MDL Variable Cheatsheet

Quick reference for variable declarations in MDL microflows.

Declaration Syntax

Type	Syntax	Example
String	declare $name string = 'value';	declare $message string = '';
Integer	declare $name integer = 0;	declare $count integer = 0;
Boolean	declare $name boolean = true;	declare $IsValid boolean = true;
Decimal	declare $name decimal = 0.0;	declare $Amount decimal = 0;
DateTime	declare $name datetime = [%CurrentDateTime%];	declare $Now datetime = [%CurrentDateTime%];
Enumeration	declare $name Enumeration(Module.Enum) = Module.Enum.Value;	declare $s Enumeration(Sales.Status) = Sales.Status.Open;
Object (Entity)	Do not declare — use a parameter, retrieve, create, or loop iterator	$Customer = create Sales.Customer (Name = $n);
List	Do not declare — use a parameter, retrieve, or create list	$Orders = create list of Sales.Order;

Key Rules

1. Primitives (String/Integer/Long/Decimal/Boolean/DateTime/Enumeration): Use declare $var type = value; — these are the only types declare (a Create Variable activity) accepts.
2. Objects (entities): Never declare an object. Mendix forbids the Create Variable activity from holding an object (CE0053/CE0038, plus CE7247 on a later set; flagged as MDL043 by mxcli check) — bare or initialized. Get the object from a microflow parameter, a retrieve … limit 1, $var = create Module.Entity(...), or a loop iterator. There is no "empty object variable" and no aliasing activity — reuse the variable you already have.
3. Lists: Never declare a list — same Create Variable restriction (CE0053/CE0038, flagged as MDL040). Get the list from a microflow parameter, a retrieve, or $var = create list of Module.Entity;
4. SET requires DECLARE: Always declare primitive variables before using SET.
5. Parameters are pre-declared: Microflow parameters don't need DECLARE (and a parameter may be an object/list type — that restriction is only on declare).

Common Mistakes

Object (Entity) "Declaration" — there is no such thing

-- WRONG: declaring an object — Mendix rejects it (CE0053/CE0038, MDL043)
declare $Product Module.Product;            -- bare form is invalid
declare $Product Module.Product = $In;      -- aliasing a parameter is invalid
declare $Product as Module.Product;         -- the AS keyword is also a parse error

-- CORRECT: get the object from a source that produces one
$Product = create Module.Product (Name = $n);     -- create
retrieve $Product from Module.Product where Code = $c limit 1;  -- retrieve
-- or accept it as a parameter, or use a loop iterator: loop $Product in $Products ...

SET Without DECLARE

-- WRONG: Variable not declared
if $value > 10 then
  set $message = 'High';  -- ERROR!
end if;

-- CORRECT: Declare first
declare $message string = '';
if $value > 10 then
  set $message = 'High';
end if;

Lists (never declared)

-- WRONG: declaring a list — Mendix rejects it (CE0053/CE0038, MDL040)
declare $Items list of Module.Item = empty;

-- CORRECT: build the list without declare
$Items = create list of Module.Item;          -- empty list to accumulate into
retrieve $Items from Module.Item where ...;    -- or populate from the database
-- or accept it as a parameter: create microflow M.Process ($Items: list of Module.Item) ...

Special Values

Value	Usage
empty	Null/empty value for any type
[%CurrentDateTime%]	Current date and time
[%CurrentUser%]	Currently logged in user object
true / false	Boolean literals

Parameter vs Variable

create microflow Module.Example (
  $Input: string,              -- Parameter: auto-declared
  $entity: Module.Customer     -- Parameter: auto-declared
)
returns boolean
begin
  -- Parameters $Input and $Entity are already available

  declare $Result boolean = true;  -- Local primitive: must declare
  -- Need a local object? Don't declare it — create/retrieve it:
  $Order = create Module.Order (Reference = $Input);  -- or retrieve … limit 1

  return $Result;
end;
/

Variable Scope

• Parameters: Available throughout the microflow
• DECLARE variables: Available from declaration point forward
• Loop variables: Only available inside the loop body

loop $item in $ItemList
begin
  -- $Item is available here (derived from list type)
  set $count = $count + 1;
end loop;
-- $Item is NOT available here