concat() support multiple string arguments

core/src/main/java/org/opensearch/sql/expression/function/BuiltinFunctionRepository.java

@@ -182,6 +182,13 @@ private FunctionBuilder getFunctionBuilder(
     if (isCastFunction(functionName) || sourceTypes.equals(targetTypes)) {
       return funcBuilder;
     }
+    // For functions with variable number of args (ex: concat())
+    // targetTypes will always be empty (as the function signature is not fixed),
+    // and failure will occur.
+    // So, in this case sourceTypes are passed instead of targetTypes to address that.
+    if (functionResolverMap.get(functionName) instanceof VarargsFunctionResolver) {
+      return castArguments(sourceTypes, sourceTypes, funcBuilder);
+    }
     return castArguments(sourceTypes,
         targetTypes, funcBuilder);
   }

core/src/main/java/org/opensearch/sql/expression/function/FunctionDSL.java

@@ -9,12 +9,14 @@
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
+import java.util.concurrent.atomic.AtomicInteger;
 import java.util.function.Function;
 import java.util.stream.Collectors;
 import lombok.experimental.UtilityClass;
 import org.apache.commons.lang3.tuple.Pair;
 import org.opensearch.sql.data.model.ExprValue;
 import org.opensearch.sql.data.model.ExprValueUtils;
+import org.opensearch.sql.data.type.ExprCoreType;
 import org.opensearch.sql.data.type.ExprType;
 import org.opensearch.sql.expression.Expression;
 import org.opensearch.sql.expression.FunctionExpression;
@@ -58,6 +60,39 @@ public static DefaultFunctionResolver define(FunctionName functionName, List<
     return builder.build();
   }
 
+  /**
+   * Define varargs function with implementation.
+   *
+   * @param functionName function name.
+   * @param functions    a list of function implementation.
+   * @return VarargsFunctionResolver.
+   */
+  public static VarargsFunctionResolver defineVarargsFunction(FunctionName functionName,
+          SerializableFunction<FunctionName,
+                  Pair<FunctionSignature, FunctionBuilder>>... functions) {
+    return defineVarargsFunction(functionName, List.of(functions));
+  }
+
+  /**
+   * Define varargs function with implementation.
+   *
+   * @param functionName function name.
+   * @param functions    a list of function implementation.
+   * @return VarargsFunctionResolver.
+   */
+  public static VarargsFunctionResolver defineVarargsFunction(FunctionName functionName, List<
+          SerializableFunction<FunctionName, Pair<FunctionSignature, FunctionBuilder>>> functions) {
+
+    VarargsFunctionResolver.VarargsFunctionResolverBuilder builder =
+            VarargsFunctionResolver.builder();
+    builder.functionName(functionName);
+    for (SerializableFunction<FunctionName, Pair<FunctionSignature, FunctionBuilder>> func
+            : functions) {
+      Pair<FunctionSignature, FunctionBuilder> functionBuilder = func.apply(functionName);
+      builder.functionBundle(functionBuilder.getKey(), functionBuilder.getValue());
+    }
+    return builder.build();
+  }
 
   /**
    * Implementation of no args function that uses FunctionProperties.
@@ -212,6 +247,56 @@ public static SerializableFunction<FunctionName, Pair<FunctionSignature, Functio
     return implWithProperties((fp, arg) -> function.apply(arg), returnType, argsType);
   }
 
