Structure

This page covers the structural elements of a microflow definition: the CREATE MICROFLOW syntax, parameters, variable declarations, return values, and annotations.

Full CREATE MICROFLOW Syntax

CREATE [OR REPLACE] MICROFLOW <Module.Name>
  [FOLDER '<path>']
BEGIN
  [<declarations>]
  [<activities>]
  [RETURN <value>;]
END;

The OR REPLACE modifier overwrites an existing microflow of the same name. The FOLDER clause organizes the microflow within the module’s folder structure.

Parameters

Parameters are declared with DECLARE at the top of the BEGIN...END block. They define the inputs to the microflow.

Primitive Parameters

DECLARE $Name String;
DECLARE $Count Integer;
DECLARE $IsActive Boolean;
DECLARE $Amount Decimal;
DECLARE $StartDate DateTime;

Supported primitive types: String, Integer, Long, Boolean, Decimal, DateTime.

Entity Parameters

Entity parameters receive a single object:

DECLARE $Customer MyModule.Customer;

Important: Do not use = empty or AS with entity declarations. The correct syntax is simply DECLARE $Var Module.Entity;.

List Parameters

List parameters receive a list of objects:

DECLARE $Orders List of Sales.Order = empty;

The = empty initializer creates an empty list. This is required for list declarations.

In MDL, all DECLARE statements at the top of a microflow are treated as parameters. Variables created by activities (such as $Var = CREATE ... or RETRIEVE $Var ...) are local variables. If you need a local variable with a default value, use DECLARE followed by SET:

DECLARE $Counter Integer = 0;
SET $Counter = 10;

Variables and Assignment

Variable Declaration

DECLARE $Message String = 'Hello';
DECLARE $Total Decimal = 0;
DECLARE $Found Boolean = false;

Assignment with SET

Change the value of an already-declared variable:

SET $Counter = $Counter + 1;
SET $FullName = $FirstName + ' ' + $LastName;
SET $IsValid = $Amount > 0;

The variable must be declared before it can be assigned.

Variables from Activities

Activities like CREATE and RETRIEVE produce result variables:

$Order = CREATE Sales.Order (Status = 'New');
RETRIEVE $Customer FROM Sales.Customer WHERE Email = $Email LIMIT 1;
$Result = CALL MICROFLOW Sales.CalculateTotal (Order = $Order);

Return Values

Every flow path should end with a RETURN statement. The return type is inferred from the returned value.

Returning a Primitive

RETURN true;
RETURN $Total;
RETURN 'Success';

Returning an Object

RETURN $Order;

Returning a List

RETURN $FilteredOrders;

Returning Nothing

If the microflow returns nothing (void), you can omit the RETURN or use:

RETURN;

Annotations

Annotations are metadata decorators placed before an activity. They control visual layout and documentation in Mendix Studio Pro.

Position

Set the canvas position of the next activity:

@position(200, 100)
$Order = CREATE Sales.Order (Status = 'New');

Caption

Set a custom caption displayed on the activity in the canvas:

@caption 'Create new order'
$Order = CREATE Sales.Order (Status = 'New');

Color

Set the background color of the activity:

@color Green
COMMIT $Order;

Annotation (Visual Note)

Attach a visual annotation note to the next activity:

@annotation 'This step validates the input before saving'
VALIDATION FEEDBACK $Customer/Email MESSAGE 'Email is required';

Combining Annotations

Multiple annotations can be stacked before a single activity:

@position(300, 200)
@caption 'Validate and save'
@color Green
@annotation 'Final step: commit the validated order'
COMMIT $Order WITH EVENTS;

Complete Example

CREATE MICROFLOW Sales.ACT_ProcessOrder
FOLDER 'Orders/Processing'
BEGIN
  -- Parameters
  DECLARE $Order Sales.Order;
  DECLARE $ApplyDiscount Boolean;

  -- Local variable
  DECLARE $Total Decimal = 0;

  -- Retrieve order lines
  RETRIEVE $Lines FROM $Order/Sales.OrderLine_Order;

  -- Calculate total
  @caption 'Calculate total'
  $Total = CALL MICROFLOW Sales.SUB_CalculateTotal (
    OrderLines = $Lines
  );

  -- Apply discount if requested
  IF $ApplyDiscount THEN
    SET $Total = $Total * 0.9;
  END IF;

  -- Update order
  CHANGE $Order (
    TotalAmount = $Total,
    Status = 'Processed'
  );
  COMMIT $Order;

  RETURN $Order;
END;

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