feat(api): support deferred objects in literal

ibis/expr/types/generic.py

@@ -2427,6 +2427,7 @@ def null(type: dt.DataType | str | None = None) -> Value:
 
 
 @public
+@deferrable
 def literal(value: Any, type: dt.DataType | str | None = None) -> Scalar:
     """Create a scalar expression from a Python value.
 
@@ -2480,6 +2481,23 @@ def literal(value: Any, type: dt.DataType | str | None = None) -> Scalar:
     Traceback (most recent call last):
       ...
     TypeError: Value 'foobar' cannot be safely coerced to int64
+
+    Literals can also be used in a deferred context.
+
+    Here's an example of constructing a table of a column's type repeated for
+    every row:
+
+    >>> from ibis import _, selectors as s
+    >>> ibis.options.interactive = True
+    >>> t = ibis.examples.penguins.fetch()
+    >>> t.select(s.across(s.all(), ibis.literal(_.type(), type=str).name(_.get_name()))).head(1)
+    ┏━━━━━━━━━┳━━━━━━━━┳━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━┳━━━┓
+    ┃ species ┃ island ┃ bill_length_mm ┃ bill_depth_mm ┃ flipper_length_mm ┃ … ┃
+    ┡━━━━━━━━━╇━━━━━━━━╇━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━╇━━━┩
+    │ string  │ string │ string         │ string        │ string            │ … │
+    ├─────────┼────────┼────────────────┼───────────────┼───────────────────┼───┤
+    │ string  │ string │ float64        │ float64       │ int64             │ … │
+    └─────────┴────────┴────────────────┴───────────────┴───────────────────┴───┘
     """
     if isinstance(value, Expr):
         node = value.op()

ibis/tests/expr/test_literal.py

@@ -8,6 +8,7 @@
 
 import ibis
 import ibis.expr.datatypes as dt
+from ibis import _
 from ibis.common.collections import frozendict
 from ibis.expr.operations import Literal
 from ibis.tests.util import assert_pickle_roundtrip
@@ -166,3 +167,11 @@ def test_timestamp_literal_without_tz():
 def test_integer_as_decimal():
     lit = ibis.literal(12, type="decimal")
     assert lit.op().value == decimal.Decimal(12)
+
+
+def test_deferred(table):
+    expr = _.g.get_name()
+    dtype = _.g.type()
+    deferred = ibis.literal(expr, type=dtype)
+    result = deferred.resolve(table)
+    assert result.op().value == "g"

ibis/tests/test_strategies.py

@@ -4,12 +4,10 @@
 import hypothesis.strategies as st
 import pytest
 
-import ibis
 import ibis.expr.datatypes as dt
 import ibis.expr.schema as sch
 import ibis.expr.types as ir
 import ibis.tests.strategies as its
-from ibis.common.annotations import ValidationError
 
 
 @h.given(its.null_dtype)
@@ -173,10 +171,4 @@ def test_memtable(memtable):
     assert isinstance(memtable.schema(), sch.Schema)
 
 
-@h.given(its.all_dtypes())
-def test_deferred_literal(dtype):
-    with pytest.raises(ValidationError):
-        ibis.literal(ibis._.a, type=dtype)
-
-
 # TODO(kszucs): we enforce field name uniqueness in the schema, but we don't for Struct datatype