+  /**
+   * Varargs Function Implementation.
+   * This implementation considers 1...n args of the same type.
+   *
+   * @param function   {@link ExprValue} based varargs function.
+   * @param returnType return type.
+   * @param argsType   argument type.
+   * @return Varargs Function Implementation.
+   */
+  public static SerializableFunction<FunctionName, Pair<FunctionSignature, FunctionBuilder>> impl(
+      SerializableVarargsFunction<ExprValue, ExprValue> function,
+      ExprType returnType,
+      ExprType argsType,
+      boolean withVarargs) {
+
+    return functionName -> {
+      AtomicInteger argsCount = new AtomicInteger(0);
+      FunctionBuilder functionBuilder =
+              (functionProperties, arguments) -> new FunctionExpression(functionName, arguments) {
+                @Override
+                public ExprValue valueOf(Environment<Expression, ExprValue> valueEnv) {
+                  argsCount.set(arguments.size());
+                  ExprValue[] args = arguments.stream()
+                          .map(arg -> arg.valueOf(valueEnv))
+                          .collect(Collectors.toList())
+                          .toArray(new ExprValue[arguments.size()]);
+
+                  return function.apply(args);
+                }
+
+                @Override
+                public ExprType type() {
+                  return returnType;
+                }
+
+                @Override
+                public String toString() {
+                  return String.format("%s(%s)", functionName, arguments.stream()
+                          .map(Object::toString)
+                          .collect(Collectors.joining(", ")));
+                }
+              };
+      ExprCoreType[] argsTypes = new ExprCoreType[argsCount.get()];
+      Arrays.fill(argsTypes, argsType);
+      FunctionSignature functionSignature =
+              new FunctionSignature(functionName, List.of(argsTypes));
+      return Pair.of(functionSignature, functionBuilder);
+    };
+  }
+
   /**
    * Binary Function Implementation.
    *
@@ -323,13 +408,29 @@ public SerializableTriFunction<ExprValue, ExprValue, ExprValue, ExprValue> nullM
     };
   }
 
+  /**
+   * Wrapper the varargs ExprValue function with default NULL and MISSING handling.
+   */
+  public SerializableVarargsFunction<ExprValue, ExprValue> nullMissingHandling(
+          SerializableVarargsFunction<ExprValue, ExprValue> function, boolean withVarargs) {
+    return (args) -> {
+      if (Arrays.stream(args).anyMatch(ExprValue::isMissing)) {
+        return ExprValueUtils.missingValue();
+      } else if (Arrays.stream(args).anyMatch(ExprValue::isNull)) {
+        return ExprValueUtils.nullValue();
+      } else {
+        return function.apply(args);
+      }
+    };
+  }
+
   /**
    * Wrapper the unary ExprValue function that is aware of FunctionProperties,
    * with default NULL and MISSING handling.
    */
   public static SerializableBiFunction<FunctionProperties, ExprValue, ExprValue>
         nullMissingHandlingWithProperties(
-      SerializableBiFunction<FunctionProperties, ExprValue, ExprValue>  implementation) {
+      SerializableBiFunction<FunctionProperties, ExprValue, ExprValue> implementation) {
     return (functionProperties, v1) -> {
       if (v1.isMissing()) {
         return ExprValueUtils.missingValue();

core/src/main/java/org/opensearch/sql/expression/function/SerializableVarargsFunction.java

@@ -0,0 +1,22 @@
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+
+package org.opensearch.sql.expression.function;
+
+import java.io.Serializable;
+
+/**
+ * Serializable Varargs Function.
+ */
+public interface SerializableVarargsFunction<T, R> extends Serializable {
+  /**
+   * Applies this function to the given arguments.
+   *
+   * @param t the function argument
+   * @return the function result
+   */
+  R apply(T... t);
+}

core/src/main/java/org/opensearch/sql/expression/function/VarargsFunctionResolver.java

