Initial support for Power Fx Modules. (#2661)

Only exposed in REPL, and must be explicitly activated.

Only new concept in core is adding **FileLocation**. We need to report
errors within a file (line,col), not just character offset in an
expression.

Also implement the Assert() function in the repl - this helps with
wrigin modules that act as unit tests.

Module support is defined via Repl. Module format is deserialized via
YamlDotNet.
Modules only support:
- a single Formulas section that behaves like app.formulas. 
- an "imports" section for importing other modules. 
The tests demonstrate the various combinations for public/private,
conflict, shadowing, etc.

Add management functions to repl:
Import(path)
DeleteModule(path) - remove from list 
ListModules() - show which moduels are loaded

src/libraries/Microsoft.PowerFx.Core/AssemblyProperties.cs

@@ -44,6 +44,7 @@ using System.Runtime.CompilerServices;
 [assembly: InternalsVisibleTo("Microsoft.PowerFx.Connectors, PublicKey=0024000004800000940000000602000000240000525341310004000001000100b5fc90e7027f67871e773a8fde8938c81dd402ba65b9201d60593e96c492651e889cc13f1415ebb53fac1131ae0bd333c5ee6021672d9718ea31a8aebd0da0072f25d87dba6fc90ffd598ed4da35e44c398c454307e8e33b8426143daec9f596836f97c8f74750e5975c64e2189f45def46b2a2b1247adc3652bf5c308055da9")]
 [assembly: InternalsVisibleTo("Microsoft.PowerFx.LanguageServerProtocol, PublicKey=0024000004800000940000000602000000240000525341310004000001000100b5fc90e7027f67871e773a8fde8938c81dd402ba65b9201d60593e96c492651e889cc13f1415ebb53fac1131ae0bd333c5ee6021672d9718ea31a8aebd0da0072f25d87dba6fc90ffd598ed4da35e44c398c454307e8e33b8426143daec9f596836f97c8f74750e5975c64e2189f45def46b2a2b1247adc3652bf5c308055da9")]
 [assembly: InternalsVisibleTo("Microsoft.PowerFx.Json, PublicKey=0024000004800000940000000602000000240000525341310004000001000100b5fc90e7027f67871e773a8fde8938c81dd402ba65b9201d60593e96c492651e889cc13f1415ebb53fac1131ae0bd333c5ee6021672d9718ea31a8aebd0da0072f25d87dba6fc90ffd598ed4da35e44c398c454307e8e33b8426143daec9f596836f97c8f74750e5975c64e2189f45def46b2a2b1247adc3652bf5c308055da9")]
+[assembly: InternalsVisibleTo("Microsoft.PowerFx.Repl, PublicKey=0024000004800000940000000602000000240000525341310004000001000100b5fc90e7027f67871e773a8fde8938c81dd402ba65b9201d60593e96c492651e889cc13f1415ebb53fac1131ae0bd333c5ee6021672d9718ea31a8aebd0da0072f25d87dba6fc90ffd598ed4da35e44c398c454307e8e33b8426143daec9f596836f97c8f74750e5975c64e2189f45def46b2a2b1247adc3652bf5c308055da9")]
 [assembly: InternalsVisibleTo("Microsoft.PowerFx.Core.Tests, PublicKey=0024000004800000940000000602000000240000525341310004000001000100b5fc90e7027f67871e773a8fde8938c81dd402ba65b9201d60593e96c492651e889cc13f1415ebb53fac1131ae0bd333c5ee6021672d9718ea31a8aebd0da0072f25d87dba6fc90ffd598ed4da35e44c398c454307e8e33b8426143daec9f596836f97c8f74750e5975c64e2189f45def46b2a2b1247adc3652bf5c308055da9")]
 [assembly: InternalsVisibleTo("Microsoft.PowerFx.Interpreter.Tests, PublicKey=0024000004800000940000000602000000240000525341310004000001000100b5fc90e7027f67871e773a8fde8938c81dd402ba65b9201d60593e96c492651e889cc13f1415ebb53fac1131ae0bd333c5ee6021672d9718ea31a8aebd0da0072f25d87dba6fc90ffd598ed4da35e44c398c454307e8e33b8426143daec9f596836f97c8f74750e5975c64e2189f45def46b2a2b1247adc3652bf5c308055da9")]
 [assembly: InternalsVisibleTo("Microsoft.PowerFx.Json.Tests, PublicKey=0024000004800000940000000602000000240000525341310004000001000100b5fc90e7027f67871e773a8fde8938c81dd402ba65b9201d60593e96c492651e889cc13f1415ebb53fac1131ae0bd333c5ee6021672d9718ea31a8aebd0da0072f25d87dba6fc90ffd598ed4da35e44c398c454307e8e33b8426143daec9f596836f97c8f74750e5975c64e2189f45def46b2a2b1247adc3652bf5c308055da9")]

