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PROPOSAL_ellipsis_placeholder_expression.md

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PROPOSAL_ellipsis_placeholder_expression.md

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Ellipsis Placeholder Expression

Status: Draft

Summary

Add a single-token expression ... that represents an unbound / intentionally-empty argument value in microflow call statements.

$Total = call java action SampleModule.Recalculate(
  CompanyId       = ...,
  RecalculateAll  = true,
  ItemList        = ...
);

... produces a parameter binding with an empty Argument string in the serialized BSON (Microflows$BasicCodeActionParameterValue.Argument = ""). Re-executing a script that contains ... reproduces the same empty binding byte-for-byte, so describe → exec → describe stays symmetric for existing Studio Pro projects that have unbound code-action parameters.

Motivation

Studio Pro's Java-action call dialog allows a developer to leave individual parameters empty — for example, when a Java action declares a parameter that the calling microflow does not yet have a meaningful value for, or when an external mapping is expected to fill the slot at runtime. The on-disk representation is a Microflows$JavaActionParameterMapping whose Value is a BasicCodeActionParameterValue with Argument: "".

Before ... existed, the describer had two options for these empty bindings:

  1. Emit '' (empty string literal). On re-exec, the visitor would round-trip to a non-empty single-quote literal whose Argument was '', not "", and Studio Pro would render the parameter as the literal string ''.
  2. Drop the parameter entirely. Studio Pro would then add a back a placeholder mapping with a generated value, breaking the round-trip.

Both lose information. ... lets the describer round-trip the empty binding without inventing a fake value.

Syntax

atomicExpression
    : literal
    | ELLIPSIS
    | ...
    ;

Where ELLIPSIS is the lexer token '...'. The token is reserved for this single use; it is not valid in arithmetic / boolean / comparison expressions.

Semantics

  • ... is recognised by the builder via isPlaceholderExpression in mdl/executor/cmd_microflows_builder_calls.go.
  • Inside a Java-action callArgument, ... produces a BasicCodeActionParameterValue with Argument: "".
  • Outside of callArgument lists, ... parses but the builder rejects it (it never resolves to a runtime value). Future statements may extend the set of contexts that accept ... — see Open Questions.

Examples

-- Java action call with two unbound and one bound argument
$Total = call java action SampleModule.Recalculate(
  CompanyId       = ...,
  RecalculateAll  = true,
  ItemList        = ...
);

The Mendix BSON for the unbound arguments is:

JavaActionParameterMapping {
  Parameter: 'SampleModule.Recalculate.CompanyId',
  Value: BasicCodeActionParameterValue { Argument: '' }
}

Tests And Examples

  • Builder coverage: TestBuildJavaAction_PlaceholderArgumentPreservesEmptyBasicValue in mdl/executor/cmd_microflows_builder_java_action_test.go.
  • Visitor coverage: atomicExpression's ELLIPSIS arm produces ast.SourceExpr{Source: "..."} (see mdl/visitor/visitor_microflow_expression.go).
  • Example script: mdl-examples/doctype-tests/ellipsis_placeholder.test.mdl.

Open Questions

  • Should ... be allowed as an argument to call microflow and call nanoflow calls as well? Today only Java actions consume the BasicCodeActionParameterValue form, so there is no symmetric BSON representation, but a future proposal could extend this.
  • Should we explicitly document ... as round-trip-only and warn the linter when an authored microflow uses ... outside of a known describe-emitted context? This would prevent users from authoring scripts that produce Studio Pro warnings ("unbound parameter") on import.
  • Should the surface syntax be ... or a clearer keyword like unspecified / default / unbound? ... was chosen because it is visually distinct, short, and matches existing "this is intentionally blank" conventions in other tools (Python's Ellipsis, TypeScript's never placeholder, etc.).