Add function argument count validation to language service

index.ts

@@ -43,8 +43,10 @@ export type {
   HoverV2,
   GetCompletionsParams,
   GetHoverParams,
+  GetDiagnosticsParams,
   HighlightToken,
-  LanguageServiceOptions
+  LanguageServiceOptions,
+  ArityInfo
 } from './src/language-service/index.js';
 
 export { createLanguageService, Expression, Parser };

samples/language-service-sample/app.js

@@ -417,6 +417,30 @@ require(['vs/editor/editor.main'], function () {
         highlightDecorations = expressionEditor.deltaDecorations(highlightDecorations, decorations);
     }
 
+    // Diagnostics - show function argument count errors
+    function applyDiagnostics() {
+        const doc = makeTextDocument(expressionModel);
+        const diagnostics = ls.getDiagnostics({ textDocument: doc });
+
+        // Convert LSP diagnostics to Monaco markers
+        const markers = diagnostics.map(d => {
+            const startPos = fromLspPosition(d.range.start);
+            const endPos = fromLspPosition(d.range.end);
+            return {
+                severity: monaco.MarkerSeverity.Error,
+                message: d.message,
+                startLineNumber: startPos.lineNumber,
+                startColumn: startPos.column,
+                endLineNumber: endPos.lineNumber,
+                endColumn: endPos.column,
+                source: d.source || 'expr-eval'
+            };
+        });
+
+        // Set markers on the model
+        monaco.editor.setModelMarkers(expressionModel, 'expr-eval', markers);
+    }
+
     // Syntax highlight JSON
     function syntaxHighlightJson(json) {
         if (typeof json !== 'string') {
@@ -599,6 +623,7 @@ require(['vs/editor/editor.main'], function () {
     // Event listeners for changes
     expressionModel.onDidChangeContent(() => {
         applyHighlighting();
+        applyDiagnostics();
         evaluate();
     });
 

samples/language-service-sample/examples.js

@@ -93,5 +93,15 @@ const exampleCases = [
                 {"rowId": 3, "state": "unchanged", "data": { "InventoryId": 9362, "Description": "Wood", "Weight": { "Unit": "kg", "Amount": 18 } }}
             ]
         }
+    },
+    {
+        id: 'diagnostics-demo',
+        title: 'Diagnostics Demo',
+        description: 'Shows error highlighting for incorrect function arguments',
+        expression: '// Try functions with wrong argument counts:\n// pow() needs 2 args, random() needs 0-1 args\npow(2) + random(1, 2, 3)',
+        context: {
+            x: 5,
+            y: 10
+        }
     }
 ];