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

@@ -39,10 +39,15 @@ namespace Microsoft.PowerFx
 
             // If this is set directly, it will skip localization. 
             set => _message = value;
-        }        
+        }
 
         /// <summary>
-        /// Source location for this error.
+        /// Optional - provide file context for where this expression is from. 
+        /// </summary>
+        public FileLocation FragmentLocation { get; set; }
+
+        /// <summary>
+        /// Source location for this error within a single expression.
         /// </summary>
         public Span Span { get; set; }
 
@@ -209,6 +214,24 @@ namespace Microsoft.PowerFx
                 return errors.Select(x => ExpressionError.New(x, locale)).ToArray();
             }
         }
+
+        // Translate Span in original text (start,end)  to something more useful for a file. 
+        internal static IEnumerable<ExpressionError> NewFragment(IEnumerable<IDocumentError> errors, string originalText, FileLocation fragmentLocation)
+        {
+            if (errors == null)
+            {
+                return Array.Empty<ExpressionError>();
+            }
+            else
+            {
+                return errors.Select(x =>
+                {
+                    var error = ExpressionError.New(x, null);
+                    error.FragmentLocation = fragmentLocation.Apply(originalText, error.Span);
+                    return error;
+                });
+            }
+        }
     }
 
     /// <summary>

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

@@ -0,0 +1,66 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using System.Linq;
+using Microsoft.PowerFx.Core.Errors;
+using Microsoft.PowerFx.Core.Localization;
+using Microsoft.PowerFx.Types;
+
+namespace Microsoft.PowerFx.Syntax
+{
+    /// <summary>
+    /// File-aware, Multi-line source span with Line and Column. 
+    /// Wheras <see cref="Span"/> is a character span within a single expression.
+    /// </summary>
+    public class FileLocation
+    {
+        public string Filename { get; init; }
+
+        // 1-based index
+        public int LineStart { get; init; }
+
+        // 1-based index. 
+        public int ColStart { get; init; }
+
+        /// <summary>
+        /// Apply a span (which is a character offset within a single expression) 
+        /// to this file offset. 
+        /// </summary>
+        /// <param name="originalText"></param>
+        /// <param name="s2"></param>
+        /// <returns></returns>
+        public FileLocation Apply(string originalText, Span s2)
+        {
+            // Convert index span to line span. 
+            int iLine = this.LineStart;
+            int iCol = ColStart;
+
+            for (int i = 0; i < s2.Min; i++)
+            {
+                if (originalText[i] == '\r')
+                {
+                    // ignore
+                }
+                else if (originalText[i] == '\n')
+                {
+                    iLine++;
+                    iCol = ColStart; // reset 
+                }
+                else
+                {
+                    iCol++;
+                }
+            }
+
+            return new FileLocation
+            {
+                Filename = Filename,
+                ColStart = iCol,
+                LineStart = iLine
+            };
+        }
+    }
+}

src/libraries/Microsoft.PowerFx.Repl/AssemblyProperties.cs

@@ -1,4 +1,8 @@
 // Copyright (c) Microsoft Corporation.
 // Licensed under the MIT license.
 
+using System.Reflection;
 using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+
+[assembly: InternalsVisibleTo("Microsoft.PowerFx.Repl.Tests, PublicKey=0024000004800000940000000602000000240000525341310004000001000100b5fc90e7027f67871e773a8fde8938c81dd402ba65b9201d60593e96c492651e889cc13f1415ebb53fac1131ae0bd333c5ee6021672d9718ea31a8aebd0da0072f25d87dba6fc90ffd598ed4da35e44c398c454307e8e33b8426143daec9f596836f97c8f74750e5975c64e2189f45def46b2a2b1247adc3652bf5c308055da9")]

