add description for each function

Signed-off-by: xinyual <xinyual@amazon.com>

Author: xinyual

core/src/main/java/org/opensearch/sql/expression/function/CollectionUDF/ArrayFunctionImpl.java

@@ -28,6 +28,11 @@
 import org.opensearch.sql.expression.function.UDFOperandMetadata;
 
 // TODO: Support array of mixture types.
+
+/**
+ * create an array with input values. We will infer a least restricted type, for example array(1,
+ * "demo") -> ["1", "demo"]
+ */
 public class ArrayFunctionImpl extends ImplementorUDF {
   public ArrayFunctionImpl() {
     super(new ArrayImplementor(), NullPolicy.ANY);

core/src/main/java/org/opensearch/sql/expression/function/CollectionUDF/ExistsFunctionImpl.java

@@ -19,6 +19,11 @@
 import org.opensearch.sql.expression.function.ImplementorUDF;
 import org.opensearch.sql.expression.function.UDFOperandMetadata;
 
+/**
+ * The function check whether existing one of element inside array can meet the lambda function. The
+ * function should also return boolean. For example, array=array(1, -2, -1), forall(array, x -> x >
+ * 0) = true
+ */
 public class ExistsFunctionImpl extends ImplementorUDF {
   public ExistsFunctionImpl() {
     super(new ExistsImplementor(), NullPolicy.ALL);

core/src/main/java/org/opensearch/sql/expression/function/CollectionUDF/FilterFunctionImpl.java

@@ -20,6 +20,10 @@
 import org.opensearch.sql.expression.function.ImplementorUDF;
 import org.opensearch.sql.expression.function.UDFOperandMetadata;
 
+/**
+ * The function filter the element in the array by the lambda function. The function should return
+ * boolean. For example, array=array(1, 2, -1) filter(array, x -> x > 0) = [1, 2]
+ */
 public class FilterFunctionImpl extends ImplementorUDF {
   public FilterFunctionImpl() {
     super(new FilterImplementor(), NullPolicy.ANY);

core/src/main/java/org/opensearch/sql/expression/function/CollectionUDF/ForallFunctionImpl.java

@@ -19,6 +19,10 @@
 import org.opensearch.sql.expression.function.ImplementorUDF;
 import org.opensearch.sql.expression.function.UDFOperandMetadata;
 
+/**
+ * The function check whether all element inside array can meet the lambda function. The function
+ * should also return boolean. For example, array=array(1, 2, -1), forall(array, x -> x > 0) = false
+ */
 public class ForallFunctionImpl extends ImplementorUDF {
   public ForallFunctionImpl() {
     super(new ForallImplementor(), NullPolicy.ALL);

core/src/main/java/org/opensearch/sql/expression/function/CollectionUDF/ReduceFunctionImpl.java

@@ -30,6 +30,11 @@
 import org.opensearch.sql.expression.function.ImplementorUDF;
 import org.opensearch.sql.expression.function.UDFOperandMetadata;
 
+/**
+ * The function will first use acc_function to go through all element and return value to the acc.
+ * Then apply reduce function to the acc if exists. For example, array=array(1, 2, 3), reduce(array,
+ * 0, (acc, x) -> acc + x) = 6, reduce(array, 0, (acc, x) -> acc + x, acc -> acc * 10) = 60
+ */
 public class ReduceFunctionImpl extends ImplementorUDF {
   public ReduceFunctionImpl() {
     super(new ReduceImplementor(), NullPolicy.ANY);

core/src/main/java/org/opensearch/sql/expression/function/CollectionUDF/TransformFunctionImpl.java

@@ -28,6 +28,12 @@
 import org.opensearch.sql.expression.function.ImplementorUDF;
 import org.opensearch.sql.expression.function.UDFOperandMetadata;
 
+/**
+ * The function transform the element of array one by one using lambda. For example, array=array(1,
+ * 2, 3), transform(array, x -> x + 2) = [3, 4, 5] Transform can accept one more argument like (x,
+ * i) -> x + i, where i is the index of element in array. For example, array=array(1, 2, 3),
+ * transform(array, (x, i) -> x + i) = [1, 3, 5]
+ */
 public class TransformFunctionImpl extends ImplementorUDF {
   public TransformFunctionImpl() {
     super(new TransformImplementor(), NullPolicy.ANY);