Added Select / Drop reusable transformations:

Author: norberttech

documentation/components/core/core.md

@@ -79,6 +79,7 @@ For detailed information about specific DataFrame operations, see the following
 
 ### Core Operations
 - **[Building Blocks](building-blocks.md)** - Understanding Rows, Entries, and basic data structures
+- **[Transformations](transformations.md)** - Reusable DataFrame transformations and the Transformation interface
 - **[Select/Drop](select-drop.md)** - Column selection and removal
 - **[Rename](rename.md)** - Column renaming strategies
 - **[Map](map.md)** - Row transformations and data mapping

documentation/components/core/transformations.md

@@ -0,0 +1,257 @@
+# Transformations
+
+- [⬅️️ Back](/documentation/components/core/core.md)
+
+## Introduction
+
+Transformations are a powerful abstraction in Flow PHP that allow you to modify DataFrames in a composable and reusable
+way. Unlike Transformers which operate on individual Rows, Transformations work at the DataFrame level, providing access
+to the full processing pipeline.
+
+Every Transformation implements the `Transformation` interface with a single method:
+
+```php
+interface Transformation
+{
+    public function transform(DataFrame $dataFrame): DataFrame;
+}
+```
+
+## Using Transformations
+
+Transformations can be applied to DataFrames using two methods:
+
+- `with()` - Applies one or more transformations
+- `transform()` - Alias for `with()`, provides semantic clarity
+
+Both methods accept `Transformation` objects directly or through convenient DSL functions.
+
+```php
+use function Flow\ETL\DSL\{df, from_array, select, drop};
+
+// Using with()
+df()
+    ->read(from_array([/* ... */]))
+    ->with(select('id', 'name'))
+    ->write(to_output())
+    ->run();
+
+// Using transform() - identical behavior
+df()
+    ->read(from_array([/* ... */]))
+    ->transform(drop('temporary_column'))
+    ->write(to_output())
+    ->run();
+```
+
+## Available Transformations
+
+### Select
+
+Select specific columns from the DataFrame, keeping only the columns you need.
+
+```php
+use function Flow\ETL\DSL\{df, from_array, select, ref};
+
+// Select columns by name
+df()
+    ->read(from_array([
+        ['id' => 1, 'name' => 'Alice', 'age' => 25, 'city' => 'New York'],
+        ['id' => 2, 'name' => 'Bob', 'age' => 30, 'city' => 'Los Angeles'],
+    ]))
+    ->with(select('id', 'name'))
+    ->write(to_output())
+    ->run();
+
+// Select using References for more control
+df()
+    ->read(from_array([/* ... */]))
+    ->with(select(ref('id'), ref('city')))
+    ->write(to_output())
+    ->run();
+```
+
+### Drop
+
+Remove unwanted columns from the DataFrame, keeping all other columns.
+
+```php
+use function Flow\ETL\DSL\{df, from_array, drop, ref};
+
+// Drop columns by name
+df()
+    ->read(from_array([
+        ['id' => 1, 'password' => 'secret', 'name' => 'Alice'],
+        ['id' => 2, 'password' => 'hidden', 'name' => 'Bob'],
+    ]))
+    ->with(drop('password'))
+    ->write(to_output())
+    ->run();
+
+// Drop using References
+df()
+    ->read(from_array([/* ... */]))
+    ->with(drop(ref('temp_column'), ref('debug_info')))
+    ->write(to_output())
+    ->run();
+```
+
+### Batch Size
+
+Control memory usage by setting the batch size for processing. Smaller batch sizes reduce memory consumption when
+processing large datasets.
+
+```php
+use function Flow\ETL\DSL\{df, from_csv, batch_size};
+
+// Process large CSV file in batches of 100 rows
+df()
+    ->read(from_csv('huge_file.csv'))
+    ->with(batch_size(100))
+    ->write(to_database('users'))
+    ->run();
+```
+
+### Add Row Index
+
+Add an index column to each row, useful for tracking row position or creating unique identifiers.
+
+```php