src/libraries/Microsoft.PowerFx.Repl/Functions/DeleteModule.cs

@@ -0,0 +1,49 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.PowerFx.Types;
+
+#pragma warning disable CS0618 // Type or member is obsolete
+
+namespace Microsoft.PowerFx.Repl.Functions
+{
+    /// <summary>
+    /// Delete a modules.
+    /// </summary>
+    internal class DeleteModuleFunction : ReflectionFunction
+    {
+        private readonly PowerFxREPL _repl;
+
+        public DeleteModuleFunction(PowerFxREPL repl)
+            : base("DeleteModule", FormulaType.Void, new[] { FormulaType.String })
+        {
+            ConfigType = typeof(IReplOutput);
+            _repl = repl;
+        }
+
+        public async Task<VoidValue> Execute(IReplOutput output, StringValue value, CancellationToken cancel)
+        {
+            if (!_repl.TryResolveModule(value.Value, out var module))
+            {
+                await output.WriteLineAsync($"Can't resolve module '{value.Value}'. Try ListModules() to see loaded modules.", OutputKind.Error, cancel)
+                .ConfigureAwait(false);
+            }
+            else
+            {
+                _repl.DeleteModule(module);
+
+                await output.WriteLineAsync("Removed module: " + module.FullPath, OutputKind.Notify, cancel)
+                .ConfigureAwait(false);
+            }
+
+            return FormulaValue.NewVoid();
+        }
+    }
+}

src/libraries/Microsoft.PowerFx.Repl/Functions/ImportFunction.cs

@@ -0,0 +1,87 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.PowerFx.Types;
+
+#pragma warning disable CS0618 // Type or member is obsolete
+
+namespace Microsoft.PowerFx.Repl.Functions
+{
+    /// <summary>
+    /// Import a module.
+    /// </summary>
+    internal class ImportFunction : ReflectionFunction
+    {
+        private readonly PowerFxREPL _repl;
+
+        public ImportFunction(PowerFxREPL repl)
+            : base("Import", FormulaType.Void, new[] { FormulaType.String })
+        {
+            ConfigType = typeof(IReplOutput);
+            _repl = repl;
+        }
+
+        public async Task<VoidValue> Execute(IReplOutput output, StringValue name, CancellationToken cancel)
+        {
+            var errors = new List<ExpressionError>();
+
+            var ctx = new ModuleLoadContext(_repl.Engine.GetCombinedEngineSymbols());
+
+            string filename = name.Value;
+            var module = await ctx.LoadFromFileAsync(filename, errors).ConfigureAwait(false);
+
+            foreach (var error in errors)
+            {
+                // Adjust to file...? 
+                var loc = error.FragmentLocation;
+
+                var shortName = Path.GetFileName(loc.Filename);
+
+                var prefix = error.IsWarning ? "Warning" : "Error";
+                var kind = error.IsWarning ? OutputKind.Warning : OutputKind.Error;
+
+                var msg = $"{prefix}: {shortName} ({loc.LineStart},{loc.ColStart}): {error.Message}";
+
+                await output.WriteLineAsync(msg, kind, cancel)
+                    .ConfigureAwait(false);
+            }
+
+            var hasErrors = errors.Where(error => !error.IsWarning).Any();
+
+            if (!hasErrors)
+            {
+                // Apply these functions to engine. 
+
+                string header = "Defined functions:";
+                await output.WriteLineAsync(header, OutputKind.Notify, cancel)
+                    .ConfigureAwait(false);
+
+                await PrintModuleAsync(module, output, cancel)
+                    .ConfigureAwait(false);
+
+                _repl.AddModule(module);
+            }
+
+            return FormulaValue.NewVoid();
+        }
+
+        internal static async Task PrintModuleAsync(Module module, IReplOutput output, CancellationToken cancel)
+        {
+            foreach (var funcName in module.Symbols.FunctionNames)
+            {
+                await output.WriteLineAsync($"  {funcName}", OutputKind.Notify, cancel)
+                .ConfigureAwait(false);
+            }
+
+            await output.WriteLineAsync(string.Empty, OutputKind.Notify, cancel)
+                .ConfigureAwait(false);
+        }
+    }
+}

src/libraries/Microsoft.PowerFx.Repl/Functions/ListModules.cs

@@ -0,0 +1,50 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.PowerFx.Types;
+
+#pragma warning disable CS0618 // Type or member is obsolete
+
+namespace Microsoft.PowerFx.Repl.Functions
+{
+    /// <summary>
+    /// List the modules that are loaded.
+    /// </summary>
+    internal class ListModulesFunction : ReflectionFunction
+    {
+        private readonly PowerFxREPL _repl;
+
+        public ListModulesFunction(PowerFxREPL repl)
+            : base("ListModules", FormulaType.Void)
+        {
+            ConfigType = typeof(IReplOutput);
+            _repl = repl;
+        }
+
+        public async Task<VoidValue> Execute(IReplOutput output, CancellationToken cancel)
+        {
+            var modules = _repl.Modules;
+
+            await output.WriteLineAsync("Modules loaded:", OutputKind.Notify, cancel)
+                .ConfigureAwait(false);
+
+            foreach (var module in modules)
+            {
+                await output.WriteLineAsync(module.FullPath, OutputKind.Notify, cancel)
+                    .ConfigureAwait(false);
+
+                await ImportFunction.PrintModuleAsync(module, output, cancel)
+                    .ConfigureAwait(false);
+            }
+
+            return FormulaValue.NewVoid();
+        }
+    }
+}

