Added coalesce scalar function

src/core/etl/src/Flow/ETL/DSL/functions.php

@@ -14,8 +14,7 @@
 use Flow\ETL\Function\ArraySort\Sort;
 use Flow\ETL\Function\Between\Boundary;
 use Flow\ETL\Function\StyleConverter\StringStyles;
-use Flow\ETL\Function\{
-    All,
+use Flow\ETL\Function\{All,
     Any,
     ArrayGet,
     ArrayGetCollection,
@@ -32,6 +31,7 @@
     CallMethod,
     Capitalize,
     Cast,
+    Coalesce,
     Collect,
     CollectUnique,
     Combine,
@@ -74,8 +74,7 @@
     ToUpper,
     Ulid,
     Uuid,
-    When
-};
+    When};
 use Flow\ETL\Loader\StreamLoader\Output;
 use Flow\ETL\Loader\{ArrayLoader, CallbackLoader, MemoryLoader, StreamLoader, TransformerLoader};
 use Flow\ETL\Memory\Memory;
@@ -834,6 +833,12 @@ function cast(mixed $value, ScalarFunction|string|Type $type) : Cast
     return new Cast($value, $type);
 }
 
+#[DocumentationDSL(module: Module::CORE, type: DSLType::SCALAR_FUNCTION)]
+function coalesce(ScalarFunction ...$values) : Coalesce
+{
+    return new Coalesce(...$values);
+}
+
 #[DocumentationDSL(module: Module::CORE, type: DSLType::AGGREGATING_FUNCTION)]
 function count(EntryReference $function) : Count
 {

src/core/etl/src/Flow/ETL/Function/Coalesce.php

@@ -0,0 +1,38 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Flow\ETL\Function;
+
+use Flow\ETL\Row;
+
+final class Coalesce extends ScalarFunctionChain
+{
+    /**
+     * @param array<ScalarFunction> $values
+     */
+    private array $values;
+
+    public function __construct(
+        ScalarFunction ...$values,
+    ) {
+        $this->values = $values;
+    }
+
+    public function eval(Row $row) : mixed
+    {
+        foreach ($this->values as $value) {
+            try {
+                $result = (new Parameter($value))->eval($row);
+            } catch (\Exception $e) {
+                continue;
+            }
+
+            if ($result !== null) {
+                return $result;
+            }
+        }
+
+        return null;
+    }
+}

src/core/etl/src/Flow/ETL/Function/ScalarFunctionChain.php

@@ -76,6 +76,11 @@ public function cast(ScalarFunction|string|Type $type) : self
         return new Cast($this, $type);
     }
 
+    public function coalesce(ScalarFunction ...$params) : self
+    {
+        return new Coalesce($this, ...$params);
+    }
+
     public function concat(ScalarFunction|string ...$params) : self
     {
         return new Concat($this, ...$params);

src/core/etl/tests/Flow/ETL/Tests/Unit/Function/CoalesceTest.php

@@ -0,0 +1,35 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Flow\ETL\Tests\Unit\Function;
+
+use function Flow\ETL\DSL\{coalesce, int_entry, lit, ref, row};
+use PHPUnit\Framework\TestCase;
+
+final class CoalesceTest extends TestCase
+{
+    public function test_coalesce_entries() : void
+    {
+        self::assertSame(
+            1,
+            coalesce(ref('name'), ref('id'), lit('N/A'))->eval(row(int_entry('id', 1)))
+        );
+    }
+
+    public function test_coalesce_on_lit_and_non_existing_entries() : void
+    {
+        self::assertSame(
+            'N/A',
+            coalesce(ref('non_existing'), ref('string'), lit('N/A'))->eval(row(int_entry('id', 1)))
+        );
+    }
+
+    public function test_coalesce_on_ref() : void
+    {
+        self::assertSame(
+            1,
+            ref('name')->coalesce(ref('id'), lit('N/A'))->eval(row(int_entry('id', 1)))
+        );
+    }
+}