@@ -0,0 +1,69 @@
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package org.opensearch.sql.expression.function;
+
+import java.util.AbstractMap;
+import java.util.Map;
+import java.util.PriorityQueue;
+import java.util.Set;
+import java.util.stream.Collectors;
+import lombok.Builder;
+import lombok.Getter;
+import lombok.RequiredArgsConstructor;
+import lombok.Singular;
+import org.apache.commons.lang3.tuple.Pair;
+import org.opensearch.sql.exception.ExpressionEvaluationException;
+
+/**
+ * The Function Resolver hold the overload {@link FunctionBuilder} implementation.
+ * is composed by {@link FunctionName} which identified the function name
+ * and a map of {@link FunctionSignature} and {@link FunctionBuilder}
+ * to represent the overloaded implementation
+ */
+@Builder
+@RequiredArgsConstructor
+public class VarargsFunctionResolver implements FunctionResolver {
+  @Getter
+  private final FunctionName functionName;
+  @Singular("functionBundle")
+  private final Map<FunctionSignature, FunctionBuilder> functionBundle;
+
+  /**
+   * Resolve the {@link FunctionBuilder} by using input {@link FunctionSignature}.
+   * If the {@link FunctionBuilder} exactly match the input {@link FunctionSignature}, return it.
+   * If applying the widening rule, found the most match one, return it.
+   * If nothing found, throw {@link ExpressionEvaluationException}
+   *
+   * @return function signature and its builder
+   */
+  @Override
+  public Pair<FunctionSignature, FunctionBuilder> resolve(FunctionSignature unresolvedSignature) {
+    PriorityQueue<Map.Entry<Integer, FunctionSignature>> functionMatchQueue = new PriorityQueue<>(
+            Map.Entry.comparingByKey());
+
+    for (FunctionSignature functionSignature : functionBundle.keySet()) {
+      functionMatchQueue.add(
+              new AbstractMap.SimpleEntry<>(unresolvedSignature.match(functionSignature),
+                      functionSignature));
+    }
+    Map.Entry<Integer, FunctionSignature> bestMatchEntry = functionMatchQueue.peek();
+    if (unresolvedSignature.getParamTypeList().isEmpty()) {
+      throw new ExpressionEvaluationException(
+              String.format("%s function expected %s, but get %s", functionName,
+                      formatFunctions(functionBundle.keySet()),
+                      unresolvedSignature.formatTypes()
+              ));
+    } else {
+      FunctionSignature resolvedSignature = bestMatchEntry.getValue();
+      return Pair.of(resolvedSignature, functionBundle.get(resolvedSignature));
+    }
+  }
+
+  private String formatFunctions(Set<FunctionSignature> functionSignatures) {
+    return functionSignatures.stream().map(FunctionSignature::formatTypes)
+            .collect(Collectors.joining(",", "{", "}"));
+  }
+}

core/src/main/java/org/opensearch/sql/expression/text/TextFunction.java

@@ -9,9 +9,12 @@
 import static org.opensearch.sql.data.type.ExprCoreType.INTEGER;
 import static org.opensearch.sql.data.type.ExprCoreType.STRING;
 import static org.opensearch.sql.expression.function.FunctionDSL.define;
+import static org.opensearch.sql.expression.function.FunctionDSL.defineVarargsFunction;
 import static org.opensearch.sql.expression.function.FunctionDSL.impl;
 import static org.opensearch.sql.expression.function.FunctionDSL.nullMissingHandling;
 
+import java.util.Arrays;
+import java.util.stream.Collectors;
 import lombok.experimental.UtilityClass;
 import org.opensearch.sql.data.model.ExprIntegerValue;
 import org.opensearch.sql.data.model.ExprStringValue;
@@ -22,7 +25,7 @@
 import org.opensearch.sql.expression.function.FunctionName;
 import org.opensearch.sql.expression.function.SerializableBiFunction;
 import org.opensearch.sql.expression.function.SerializableTriFunction;
-
+import org.opensearch.sql.expression.function.VarargsFunctionResolver;
 
 /**
  * The definition of text functions.
@@ -141,16 +144,16 @@ private DefaultFunctionResolver upper() {
   }
 
   /**
-   * TODO: https://github.com/opendistro-for-elasticsearch/sql/issues/710
-   *  Extend to accept variable argument amounts.
    * Concatenates a list of Strings.
    * Supports following signatures:
-   * (STRING, STRING) -> STRING
+   * (STRING, STRING, ...., STRING) -> STRING
    */
-  private DefaultFunctionResolver concat() {
-    return define(BuiltinFunctionName.CONCAT.getName(),
-        impl(nullMissingHandling((str1, str2) ->
-            new ExprStringValue(str1.stringValue() + str2.stringValue())), STRING, STRING, STRING));
+  private VarargsFunctionResolver concat() {
+    return defineVarargsFunction(BuiltinFunctionName.CONCAT.getName(),
+        impl(nullMissingHandling(strings ->
+            new ExprStringValue(Arrays.stream(strings)
+                    .map(ExprValue::stringValue)
+                    .collect(Collectors.joining())), true), STRING, STRING, true));
   }
 