src/libraries/Microsoft.PowerFx.Repl/Microsoft.PowerFx.Repl.csproj

@@ -24,6 +24,7 @@
   <ItemGroup>
     <ProjectReference Include="..\Microsoft.PowerFx.Core\Microsoft.PowerFx.Core.csproj" />
     <ProjectReference Include="..\Microsoft.PowerFx.Interpreter\Microsoft.PowerFx.Interpreter.csproj" />
+    <PackageReference Include="YamlDotNet" Version="13.4.0" />
   </ItemGroup>
 
   <ItemGroup>

src/libraries/Microsoft.PowerFx.Repl/Modules/ConflictTracker.cs

@@ -0,0 +1,33 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+using System;
+using System.Collections.Generic;
+
+namespace Microsoft.PowerFx.Repl
+{
+    // Ensure uniqueness 
+    internal class ConflictTracker
+    {
+        // Map from symbol Name to the module it was defined in. 
+        private readonly Dictionary<string, Module> _defined = new Dictionary<string, Module>();
+
+        /// <summary>
+        /// Verify that the symbols added by Module are unique. 
+        /// </summary>
+        /// <param name="module"></param>
+        /// <exception cref="InvalidOperationException">If a symbol is already defined.</exception>
+        public void VerifyUnique(Module module)
+        {
+            foreach (var name in module.Symbols.FunctionNames)
+            {
+                if (_defined.TryGetValue(name, out var original))
+                {
+                    throw new InvalidOperationException($"Symbol '{name}' is already defined in both '{original.FullPath}' and '{module.FullPath}'");
+                }
+
+                _defined[name] = module;
+            }
+        }
+    }
+}

src/libraries/Microsoft.PowerFx.Repl/Modules/FileLoader.cs

@@ -0,0 +1,57 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using System.IO;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Microsoft.PowerFx.Core.Binding;
+using Microsoft.PowerFx.Core.Errors;
+using Microsoft.PowerFx.Core.Functions;
+using Microsoft.PowerFx.Core.Glue;
+using Microsoft.PowerFx.Core.Utils;
+using Microsoft.PowerFx.Syntax;
+using YamlDotNet.Core;
+using YamlDotNet.Core.Events;
+using YamlDotNet.Serialization;
+using YamlDotNet.Serialization.NodeDeserializers;
+
+namespace Microsoft.PowerFx.Repl
+{
+    internal class FileLoader : IFileLoader
+    {
+        // Root directory that we load from.
+        private readonly string _root;
+
+        public FileLoader(string root)
+        {
+            if (!Path.IsPathRooted(root))
+            {
+                throw new ArgumentException($"Path must be rooted: {root}", nameof(root));
+            }
+
+            _root = root;
+        }
+
+        public async Task<(ModulePoco, IFileLoader)> LoadAsync(string name)
+        {
+            name = name.Trim();
+            var fullPath = Path.Combine(_root, name);
+            
+            var txt = File.ReadAllText(fullPath);
+
+            // Deserialize.
+            var deserializer = new DeserializerBuilder()                
+                .WithTypeConverter(new StringWithSourceConverter(fullPath, txt))
+                .Build();
+            var modulePoco = deserializer.Deserialize<ModulePoco>(txt);
+
+            modulePoco.Src_Filename = fullPath;
+
+            return (modulePoco, this);
+        }
+    }
+}

src/libraries/Microsoft.PowerFx.Repl/Modules/IFileLoader.cs

@@ -0,0 +1,20 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+using System.Threading.Tasks;
+
+namespace Microsoft.PowerFx.Repl
+{
+    /// <summary>
+    /// A load can resolve a name to Module contents. 
+    /// </summary>
+    internal interface IFileLoader
+    {
+        /// <summary>
+        /// Load a module poco from the given fulename. 
+        /// </summary>
+        /// <param name="filename">a filename. Could be full path or relative. The loader will resolve.</param>
+        /// <returns>The loaded module contents and a new file loader for resolving any subsequent imports in this module.</returns>
+        Task<(ModulePoco, IFileLoader)> LoadAsync(string filename);
+    }
+}

