diff --git a/velox/docs/functions/spark/string.rst b/velox/docs/functions/spark/string.rst
index 16c0e0c80e2c..82095514539b 100644
--- a/velox/docs/functions/spark/string.rst
+++ b/velox/docs/functions/spark/string.rst
@@ -22,11 +22,11 @@ Unless specified otherwise, all functions return NULL if at least one of the arg
 
 .. spark:function:: concat_ws(separator, [string]/[array<string>], ...) -> varchar
 
-   Returns the concatenation for ``string`` and all elements in ``array<string>``, separated
+   Returns the concatenation of ``string`` and all elements in ``array<string>``, separated
    by ``separator``. ``separator`` can be empty string. It takes variable number of remaining
    arguments. And ``string`` & ``array<string>`` can be used in combination. NULL element is
    skipped in the concatenation. If ``separator`` is NULL, returns NULL, regardless of the
-   following inputs. For non-NULL ``separator``, if only it is provided or all remaining inputs
+   following inputs. For non-NULL ``separator``, if no remaining input or all remaining inputs
    are NULL, returns an empty string. ::
 
         SELECT concat_ws('~', 'a', 'b', 'c'); -- 'a~b~c'
diff --git a/velox/expression/tests/SparkExpressionFuzzerTest.cpp b/velox/expression/tests/SparkExpressionFuzzerTest.cpp
index 9ac8381a66b8..18262da2df80 100644
--- a/velox/expression/tests/SparkExpressionFuzzerTest.cpp
+++ b/velox/expression/tests/SparkExpressionFuzzerTest.cpp
@@ -55,9 +55,10 @@ int main(int argc, char** argv) {
       "replace",
       "might_contain",
       "unix_timestamp",
-      // Skip concat_ws as it triggers a test failure due to an incorrect
-      // expression generation from fuzzer:
-      // https://github.com/facebookincubator/velox/issues/6590
+      // Skip concat_ws due to the below issue:
+      // We use "any" type in its signature to allow mixed
+      // using of VARCHAR & ARRAY<VARCHAR>. But the fuzzer
+      // couldn't generate correct expressions for it.
       "concat_ws"};
 
   // Required by spark_partition_id function.
diff --git a/velox/functions/sparksql/String.cpp b/velox/functions/sparksql/String.cpp
index c8b487c3ad34..ad59b23f7fe5 100644
--- a/velox/functions/sparksql/String.cpp
+++ b/velox/functions/sparksql/String.cpp
@@ -112,9 +112,9 @@ void doApply(
   std::vector<column_index_t> argMapping;
   std::vector<std::string> constantStrings;
   auto numArgs = args.size();
-  // Save constant values to constantStrings_.
-  // Identify and combine consecutive constant inputs.
   argMapping.reserve(numArgs - 1);
+  // Save intermediate result for consecutive constant string args.
+  // They are concatenated in advance.
   constantStrings.reserve(numArgs - 1);
   // For each array arg, save rawSizes, rawOffsets, indices, and elements
   // BaseBector.
@@ -142,8 +142,8 @@ void doApply(
     }
     // Handles string arg.
     argMapping.push_back(i);
-    // Cannot concat string args in advance.
     if (!separator.has_value()) {
+      // Cannot concat consecutive constant string args in advance.
       constantStrings.push_back("");
       continue;
     }
@@ -169,7 +169,7 @@ void doApply(
     }
   }
 
-  // Number of string columns after combined constant ones.
+  // Number of string columns after combined consecutive constant ones.
   auto numStringCols = constantStrings.size();
   // For column string arg decoding.
   std::vector<exec::LocalDecodedVector> decodedStringArgs;
@@ -369,7 +369,9 @@ std::shared_ptr<exec::VectorFunction> makeLength(
 
 std::vector<std::shared_ptr<exec::FunctionSignature>> concatWsSignatures() {
   return {// The second and folowing arguments are varchar or array(varchar).
-          // The argument type will be checked in makeConcatWs.
+          // Use "any" to allow the mixed using of these two types. The
+          // argument type will be checked in makeConcatWs.
+          //
           // varchar, [varchar], [array(varchar)], ... -> varchar.
           exec::FunctionSignatureBuilder()
               .returnType("varchar")