   /**

core/src/test/java/org/opensearch/sql/expression/function/BuiltinFunctionRepositoryTest.java

@@ -238,6 +238,37 @@ void resolve_unregistered() {
     assertEquals("unsupported function name: unknown", exception.getMessage());
   }
 
+  @Test
+  void resolve_should_cast_arguments_for_varargs_function() {
+    FunctionSignature unresolvedSignature = new FunctionSignature(
+            mockFunctionName, ImmutableList.of(STRING, STRING, STRING));
+    FunctionSignature resolvedSignature = new FunctionSignature(
+            mockFunctionName, Collections.emptyList());
+
+    VarargsFunctionResolver varargsFunctionResolver = mock(VarargsFunctionResolver.class);
+    FunctionBuilder funcBuilder = mock(FunctionBuilder.class);
+
+    when(mockFunctionName.getFunctionName()).thenReturn("mockFunction");
+    when(mockExpression.toString()).thenReturn("string");
+    when(mockNamespaceMap.get(DEFAULT_NAMESPACE)).thenReturn(mockMap);
+    when(mockNamespaceMap.containsKey(DEFAULT_NAMESPACE)).thenReturn(true);
+    when(mockMap.containsKey(eq(mockFunctionName))).thenReturn(true);
+    when(mockMap.get(eq(mockFunctionName))).thenReturn(varargsFunctionResolver);
+    when(varargsFunctionResolver.resolve(eq(unresolvedSignature))).thenReturn(
+            Pair.of(resolvedSignature, funcBuilder));
+    repo.register(varargsFunctionResolver);
+    // Relax unnecessary stubbing check because error case test doesn't call this
+    lenient().doAnswer(invocation ->
+            new FakeFunctionExpression(mockFunctionName, invocation.getArgument(1))
+    ).when(funcBuilder).apply(eq(functionProperties), any());
+
+    FunctionImplementation function =
+            repo.resolve(Collections.singletonList(DEFAULT_NAMESPACE), unresolvedSignature)
+                    .apply(functionProperties,
+                            ImmutableList.of(mockExpression, mockExpression, mockExpression));
+    assertEquals("mockFunction(string, string, string)", function.toString());
+  }
+
   private FunctionSignature registerFunctionResolver(FunctionName funcName,
                                                      ExprType sourceType,
                                                      ExprType targetType) {

core/src/test/java/org/opensearch/sql/expression/function/FunctionDSLTestBase.java

@@ -59,6 +59,8 @@ public int compareTo(ExprValue o) {
       twoArgs = (v1, v2) -> ANY;
   static final SerializableTriFunction<ExprValue, ExprValue, ExprValue, ExprValue>
       threeArgs = (v1, v2, v3) -> ANY;
+  static final SerializableVarargsFunction<ExprValue, ExprValue>
+      varrgs = (v1) -> ANY;
   @Mock
   FunctionProperties mockProperties;
 }

core/src/test/java/org/opensearch/sql/expression/function/FunctionDSLimplVarargsTest.java

@@ -0,0 +1,32 @@
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package org.opensearch.sql.expression.function;
+
+import static org.opensearch.sql.expression.function.FunctionDSL.impl;
+
+import java.util.List;
+import org.apache.commons.lang3.tuple.Pair;
+import org.opensearch.sql.expression.DSL;
+import org.opensearch.sql.expression.Expression;
+
+class FunctionDSLimplVarargsTest extends FunctionDSLimplTestBase {
+
+  @Override
+  SerializableFunction<FunctionName, Pair<FunctionSignature, FunctionBuilder>>
+      getImplementationGenerator() {
+    return impl(varrgs, ANY_TYPE, ANY_TYPE, true);
+  }
+
+  @Override
+  List<Expression> getSampleArguments() {
+    return List.of(DSL.literal(ANY));
+  }
+
+  @Override
+  String getExpected_toString() {
+    return "sample(ANY)";
+  }
+}