Public API to get list of functions call (#2474)

Issue https://github.com/microsoft/Power-Fx/issues/2455.

src/libraries/Microsoft.PowerFx.Core/Public/CheckResult.cs

@@ -706,6 +706,11 @@ namespace Microsoft.PowerFx
 
             return summary;
         }
+
+        public IEnumerable<string> GetFunctionNames()
+        {
+            return ListFunctionVisitor.Run(ApplyParse());
+        }
     }
 
     // Internal interface to ensure that Result objects have a common contract

src/libraries/Microsoft.PowerFx.Core/Syntax/Visitors/ListFunctionVisitor.cs

@@ -0,0 +1,35 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+using System.Collections.Generic;
+
+namespace Microsoft.PowerFx.Syntax
+{
+    internal class ListFunctionVisitor : IdentityTexlVisitor
+    {
+        // FullName --> Name 
+        // Use Fullname as key because it's unique. 
+        private readonly HashSet<string> _functionNames = new HashSet<string>();
+
+        public static IEnumerable<string> Run(ParseResult parse)
+        {
+            var visitor = new ListFunctionVisitor();
+            parse.Root.Accept(visitor);
+            return visitor._functionNames;
+        }
+
+        public override bool PreVisit(CallNode node)
+        {
+            var hasNamespace = node.Head.Namespace.Length > 0;
+
+            var name = node.Head.Name;
+            var fullName = hasNamespace ?
+                    node.Head.Namespace + "." + name :
+                    name;
+
+            _functionNames.Add(fullName);
+
+            return base.PreVisit(node);
+        }
+    }
+}

src/tests/Microsoft.PowerFx.Core.Tests.Shared/ListFunctionVisitorTests.cs

@@ -0,0 +1,63 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+using System;
+using Xunit;
+
+namespace Microsoft.PowerFx.Core.Tests
+{
+    public class ListFunctionVisitorTests : PowerFxTest
+    {
+        [Theory]
+        [InlineData("Abs(1)", "Abs")]
+        [InlineData("Abs(Abs(Abs(Abs(Abs(1)))))", "Abs")]
+        [InlineData("With({x:Today()}, x+1)", "With,Today")]
+        [InlineData("SomeNameSpace.Foo() + SomeNameSpace.Bar()", "SomeNameSpace.Foo,SomeNameSpace.Bar")]
+        [InlineData("true And true", "")]
+        [InlineData("If(true, Blank(),Error())", "If,Blank,Error")]
+        public void ListFunctionNamesTest(string expression, string expectedNames)
+        {
+            foreach (var textFirst in new bool[] { false, true })
+            {
+                if (textFirst)
+                {
+                    expression = $"={expression}";
+                }
+
+                CheckFunctionNames(textFirst, expression, expectedNames);
+            }
+        }
+
+        [Theory]
+        [InlineData("Hello ${Sum(1,Sqrt(2))} world", "Sum,Sqrt")]
+        [InlineData("3 ' {} ${Upper(3+3)} \" ${Lower($\"{7+7}\")}", "Upper,Lower")]
+        public void ListFunctionNamesTextFirstTest(string expression, string expectedNames)
+        {
+            CheckFunctionNames(true, expression, expectedNames);
+        }
+
+        [Fact]
+        public void ListFunctionNamesErrorTest()
+        {
+            var checkResult = new CheckResult(new Engine());
+            Assert.Throws<InvalidOperationException>(() => checkResult.GetFunctionNames());
+        }
+
+        private static void CheckFunctionNames(bool textFirst, string expression, string expectedNames)
+        {
+            var options = new ParserOptions() { TextFirst = textFirst };
+            var engine = new Engine();
+            var check = engine.Check(expression, options);
+            var checkResult = new CheckResult(engine).SetText(expression, options);
+
+            var functionsNames1 = check.GetFunctionNames();
+            var functionsNames2 = checkResult.GetFunctionNames();
+
+            var actualNames1 = string.Join(",", functionsNames1);
+            var actualNames2 = string.Join(",", functionsNames2);
+
+            Assert.Equal(expectedNames, actualNames1);
+            Assert.Equal(expectedNames, actualNames2);
+        }
+    }
+}