+use function Flow\ETL\DSL\{df, from_array, add_row_index};
+use Flow\ETL\Transformation\AddRowIndex\StartFrom;
+
+// Add default index starting from 0
+df()
+    ->read(from_array([
+        ['name' => 'Alice'],
+        ['name' => 'Bob'],
+    ]))
+    ->with(add_row_index())
+    ->write(to_output())
+    ->run();
+// Output: [['index' => 0, 'name' => 'Alice'], ['index' => 1, 'name' => 'Bob']]
+
+// Custom column name and start from 1
+df()
+    ->read(from_array([/* ... */]))
+    ->with(add_row_index('row_number', StartFrom::ONE))
+    ->write(to_output())
+    ->run();
+```
+
+### Limit
+
+Restrict the number of rows processed, useful for debugging or sampling data.
+
+```php
+use function Flow\ETL\DSL\{df, from_database, limit};
+
+// Process only first 1000 rows
+df()
+    ->read(from_database('large_table'))
+    ->with(limit(1000))
+    ->write(to_csv('sample.csv'))
+    ->run();
+
+// Remove limit (process all rows)
+df()
+    ->read(from_array([/* ... */]))
+    ->with(limit(null))
+    ->write(to_output())
+    ->run();
+```
+
+### Mask Columns
+
+Replace column values with a mask string, useful for hiding sensitive information.
+
+```php
+use function Flow\ETL\DSL\{df, from_array, mask_columns};
+
+// Mask sensitive columns with default mask
+df()
+    ->read(from_array([
+        ['name' => 'Alice', 'ssn' => '123-45-6789', 'salary' => 50000],
+        ['name' => 'Bob', 'ssn' => '987-65-4321', 'salary' => 60000],
+    ]))
+    ->with(mask_columns(['ssn', 'salary']))
+    ->write(to_output())
+    ->run();
+// Output: [['name' => 'Alice', 'ssn' => '******', 'salary' => '******'], ...]
+
+// Use custom mask
+df()
+    ->read(from_array([/* ... */]))
+    ->with(mask_columns(['credit_card'], '[REDACTED]'))
+    ->write(to_output())
+    ->run();
+```
+
+## Chaining Transformations
+
+Transformations can be chained together to create complex data processing pipelines:
+
+```php
+use function Flow\ETL\DSL\{df, from_csv, select, add_row_index, limit, batch_size};
+
+df()
+    ->read(from_csv('users.csv'))
+    ->with(select('id', 'name', 'email'))     // Keep only needed columns
+    ->with(add_row_index('row_num'))          // Add row numbers
+    ->with(limit(1000))                       // Process only first 1000
+    ->with(batch_size(50))                    // Process in batches of 50
+    ->write(to_json('users_sample.json'))
+    ->run();
+```
+
+## Using with to_transformation Loader
+
+The `to_transformation` loader allows you to apply transformations as part of the loading phase, enabling complex ETL
+patterns:
+
+```php
+use function Flow\ETL\DSL\{df, from_array, to_transformation, to_csv, select};
+
+// Apply transformation before loading
+df()
+    ->read(from_array([/* ... */]))
+    ->write(
+        to_transformation(
+            select('id', 'name'),      // Transform data
+            to_csv('output.csv')       // Then write to CSV
+        )
+    )
+    ->run();
+```
+
+This pattern is particularly useful when you need to:
+
+- Apply different transformations to the same data for multiple outputs
+- Create transformation pipelines that can be reused
+- Separate transformation logic from extraction and loading
+
+## Creating Custom Transformations
+
+You can create custom transformations by implementing the `Transformation` interface:
+
+```php
+use Flow\ETL\{DataFrame, Transformation};
+
+final class UppercaseNames implements Transformation
+{
+    public function transform(DataFrame $dataFrame): DataFrame
+    {
+        return $dataFrame->withEntry(
+            'name',
+            ref('name')->upper()
+        );
+    }
+}
+
+// Use custom transformation
+df()
+    ->read(from_array([/* ... */]))
+    ->with(new UppercaseNames())
+    ->write(to_output())
+    ->run();
+```

