[split/13] move rest of expr to expr_fn in datafusion-expr module (#1794)

* move expr functions to datafusion-expr expr_fn

* add module level comments

Author: jimexist

datafusion-expr/src/accumulator.rs

@@ -15,6 +15,8 @@
 // specific language governing permissions and limitations
 // under the License.
 
+//! Accumulator module contains the trait definition for aggregation function's accumulators.
+
 use arrow::array::ArrayRef;
 use datafusion_common::{Result, ScalarValue};
 use std::fmt::Debug;

datafusion-expr/src/aggregate_function.rs

@@ -15,6 +15,8 @@
 // specific language governing permissions and limitations
 // under the License.
 
+//! Aggregate function module contains all built-in aggregate functions definitions
+
 use datafusion_common::{DataFusionError, Result};
 use std::{fmt, str::FromStr};
 

datafusion-expr/src/built_in_function.rs

@@ -15,7 +15,7 @@
 // specific language governing permissions and limitations
 // under the License.
 
-//! Built-in functions
+//! Built-in functions module contains all the built-in functions definitions.
 
 use crate::Volatility;
 use datafusion_common::{DataFusionError, Result};

datafusion-expr/src/columnar_value.rs

@@ -15,6 +15,8 @@
 // specific language governing permissions and limitations
 // under the License.
 
+//! Columnar value module contains a set of types that represent a columnar value.
+
 use arrow::array::ArrayRef;
 use arrow::array::NullArray;
 use arrow::datatypes::DataType;

datafusion-expr/src/expr.rs

@@ -15,6 +15,8 @@
 // specific language governing permissions and limitations
 // under the License.
 
+//! Expr module contains core type definition for `Expr`.
+
 use crate::aggregate_function;
 use crate::built_in_function;
 use crate::expr_fn::binary_expr;
@@ -696,3 +698,28 @@ fn create_name(e: &Expr, input_schema: &DFSchema) -> Result<String> {
         )),
     }
 }
+
+#[cfg(test)]
+mod test {
+    use crate::expr_fn::col;
+    use crate::lit;
+
+    #[test]
+    fn test_not() {
+        assert_eq!(lit(1).not(), !lit(1));
+    }
+
+    #[test]
+    fn test_partial_ord() {
+        // Test validates that partial ord is defined for Expr using hashes, not
+        // intended to exhaustively test all possibilities
+        let exp1 = col("a") + lit(1);
+        let exp2 = col("a") + lit(2);
+        let exp3 = !(col("a") + lit(2));
+
+        assert!(exp1 < exp2);
+        assert!(exp2 > exp1);
+        assert!(exp2 > exp3);
+        assert!(exp3 < exp2);
+    }
+}