feat: Expression system. (apache#132)

* feat: Expressions

* Fix comments

* Refactor expression to be more similar to iceberg model

* Fix typo

crates/iceberg/src/expr/mod.rs

@@ -0,0 +1,42 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+//! This module contains expressions.
+
+mod term;
+pub use term::*;
+mod predicate;
+pub use predicate::*;
+
+/// Predicate operators used in expressions.
+#[allow(missing_docs)]
+pub enum PredicateOperator {
+    IsNull,
+    NotNull,
+    IsNan,
+    NotNan,
+    LessThan,
+    LessThanOrEq,
+    GreaterThan,
+    GreaterThanOrEq,
+    Eq,
+    NotEq,
+    In,
+    NotIn,
+    StartsWith,
+    NotStartsWith,
+}

crates/iceberg/src/expr/predicate.rs

@@ -0,0 +1,93 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+//! This module contains predicate expressions.
+//! Predicate expressions are used to filter data, and evaluates to a boolean value. For example,
+//! `a > 10` is a predicate expression, and it evaluates to `true` if `a` is greater than `10`,
+
+use crate::expr::{BoundReference, PredicateOperator, UnboundReference};
+use crate::spec::Literal;
+use std::collections::HashSet;
+
+/// Logical expression, such as `AND`, `OR`, `NOT`.
+pub struct LogicalExpression<T, const N: usize> {
+    inputs: [Box<T>; N],
+}
+
+/// Unary predicate, for example, `a IS NULL`.
+pub struct UnaryExpression<T> {
+    /// Operator of this predicate, must be single operand operator.
+    op: PredicateOperator,
+    /// Term of this predicate, for example, `a` in `a IS NULL`.
+    term: T,
+}
+
+/// Binary predicate, for example, `a > 10`.
+pub struct BinaryExpression<T> {
+    /// Operator of this predicate, must be binary operator, such as `=`, `>`, `<`, etc.
+    op: PredicateOperator,
+    /// Term of this predicate, for example, `a` in `a > 10`.
+    term: T,
+    /// Literal of this predicate, for example, `10` in `a > 10`.
+    literal: Literal,
+}
+
+/// Set predicates, for example, `a in (1, 2, 3)`.
+pub struct SetExpression<T> {
+    /// Operator of this predicate, must be set operator, such as `IN`, `NOT IN`, etc.
+    op: PredicateOperator,
+    /// Term of this predicate, for example, `a` in `a in (1, 2, 3)`.
+    term: T,
+    /// Literals of this predicate, for example, `(1, 2, 3)` in `a in (1, 2, 3)`.
+    literals: HashSet<Literal>,
+}
+
+/// Unbound predicate expression before binding to a schema.
+pub enum UnboundPredicate {
+    /// And predicate, for example, `a > 10 AND b < 20`.
+    And(LogicalExpression<UnboundPredicate, 2>),
+    /// Or predicate, for example, `a > 10 OR b < 20`.
+    Or(LogicalExpression<UnboundPredicate, 2>),
+    /// Not predicate, for example, `NOT (a > 10)`.
+    Not(LogicalExpression<UnboundPredicate, 1>),
+    /// Unary expression, for example, `a IS NULL`.
+    Unary(UnaryExpression<UnboundReference>),
+    /// Binary expression, for example, `a > 10`.
+    Binary(BinaryExpression<UnboundReference>),
+    /// Set predicates, for example, `a in (1, 2, 3)`.
+    Set(SetExpression<UnboundReference>),
+}
+
+/// Bound predicate expression after binding to a schema.
+pub enum BoundPredicate {
+    /// An expression always evaluates to true.
+    AlwaysTrue,
+    /// An expression always evaluates to false.
+    AlwaysFalse,
+    /// An expression combined by `AND`, for example, `a > 10 AND b < 20`.
+    And(LogicalExpression<BoundPredicate, 2>),
+    /// An expression combined by `OR`, for example, `a > 10 OR b < 20`.
+    Or(LogicalExpression<BoundPredicate, 2>),
+    /// An expression combined by `NOT`, for example, `NOT (a > 10)`.
+    Not(LogicalExpression<BoundPredicate, 1>),
+    /// Unary expression, for example, `a IS NULL`.
+    Unary(UnaryExpression<BoundReference>),
+    /// Binary expression, for example, `a > 10`.
+    Binary(BinaryExpression<BoundReference>),
+    /// Set predicates, for example, `a in (1, 2, 3)`.
+    Set(SetExpression<BoundReference>),
+}

crates/iceberg/src/expr/term.rs

@@ -0,0 +1,37 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+//! Term definition.
+
+use crate::spec::NestedFieldRef;
+
+/// Unbound term before binding to a schema.
+pub type UnboundTerm = UnboundReference;
+
+/// A named reference in an unbound expression.
+/// For example, `a` in `a > 10`.
+pub struct UnboundReference {
+    name: String,
+}
+
+/// A named reference in a bound expression after binding to a schema.
+pub struct BoundReference {
+    field: NestedFieldRef,
+}
+
+/// Bound term after binding to a schema.
+pub type BoundTerm = BoundReference;

crates/iceberg/src/lib.rs

@@ -28,6 +28,7 @@ pub use error::ErrorKind;
 pub use error::Result;
 
 mod catalog;
+
 pub use catalog::Catalog;
 pub use catalog::Namespace;
 pub use catalog::NamespaceIdent;
@@ -44,5 +45,7 @@ mod avro;
 pub mod io;
 pub mod spec;
 
+#[allow(dead_code)]
+pub mod expr;
 pub mod transaction;
 pub mod transform;