[GitHub] [iceberg] rdblue commented on a diff in pull request #4816: remove operations from base expressions; add in expression

[GitBox <gi...@apache.org>]

rdblue commented on code in PR #4816:
URL: https://github.com/apache/iceberg/pull/4816#discussion_r917458529


##########
python/pyiceberg/expressions/base.py:
##########
@@ -131,11 +75,115 @@ def __ge__(self, other):
 
 
 class BooleanExpression(ABC):
-    """base class for all boolean expressions"""
+    """Represents a boolean expression tree."""
 
     @abstractmethod
     def __invert__(self) -> "BooleanExpression":
-        ...
+        """Transform the Expression into its negated version."""
+
+
+class Bound(Generic[T], ABC):
+    """Represents a bound value expression."""
+
+    def eval(self, struct: StructProtocol):  # pylint: disable=W0613
+        ...  # pragma: no cover
+
+
+class Unbound(Generic[T, B], ABC):
+    """Represents an unbound expression node."""
+
+    @abstractmethod
+    def bind(self, schema: Schema, case_sensitive: bool) -> B:
+        ...  # pragma: no cover
+
+
+class Term(ABC):
+    """An expression that evaluates to a value."""
+
+
+class BaseReference(Generic[T], Term, ABC):
+    """Represents a variable reference in an expression."""
+
+
+class BoundTerm(Bound[T], Term):
+    """Represents a bound term."""
+
+
+class UnboundTerm(Unbound[T, BoundTerm[T]], Term):
+    """Represents an unbound term."""
+
+
+@dataclass(frozen=True)
+class BoundReference(BoundTerm[T], BaseReference[T]):
+    """A reference bound to a field in a schema
+
+    Args:
+        field (NestedField): A referenced field in an Iceberg schema
+        accessor (Accessor): An Accessor object to access the value at the field's position
+    """
+
+    field: NestedField
+    accessor: Accessor
+
+    def eval(self, struct: StructProtocol) -> Any:

Review Comment:
   I think that this should return `T`. That's the type that this reference produces. That may be a little difficult since `Accessor` is not parameterized by the type that it returns. If it is hard, we can fix it later.



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