improves java developer experience with WfRunVariable assignments, de…

docs/docs/08-api.md

@@ -776,6 +776,22 @@ have to worry about this in daily LittleHorse usage.
 | `literal_value` | oneof `source`| [VariableValue](#variablevalue) | Assign a literal value |
 | `format_string` | oneof `source`| [VariableAssignment.FormatString](#variableassignmentformatstring) | Assign a format string |
 | `node_output` | oneof `source`| [VariableAssignment.NodeOutputReference](#variableassignmentnodeoutputreference) | Assign the value of a NodeOutput. |
+| `expression` | oneof `source`| [VariableAssignment.Expression](#variableassignmentexpression) | Assign the value of an Expression. |
+ <!-- end Fields -->
+ <!-- end HasFields -->
+
+
+
+### Message `VariableAssignment.Expression` {#variableassignmentexpression}
+
+An Expression allows you to combine multiple values into one.
+
+
+| Field | Label | Type | Description |
+| ----- | ----  | ---- | ----------- |
+| `lhs` | | [VariableAssignment](#variableassignment) | The left-hand-side of the expression. |
+| `operation` | | [VariableMutationType](#variablemutationtype) | The operator in the expression. |
+| `rhs` | | [VariableAssignment](#variableassignment) | The right-hand-side of the expression. |
  <!-- end Fields -->
  <!-- end HasFields -->
 

schemas/littlehorse/common_wfspec.proto

@@ -39,6 +39,16 @@ message VariableAssignment {
     string node_name = 1;
   }
 
+  // An Expression allows you to combine multiple values into one.
+  message Expression {
+    // The left-hand-side of the expression.
+    VariableAssignment lhs = 1;
+    // The operator in the expression.
+    VariableMutationType operation = 2;
+    // The right-hand-side of the expression.
+    VariableAssignment rhs = 3;
+  }
+
   // If you provide a `variable_name` and the specified variable is JSON_OBJ or
   // JSON_ARR type, then you may also provide a json_path which makes the VariableAssignment
   // resolve to the specified field.
@@ -54,6 +64,8 @@ message VariableAssignment {
     FormatString format_string = 4;
     // Assign the value of a NodeOutput.
     NodeOutputReference node_output = 5;
+    // Assign the value of an Expression.
+    Expression expression = 6;
   }
 }