CALL MICROFLOW: allow assigning return value to an existing variable (avoid CE0111)

[ako]

Problem

RETRIEVE implicitly declares its output variable, but CALL MICROFLOW assigning to an already-used name is rejected as a duplicate (CE0111). Writing $R = CALL MICROFLOW ... repeatedly forces unique names ($R1…$R10) as a workaround.

Current behavior (partially addressed)

Detection shipped in v0.11.0: mxcli check --references flags reuse as CE0111 (mdl/executor/cmd_microflows_builder_validate.go:156,364-366; the output var is registered at :162). So the error is now surfaced before execution — but "reuse the existing variable on assignment" is still not implemented, so the unique-name workaround remains.

Request

Allow a CALL MICROFLOW return value to be assigned to an existing variable in scope (treat re-assignment as assignment, not re-declaration), matching how RETRIEVE behaves.

Touch points

• mdl/executor/cmd_microflows_builder_validate.go:360-367 — distinguish "first assignment" (declare) from "re-assignment to existing var" (allowed) instead of always emitting CE0111.

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Source: customer feedback batch (2026-06). Reporter: Miwa. Reporter severity: 4/5.
Filed after codebase investigation at 297e0b0a.

[github-actions]

Issue Triage

Summary:
The issue requests that CALL MICROFLOW be allowed to assign its return value to an already‑declared variable (re‑assignment) instead of triggering a duplicate‑variable error (CE0111), mirroring the behavior of RETRIEVE.

Checklist for feature request completeness

Required info	Present?	Details
Use case – why the feature is needed / problem it solves	✅	Describes the need to avoid forced unique‑variable workarounds ($R1…$R10) when repeatedly calling a microflow.
Proposed syntax or behavior (if applicable)	✅ (implicit)	States that re‑assignment to an existing variable should be treated as a normal assignment, not a redeclaration.
Mendix version relevance (if version‑specific)	❌	No mention of which Mendix versions the change should apply to or any version‑specific constraints.

Missing information:
Please indicate the Mendix version(s) of the projects where this behavior is desired (e.g., 9.x, 10.x, or all versions). This helps assess any version‑specific impacts.

Thank you for the detailed request! Once the Mendix version relevance is added, the issue will be complete.

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[github-actions]

Enhancement Implementation Guide

Completeness Assessment

The issue is well-defined with specific technical details:

• Problem: CALL MICROFLOW rejects assignment to existing variables (CE0111) unlike RETRIEVE
• Current state: Detection exists (v0.11.0) but reassignment not implemented
• Touch point: mdl/executor/cmd_microflows_builder_validate.go:360-367 (output var registered at :162)
• Request: Treat reassignment as assignment (not redeclaration) matching RETRIEVE behavior

No additional questions needed - the issue provides sufficient detail for implementation.

Overlap Check

Existing functionality that partially covers this:

• RETRIEVE statement already allows reassignment to existing variables (implicitly declared output var)
• Variable declaration/checking logic exists in the microflow executor
• The VariableExists() method is already used elsewhere for scoping checks
• No existing MDL syntax achieves this - it's a semantic change to CALL MICROFLOW assignment

Proposed MDL Syntax

No syntax change needed - modifying existing semantics. Current syntax remains:

$ExistingVar = call microflow MyModule.MyMF($Param = $value)

Examples showing allowed reassignment:

// First assignment (declares $Result)
$Result = call microflow GetUser(id = 42)

// Reassignment (uses existing $Result)
$Result = call microflow UpdateUser(user = $Result)

// Multiple reassignments in sequence
$Temp = call microflow GetConfig()
$Temp = call microflow ProcessConfig(config = $Temp)
$Result = call microflow ApplySettings(settings = $Temp)

Implementation Roadmap

1. Modify validation logic in mdl/executor/cmd_microflows_builder_validate.go:
   • Locate validateCallMicroflowStatement function (or similar)
   • Find lines 360-367 where CE0111 is currently emitted
   • Replace unconditional error with conditional variable registration:

     // BEFORE (lines 360-367)
     if mb.VariableExists(stmt.OutputVariable) {
         return mdlerrors.NewVariableAlreadyDeclared(stmt.OutputVariable)
     }
     mb.RegisterVariable(stmt.OutputVariable, variableInfo)
     
     // AFTER
     if !mb.VariableExists(stmt.OutputVariable) {
         mb.RegisterVariable(stmt.OutputVariable, variableInfo)
     }
     // Proceed with call validation regardless

2. Ensure consistent behavior with RETRIEVE:
   • Verify RETRIEVE validation uses same pattern (declare-if-not-exists)
   • If not, apply same fix to RETRIEVE for consistency
3. Update variable registration:
   • Confirm RegisterVariable handles redeclaration safely (should be no-op if exists)
   • Check that variable type/mutability is preserved on reassignment
4. Regenerate any cached artifacts (none needed - pure Go change)

BSON Investigation

Not required - this is purely a scoping/validation change:

• No BSON structure changes involved
• Microflow execution logic unchanged (only validation)
• Output variable handling in BSON remains identical
• Focus is on preventing false-positive CE0111 during validation

Testing Strategy

1. Add doctype test in mdl-examples/doctype-tests/02-microflow-examples.mdl:

   create microflow Test.ReassignVar() returns string begin
     $X = call microflow GetInitialValue() returns string;
     $X = call microflow AppendSuffix(value = $X) returns string;
     return
   
   

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