(Parametric, MapSet):
-    """Structured values."""
+    """A nested type with ordered fields of any type, analogous to a dataclass in Python.
+
+    eg a Struct might have a field a of type int64 and a field b of type string.
+    """
 
     fields: FrozenOrderedDict[str, DataType]
 
     scalar = "StructScalar"
     column = "StructColumn"
 
+    def __init__(self, fields, nullable=True):
+        # We could do this validation in a type annotation, but I thought this
+        # was common enough to warrant a nicer error message.
+        if not fields:
+            raise TypeError("Structs require at least one field")
+        super().__init__(fields=fields, nullable=nullable)
+
     @classmethod
     def from_tuples(
         cls, pairs: Iterable[tuple[str, str | DataType]], nullable: bool = True

ibis/expr/datatypes/value.py

@@ -42,8 +42,6 @@ def infer(value: Any) -> dt.DataType:
 @infer.register(collections.OrderedDict)
 def infer_struct(value: Mapping[str, Any]) -> dt.Struct:
     """Infer the [`Struct`](./datatypes.qmd#ibis.expr.datatypes.Struct) type of `value`."""
-    if not value:
-        raise TypeError("Empty struct type not supported")
     fields = {name: infer(val) for name, val in value.items()}
     return dt.Struct(fields)
 

ibis/tests/expr/test_struct.py

@@ -22,6 +22,13 @@ def s():
     return ibis.table(dict(a="struct<f: float, g: string>"), name="s")
 
 
+@pytest.mark.parametrize("val", [{}, []])
+@pytest.mark.parametrize("typ", [None, "struct<>"])
+def test_struct_factory_empty(val, typ):
+    with pytest.raises(TypeError):
+        ibis.struct(val, type=typ)
+
+
 def test_struct_operations():
     value = OrderedDict(
         [