src/libraries/Microsoft.PowerFx.Repl/Modules/Module.cs

@@ -0,0 +1,32 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+using System;
+
+namespace Microsoft.PowerFx.Repl
+{
+    internal class Module
+    {
+        /// <summary>
+        /// Public symbols exported by this module. 
+        /// </summary>
+        public ReadOnlySymbolTable Symbols { get; private set; }
+
+        /// <summary>
+        /// Identity of the module. We should never have two different modules with the same identity.
+        /// </summary>
+        public ModuleIdentity Identity { get; init; }
+
+        /// <summary>
+        /// Optional Full path that this module was loaded from. Null if not loaded from a file.
+        /// Primarily useful for helping makers debugging ("where did this module come from").
+        /// </summary>
+        public string FullPath { get; init; }
+
+        internal Module(ModuleIdentity identity, ReadOnlySymbolTable exports)
+        {
+            this.Identity = identity;
+            this.Symbols = exports ?? throw new ArgumentNullException(nameof(exports));
+        }        
+    }
+}

src/libraries/Microsoft.PowerFx.Repl/Modules/ModuleIdentity.cs

@@ -0,0 +1,70 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.IO;
+using System.Text;
+using Microsoft.CodeAnalysis.VisualBasic.Syntax;
+
+namespace Microsoft.PowerFx.Repl
+{
+    /// <summary>
+    /// A comparable handle representing module identity. 
+    /// </summary>
+    [DebuggerDisplay("{_value}")]
+    internal struct ModuleIdentity
+    {        
+        private readonly string _value;
+
+        private ModuleIdentity(string value)
+        {
+            _value = value;
+        }
+
+        /// <summary>
+        /// Get an identify for a file. 
+        /// </summary>
+        /// <param name="fullPath">full path to file. File does not need to actually exist.</param>
+        /// <returns>An identity.</returns>
+        public static ModuleIdentity FromFile(string fullPath)
+        {
+            if (fullPath == null)
+            {
+                throw new ArgumentNullException(nameof(fullPath));
+            }
+
+            if (!Path.IsPathRooted(fullPath))
+            {
+                throw new ArgumentException("Path must be rooted", nameof(fullPath));
+            }
+
+            // GetFullPath will normalize ".." to a canonical representation. 
+            // Path does not need to actually exist. 
+            string canonicalPath = Path.GetFullPath(fullPath);
+
+            // Lower to avoid case sensitivity.
+            var value = canonicalPath.ToLowerInvariant();
+
+            return new ModuleIdentity(value);
+        }
+
+        public override bool Equals(object obj)
+        {
+            if (obj is ModuleIdentity other)
+            {
+                return _value == other._value;
+            }
+
+            return false; 
+        }
+        
+        public override int GetHashCode()
+        {
+            return _value == null ? 0 : _value.GetHashCode();
+        }
+
+        // Do not implement ToString - identity should be treated opaque. 
+    }
+}

