feat: allow empty structs

working towards ibis-project#8289

I'm not sure how useful empty structs are, since it seems like
not many backends actually support them.
But still, if you stay in ibis-land, perhaps it is useful.
Not that hard for us to support it, so why not.

I'm not sure of the history of the specific disallowment
that I am removing from the type inference.

Relevant context:

- ibis-project#8876
- https://github.com/ibis-project/ibis/issues?q=empty+struct

ibis/backends/tests/test_struct.py

@@ -14,7 +14,9 @@
 import ibis.expr.datatypes as dt
 from ibis import util
 from ibis.backends.tests.errors import (
+    DuckDBParserException,
     PolarsColumnNotFoundError,
+    PolarsComputeError,
     PsycoPg2InternalError,
     PsycoPg2SyntaxError,
     Py4JJavaError,
@@ -29,6 +31,14 @@
 ]
 
 
+@pytest.mark.notimpl("duckdb", raises=DuckDBParserException)
+@pytest.mark.notimpl("polars", raises=PolarsComputeError)
+def test_struct_factory_empty(con):
+    s = ibis.struct({})
+    result = con.execute(s)
+    assert result == {}
+
+
 @pytest.mark.notimpl(["dask"])
 @pytest.mark.parametrize(
     ("field", "expected"),

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/expr/operations/structs.py

@@ -4,6 +4,7 @@
 
 from public import public
 
+import ibis.expr.datashape as ds
 import ibis.expr.datatypes as dt
 import ibis.expr.rules as rlz
 from ibis.common.annotations import ValidationError, attribute
@@ -38,8 +39,6 @@ class StructColumn(Value):
     names: VarTuple[str]
     values: VarTuple[Value]
 
-    shape = rlz.shape_like("values")
-
     def __init__(self, names, values):
         if len(names) != len(values):
             raise ValidationError(
@@ -52,3 +51,9 @@ def __init__(self, names, values):
     def dtype(self) -> dt.DataType:
         dtypes = (value.dtype for value in self.values)
         return dt.Struct.from_tuples(zip(self.names, dtypes))
+
+    @attribute
+    def shape(self) -> ds.DataShape:
+        if len(self.values) == 0:
+            return ds.scalar
+        return rlz.highest_precedence_shape(self.values)

ibis/tests/expr/test_struct.py

@@ -5,9 +5,12 @@
 import pytest
 
 import ibis
+import ibis.expr.datashape as ds
+import ibis.expr.datatypes as dt
 import ibis.expr.operations as ops
 import ibis.expr.types as ir
 from ibis import _
+from ibis.common.annotations import ValidationError
 from ibis.expr.tests.test_newrels import join_tables
 from ibis.tests.util import assert_pickle_roundtrip
 
@@ -22,6 +25,17 @@ def s():
     return ibis.table(dict(a="struct<f: float, g: string>"), name="s")
 
 
+@pytest.mark.parametrize("val", [{}, []])
+@pytest.mark.parametrize("typ", [None, "struct<>", dt.Struct.from_tuples([])])
+def test_struct_factory_empty(val, typ):
+    with pytest.raises(ValidationError):
+        ibis.struct(val, type="struct<a: float64, b: float64>")
+    s = ibis.struct(val, type=typ)
+    assert s.names == tuple()
+    assert s.type() == dt.Struct.from_tuples([])
+    assert s.op().shape == ds.scalar
+
+
 def test_struct_operations():
     value = OrderedDict(
         [