CALL EXTERNAL ACTION — no way to pass empty/null binding for optional parameters; results in CE7252

[acarum]

Category: MDL parser / CALL EXTERNAL ACTION syntax

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Summary

When an OData action declares a parameter (e.g. additional: AdditionalArgumentsDefinition) that must be bound to an empty/null value, there is no valid MDL syntax to express this. The result is that any microflow created via mxcli exec with a non-null binding for that
parameter produces a permanent CE7252 in Studio Pro that can only be fixed manually inside the IDE.

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Steps to reproduce

1. Import an OData action that declares two parameters: command and additional.
2. Write a microflow that calls the action. Attempt any of these bindings for additional:

-- Attempt 1: null keyword
CALL EXTERNAL ACTION Mod.Service.ActionName (command = $Cmd, additional = null);
-- → Parse error

-- Attempt 2: empty keyword
CALL EXTERNAL ACTION Mod.Service.ActionName (command = $Cmd, additional = empty);
-- → Parse error

-- Attempt 3: blank (no value)
CALL EXTERNAL ACTION Mod.Service.ActionName (command = $Cmd, additional = );
-- → Parse error: mismatched input ')' expecting the start of a statement

-- Attempt 4: omit the parameter entirely
CALL EXTERNAL ACTION Mod.Service.ActionName (command = $Cmd);
-- → Reference error: "declared but not supplied: additional"
-- The action declares: command, additional

None of the four variants are accepted.

3. The only workaround is to pass a real object:

$Additional = CREATE Mod.AdditionalArgumentsDefinition ();
CALL EXTERNAL ACTION Mod.Service.ActionName (command = $Cmd, additional = $Additional);

This passes mxcli validation but produces CE7252 in Studio Pro because the internal binding type does not match the contract.

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What Studio Pro serializes (observed via DESCRIBE MICROFLOW)

When a working microflow is round-tripped through DESCRIBE MICROFLOW, mxcli itself outputs:

call external action Mod.Service.ActionName(command = $Cmd, additional = );

So additional = (empty RHS) is the canonical representation of a null binding — but the MDL parser rejects this syntax on input.

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Expected behavior

One of the following should be valid MDL and round-trip correctly:

-- Option A: bare empty RHS (already used in DESCRIBE output)
CALL EXTERNAL ACTION Mod.Service.ActionName (command = $Cmd, additional = );

-- Option B: explicit null/empty keyword
CALL EXTERNAL ACTION Mod.Service.ActionName (command = $Cmd, additional = empty);

The reference validator should accept a null binding for a parameter that is declared in the contract, without requiring the parameter to be omitted entirely.

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Environment

• mxcli version: (0.20.0)
• Mendix version: 11.12.0
• OData service validation status: No (service not reachable at authoring time)
• Observed via: mxcli exec, mxcli check --references, mx check

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Impact

Any migration or code-generation workflow that produces CALL EXTERNAL ACTION calls with optional/null parameter bindings is broken — the generated microflow requires manual IDE intervention to clear CE7252, defeating the purpose of automated MDL generation.