src/libraries/Microsoft.PowerFx.Repl/Modules/ModuleLoadContext.cs

@@ -0,0 +1,229 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using System.IO;
+using System.Linq;
+using System.Runtime.CompilerServices;
+using System.Text;
+using System.Threading.Tasks;
+using Microsoft.PowerFx.Core.Binding;
+using Microsoft.PowerFx.Core.Errors;
+using Microsoft.PowerFx.Core.Functions;
+using Microsoft.PowerFx.Core.Glue;
+using Microsoft.PowerFx.Core.Types;
+using Microsoft.PowerFx.Syntax;
+using YamlDotNet.Serialization;
+
+namespace Microsoft.PowerFx.Repl
+{
+    internal static class ModuleLoadContextExtentions
+    {   
+        // Load a module from disk. 
+        // This may throw on hard error (missing file), or return null with compile errors. 
+        public static async Task<Module> LoadFromFileAsync(this ModuleLoadContext context, string path, List<ExpressionError> errors)
+        {
+            var dir = Path.GetDirectoryName(path);
+            var name = Path.GetFileName(path);
+
+            var loader = new FileLoader(dir);
+
+            return await context.LoadAsync(name, loader, errors).ConfigureAwait(false);
+        }
+    }
+
+    /// <summary>
+    /// Context for loading a single module. 
+    /// This can ensure the module dependencies don't have cycles. 
+    /// </summary>
+    internal class ModuleLoadContext
+    {
+        // Core symbols we resolve against. 
+        // This is needed for all the builtins. 
+        private readonly ReadOnlySymbolTable _commonIncomingSymbols;
+
+        // Set of modules we visited - used to detect cycles. 
+        // Key String is the module's full path.
+        // Value is null if module is in-progress of being loaded (this detects cycles). 
+        // We still allow diamond dependencies. 
+        private readonly Dictionary<ModuleIdentity, Module> _alreadyLoaded = new Dictionary<ModuleIdentity, Module>();
+
+        public ModuleLoadContext(ReadOnlySymbolTable commonIncomingSymbols)
+        {
+            _commonIncomingSymbols = commonIncomingSymbols;
+        }
+
+        /// <summary>
+        /// Load and return a resolved module.  May throw on errors. 
+        /// </summary>
+        /// <param name="name">name to resolve via loader parameter.</param>
+        /// <param name="loader">file loader to resolve the name and any further imports.</param>
+        /// <param name="errors">error collection to populate if there are errors in parsing, binding, etc. </param>
+        /// <returns>A module. Or possible null if there are errors. Or may throw. </returns>        
+        public async Task<Module> LoadAsync(string name, IFileLoader loader, List<ExpressionError> errors)
+        {
+            (var poco, var loader2) = await loader.LoadAsync(name).ConfigureAwait(false);
+
+            ModuleIdentity fullPathIdentity = poco.GetIdentity();
+            if (_alreadyLoaded.TryGetValue(fullPathIdentity, out var existing))
+            {
+                if (existing != null)
+                {
+                    // If we fully loaded the module, then it's not circular. 
+                    return existing;
+                }                
+                
+                // Circular reference. 
+                throw new InvalidOperationException($"Circular reference: {name}");                
+            }
+
+            _alreadyLoaded.Add(fullPathIdentity, null); // In-progress
+
+            // Load all imports first. 
+
+            var symbolList = new List<ReadOnlySymbolTable>
+            {
+                _commonIncomingSymbols
+            };
+            var unique = new ConflictTracker();
+
+            if (poco.Imports != null)
+            {
+                foreach (var import in poco.Imports)
+                {
+                    string file = import.File;
+                    if (file != null)
+                    {
+                        var m2 = await LoadAsync(file, loader2, errors).ConfigureAwait(false);
+                        if (m2 == null)
+                        {
+                            return null;
+                        }
+
+                        unique.VerifyUnique(m2);
+                        symbolList.Add(m2.Symbols);
+                    }
+                }
+            }
+
+            var incomingSymbols = ReadOnlySymbolTable.Compose(symbolList.ToArray());
+
+            var moduleExports = new SymbolTable { DebugName = name };
+
+            bool ok = ResolveBody(poco, moduleExports, incomingSymbols, errors);
+
+            if (!ok)
+            {
+                // On errors
+                return null;
+            }
+
+            var module = new Module(fullPathIdentity, moduleExports)
+            {
+                FullPath = poco.Src_Filename
+            };
+
+            _alreadyLoaded[fullPathIdentity] = module; // done loading. 
+
+            return module;
+        }
+
+        // poco - the yaml file contents. 
+        // incomingSymbols - what body can bind to. Builtins, etc. 
+        //   any module dependencies should already be resolved and included here. 
+        // moduleExports - symbols to add to. 
+        // Return true on successful load, false on failure
+        private bool ResolveBody(ModulePoco poco, SymbolTable moduleExports, ReadOnlySymbolTable incomingSymbols, List<ExpressionError> errors)
+        {
+            var str = poco.Formulas.Value;
+
+            bool allowSideEffects = true;
+            var options = new ParserOptions
+            {
+                AllowParseAsTypeLiteral = true,
+                AllowsSideEffects = true
+            };
+
+            var parseResult = UserDefinitions.Parse(str, options);
+
+            var fragmentLocation = poco.Formulas.Location;
+
+            // Scan for errors.
+            errors.AddRange(ExpressionError.NewFragment(parseResult.Errors, str, fragmentLocation));
+
+            if (errors.Any(x => !x.IsWarning))
+            {
+                // Abort on errors. 
+                return false;
+            }
+
+            {
+                var errors2 = parseResult.UDFs.Where(udf => !udf.IsParseValid).FirstOrDefault();
+                if (errors2 != null)
+                {
+                    // Should have been caught above. 
+                    throw new InvalidOperationException($"Errors should have already been caught");
+                }
+            }
+
+            var definedTypes = parseResult.DefinedTypes;
+            if (definedTypes != null && definedTypes.Any())
+            {
+                var resolvedTypes = DefinedTypeResolver.ResolveTypes(definedTypes, incomingSymbols, out var errors4);
+                errors.AddRange(ExpressionError.NewFragment(errors4, str, fragmentLocation));
+                if (errors.Any(x => !x.IsWarning))
+                {
+                    // Abort on errors. 
+                    return false;
+                }
+
+                moduleExports.AddTypes(resolvedTypes);
+            }
+
+            var s2 = ReadOnlySymbolTable.Compose(moduleExports, incomingSymbols);
+
+            // Convert parse --> TexlFunctions
+            // fail if duplicates detected within this batch. 
+            // fail if any names are resverd
+            // Basic type checking 
+            IEnumerable<UserDefinedFunction> udfs = UserDefinedFunction.CreateFunctions(parseResult.UDFs, s2, out var errors3);
+
+            errors.AddRange(ExpressionError.NewFragment(errors3, str, fragmentLocation));
+            if (errors.Any(x => !x.IsWarning))
+            {
+                return false;
+            }
+
+            // Body can refer to other functions defined in this batch. So we need 2 pass.
+            // First add all definitions. 
+            foreach (var udf in udfs)
+            {
+                moduleExports.AddFunction(udf);
+            }
+
+            // Then bind all bodies.
+            foreach (var udf in udfs)
+            {
+                var config = new BindingConfig(allowsSideEffects: allowSideEffects, useThisRecordForRuleScope: false, numberIsFloat: false);
+
+                Features features = Features.PowerFxV1;
+
+                var binding = udf.BindBody(s2, new Glue2DocumentBinderGlue(), config, features);
+
+                List<TexlError> bindErrors = new List<TexlError>();
+
+                binding.ErrorContainer.GetErrors(ref bindErrors);
+                errors.AddRange(ExpressionError.NewFragment(bindErrors, str, fragmentLocation));
+
+                if (errors.Any(x => !x.IsWarning))
+                {
+                    return false;
+                }
+            }
+
+            return true;
+        }
+    }
+}