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

@@ -169,6 +169,8 @@
 use Flow\ETL\Schema\Metadata;
 use Flow\ETL\Schema\Validator\{EvolvingValidator, SelectiveValidator, StrictValidator};
 use Flow\ETL\Time\{Duration, Sleep, SystemSleep};
+use Flow\ETL\Transformation\AddRowIndex\StartFrom;
+use Flow\ETL\Transformation\{AddRowIndex, BatchSize, Drop, Limit, MaskColumns, Select};
 use Flow\ETL\Transformer\OrderEntries\{CombinedComparator, Comparator, NameComparator, Order, TypeComparator, TypePriorities};
 use Flow\ETL\Transformer\Rename\{RenameCaseEntryStrategy, RenameReplaceEntryStrategy};
 use Flow\Filesystem\{Filesystem, Local\NativeLocalFilesystem, Partition, Partitions, Path};
@@ -1065,6 +1067,48 @@ function refs(string|Reference ...$entries) : References
     return new References(...$entries);
 }
 
+#[DocumentationDSL(module: Module::CORE, type: DSLType::TRANSFORMER)]
+function select(string|Reference ...$entries) : Select
+{
+    return new Select(...$entries);
+}
+
+#[DocumentationDSL(module: Module::CORE, type: DSLType::TRANSFORMER)]
+function drop(string|Reference ...$entries) : Drop
+{
+    return new Drop(...$entries);
+}
+
+#[DocumentationDSL(module: Module::CORE, type: DSLType::TRANSFORMER)]
+function add_row_index(string $column = 'index', StartFrom $startFrom = StartFrom::ZERO) : AddRowIndex
+{
+    return new AddRowIndex($column, $startFrom);
+}
+
+/**
+ * @param int<1, max> $size
+ */
+#[DocumentationDSL(module: Module::CORE, type: DSLType::TRANSFORMER)]
+function batch_size(int $size) : BatchSize
+{
+    return new BatchSize($size);
+}
+
+#[DocumentationDSL(module: Module::CORE, type: DSLType::TRANSFORMER)]
+function limit(?int $limit) : Limit
+{
+    return new Limit($limit);
+}
+
+/**
+ * @param array<int, string> $columns
+ */
+#[DocumentationDSL(module: Module::CORE, type: DSLType::TRANSFORMER)]
+function mask_columns(array $columns = [], string $mask = '******') : MaskColumns
+{
+    return new MaskColumns($columns, $mask);
+}
+
 #[DocumentationDSL(module: Module::CORE, type: DSLType::SCALAR_FUNCTION)]
 function optional(ScalarFunction $function) : Optional
 {

src/core/etl/src/Flow/ETL/Transformation/Drop.php

@@ -0,0 +1,23 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Flow\ETL\Transformation;
+
+use Flow\ETL\{DataFrame, Transformation};
+use Flow\ETL\Row\{Reference, References};
+
+final readonly class Drop implements Transformation
+{
+    private References $references;
+
+    public function __construct(string|Reference ...$entries)
+    {
+        $this->references = References::init(...$entries);
+    }
+
+    public function transform(DataFrame $dataFrame) : DataFrame
+    {
+        return $dataFrame->drop(...$this->references->all());
+    }
+}

src/core/etl/src/Flow/ETL/Transformation/Select.php

@@ -0,0 +1,23 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Flow\ETL\Transformation;
+
+use Flow\ETL\{DataFrame, Transformation};
+use Flow\ETL\Row\{Reference, References};
+
+final readonly class Select implements Transformation
+{
+    private References $references;
+
+    public function __construct(string|Reference ...$entries)
+    {
+        $this->references = References::init(...$entries);
+    }
+
+    public function transform(DataFrame $dataFrame) : DataFrame
+    {
+        return $dataFrame->select(...$this->references->all());
+    }
+}