OpenModelica新前端编译报错：离散变量未在when语句左值的解决方法

问题说明 #

使用OpenModelica 1.22.1-64bit版本时，SimpleTest2模型在旧前端可正常编译，但新前端编译时报错：

Following variable is discrete, but does not appear on the LHS of a when-statement: 'x'

原模型代码 #

model SimpleTest2 "just a simple model - Compilation etc."
  Modelica.Blocks.Interfaces.RealInput u annotation(Placement(visible = true, transformation(origin = {-100, 40}, extent = {{-10, -10}, {10, 10}}, rotation = 0), iconTransformation(origin = {-80, 40}, extent = {{-10, -10}, {10, 10}}, rotation = 0)));
  Modelica.Blocks.Interfaces.RealOutput y annotation(Placement(visible = true, transformation(origin = {100, 20}, extent = {{-10, -10}, {10, 10}}, rotation = 0), iconTransformation(origin = {100, 20}, extent = {{-10, -10}, {10, 10}}, rotation = 0)));
protected 
  discrete Real x;
algorithm
  x := u;
  when change(x) then
    y := y + 2;
  end when;
  annotation(Icon(coordinateSystem(extent = {{-100, -100}, {100, 100}}, preserveAspectRatio = true, initialScale = 0.1, grid = {2, 2})), Diagram(coordinateSystem(extent = {{-100, -100}, {100, 100}}, preserveAspectRatio = true, initialScale = 0.1, grid = {2, 2}), graphics = {Rectangle(origin = {-6.15, 2.93}, fillColor = {0, 133, 199}, fillPattern = FillPattern.HorizontalCylinder, extent = {{-77.89, 83.75}, {88.14, -92.53}})}));
end SimpleTest2;

报错原因 #

新前端严格执行Modelica规范：离散变量必须在when语句的左侧（LHS）赋值，确保其仅在事件触发点更新。原代码中discrete Real x直接在algorithm块的非when部分赋值，不符合规范要求。

修改方案 #

将x的赋值逻辑移至when语句中，同时可以合并冗余的事件检查逻辑，修改后的代码如下：

model SimpleTest2 "just a simple model - Compilation etc."
  Modelica.Blocks.Interfaces.RealInput u annotation(Placement(visible = true, transformation(origin = {-100, 40}, extent = {{-10, -10}, {10, 10}}, rotation = 0), iconTransformation(origin = {-80, 40}, extent = {{-10, -10}, {10, 10}}, rotation = 0)));
  Modelica.Blocks.Interfaces.RealOutput y annotation(Placement(visible = true, transformation(origin = {100, 20}, extent = {{-10, -10}, {10, 10}}, rotation = 0), iconTransformation(origin = {100, 20}, extent = {{-10, -10}, {10, 10}}, rotation = 0)));
protected 
  discrete Real x;
algorithm
  when change(u) then
    x := u;
    y := y + 2;
  end when;
  annotation(Icon(coordinateSystem(extent = {{-100, -100}, {100, 100}}, preserveAspectRatio = true, initialScale = 0.1, grid = {2, 2})), Diagram(coordinateSystem(extent = {{-100, -100}, {100, 100}}, preserveAspectRatio = true, initialScale = 0.1, grid = {2, 2}), graphics = {Rectangle(origin = {-6.15, 2.93}, fillColor = {0, 133, 199}, fillPattern = FillPattern.HorizontalCylinder, extent = {{-77.89, 83.75}, {88.14, -92.53}})}));
end SimpleTest2;

关键修改点 #

• 把x := u移到when change(u)的语句块内，让离散变量x仅在u变化时更新，符合Modelica规范
• 合并原有的when change(x)逻辑到同一个when块中，既保留原模型功能（u变化时y加2），又避免冗余的事件检查

如果需要保留原有的change(x)触发逻辑，也可以采用以下修改方式：

model SimpleTest2 "just a simple model - Compilation etc."
  Modelica.Blocks.Interfaces.RealInput u annotation(Placement(visible = true, transformation(origin = {-100, 40}, extent = {{-10, -10}, {10, 10}}, rotation = 0), iconTransformation(origin = {-80, 40}, extent = {{-10, -10}, {10, 10}}, rotation = 0)));
  Modelica.Blocks.Interfaces.RealOutput y annotation(Placement(visible = true, transformation(origin = {100, 20}, extent = {{-10, -10}, {10, 10}}, rotation = 0), iconTransformation(origin = {100, 20}, extent = {{-10, -10}, {10, 10}}, rotation = 0)));
protected 
  discrete Real x;
algorithm
  when change(u) then
    x := u;
  end when;
  when change(x) then
    y := y + 2;
  end when;
  annotation(Icon(coordinateSystem(extent = {{-100, -100}, {100, 100}}, preserveAspectRatio = true, initialScale = 0.1, grid = {2, 2})), Diagram(coordinateSystem(extent = {{-100, -100}, {100, 100}}, preserveAspectRatio = true, initialScale = 0.1, grid = {2, 2}), graphics = {Rectangle(origin = {-6.15, 2.93}, fillColor = {0, 133, 199}, fillPattern = FillPattern.HorizontalCylinder, extent = {{-77.89, 83.75}, {88.14, -92.53}})}));
end SimpleTest2;

内容的提问来源于stack exchange，提问作者Prims