src/libraries/Microsoft.PowerFx.Repl/Modules/ModulePoco.cs

@@ -0,0 +1,46 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+using Microsoft.PowerFx.Syntax;
+
+namespace Microsoft.PowerFx.Repl
+{
+    /// <summary>
+    /// The yaml representation of a module. 
+    /// Properties with 'Src_' prefix are set by deserializer and not part of the yaml file contents.
+    /// </summary>
+    internal class ModulePoco
+    {
+        /// <summary>
+        /// Full path of file this was loaded from. 
+        /// This is set by the deserializer. 
+        /// </summary>
+        public string Src_Filename { get; set; }
+
+        public ModuleIdentity GetIdentity()
+        {
+            return ModuleIdentity.FromFile(Src_Filename);
+        }
+
+        /// <summary>
+        /// Contain Power Fx UDF declarations. 
+        /// </summary>
+        public StringWithSource Formulas { get; set; }
+
+        /// <summary>
+        /// Set of modules that this depends on. 
+        /// </summary>
+        public ImportPoco[] Imports { get; set; }
+    }
+
+    internal class ImportPoco
+    {
+        /// <summary>
+        /// File path to import from. "foo.fx.yml" or ".\path\foo.fx.yml".
+        /// </summary>
+        public string File { get; set; }
+
+        // identifier resolved against the host. 
+        public string Host { get; set; }
+    }
+}

src/libraries/Microsoft.PowerFx.Repl/Modules/StringWithSource.cs

@@ -0,0 +1,25 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+using Microsoft.PowerFx.Syntax;
+
+namespace Microsoft.PowerFx.Repl
+{
+    /// <summary>
+    /// This is a string object, but also tagged with a solution location for where the string contents start. 
+    /// This is restircted to Inline strings (and rejects other formats like fodling, quotes, etc). 
+    /// Use a custom converter to populate the source location of the string. See <see cref="StringWithSourceConverter"/> .
+    /// </summary>
+    internal class StringWithSource
+    {
+        /// <summary>
+        /// The contents of the string from the yaml. 
+        /// </summary>
+        public string Value { get; set; }
+
+        /// <summary>
+        /// The location in the file for where the contents start. Useful for error reporting. 
+        /// </summary>
+        public FileLocation Location { get; set; }
+    }
+}

src/libraries/Microsoft.PowerFx.Repl/Modules/StringWithSourceConverter.cs

@@ -0,0 +1,110 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using System.IO;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Microsoft.PowerFx.Core.Binding;
+using Microsoft.PowerFx.Core.Errors;
+using Microsoft.PowerFx.Core.Functions;
+using Microsoft.PowerFx.Core.Glue;
+using Microsoft.PowerFx.Core.Utils;
+using Microsoft.PowerFx.Syntax;
+using YamlDotNet.Core;
+using YamlDotNet.Core.Events;
+using YamlDotNet.Serialization;
+using YamlDotNet.Serialization.NodeDeserializers;
+
+namespace Microsoft.PowerFx.Repl
+{
+    /// <summary>
+    /// Yaml converter to parse a <see cref="StringWithSource"/> and tag 
+    /// it with the source location. 
+    /// </summary>
+    internal class StringWithSourceConverter : IYamlTypeConverter
+    {
+        private readonly string _filename;
+        private readonly string[] _lines; // to infer indent level.
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="StringWithSourceConverter"/> class.
+        /// </summary>
+        /// <param name="filename">filename to tag with.</param>
+        /// <param name="contents">contents of the file - this is used to infer indenting depth that can't be gotten from th yaml parser.</param>
+        public StringWithSourceConverter(string filename, string contents)
+        {
+            _filename = filename;
+
+            _lines = contents.Split(new[] { Environment.NewLine }, StringSplitOptions.None);
+        }
+
+        public bool Accepts(Type type)
+        {
+            return type == typeof(StringWithSource);
+        }
+
+        public object ReadYaml(IParser parser, Type type)
+        {
+            var val = parser.Consume<Scalar>();
+
+            // Empty case 
+            if (val.Style == ScalarStyle.Plain && 
+                val.Value.Length == 0)
+            {
+                return new StringWithSource
+                {
+                    Value = null, // Preserve same semantics as String deserialization.
+                    Location = new FileLocation
+                    {
+                        Filename = _filename,
+                        LineStart = val.Start.Line,
+                        ColStart = val.Start.Column
+                    }
+                };
+            }
+
+            // https://stackoverflow.com/questions/3790454/how-do-i-break-a-string-in-yaml-over-multiple-lines
+            // Yaml has many multi-line encodings. Fortunately, we can determine this based on ScalarStyle 
+            // We want to force | , which is Literal. This means the fx is copy & paste into yaml, with an indent level
+            // and no other mutation.  
+            //
+            // This will prevent all other forms, including:
+            // > is "fold" which will remove newlines. 
+            // "" is double quotes. 
+            // plain where the string is single-line embedded.
+
+            if (val.Style != ScalarStyle.Literal)
+            {
+                // This is important if we want to preserve character index mapping. 
+                throw new InvalidOperationException($"must be literal string encoding at {val.Start.Line}");
+            }
+
+            int line1PropertyName = val.Start.Line; // 1-based Line that the "Formulas" property is on. Content starts on next line. 
+
+            // YamlParser does not tell us indent level. Must discern it. 
+            var line = _lines[line1PropertyName];
+            var colStart0 = line.FindIndex(x => !char.IsWhiteSpace(x));
+
+            return new StringWithSource
+            {
+                Value = val.Value,
+                Location = new FileLocation
+                {
+                    Filename = _filename,
+                    LineStart = line1PropertyName + 1,
+                    ColStart = colStart0 + 1
+                }
+            };
+        }
+
+        public void WriteYaml(IEmitter emitter, object value, Type type)
+        {
+            // Only reads. 
+            throw new NotImplementedException();
+        }
+    } // end class 
+}

src/libraries/Microsoft.PowerFx.Repl/Repl.cs

@@ -37,6 +37,13 @@ namespace Microsoft.PowerFx
         // Allow repl to create new UserDefinedFunctions. 
         public bool AllowUserDefinedFunctions { get; set; }
 
+        /// <summary>
+        /// Enable the Import() function for importing modules. 
+        /// Defaults to false. 
+        /// </summary>
+        [Obsolete("preview")]
+        public bool AllowImport { get; set; }
+
         // Do we print each command before evaluation?
         // Useful if we're running a file and are debugging, or if input UI is separated from output UI. 
         public bool Echo { get; set; } = false;
@@ -76,6 +83,11 @@ namespace Microsoft.PowerFx
         /// </summary>
         public ReadOnlySymbolValues ExtraSymbolValues { get; set; }
 
+        // Map from Module full path to Module.
+        private readonly Dictionary<string, Module> _loadedModules = new Dictionary<string, Module>(StringComparer.OrdinalIgnoreCase);
+
+        internal IEnumerable<Module> Modules => _loadedModules.Values;
+
         /// <summary>
         /// Get sorted names of all functions. This includes functions from the <see cref="Engine"/> as well as <see cref="MetaFunctions"/>.
         /// </summary>
@@ -94,6 +106,68 @@ namespace Microsoft.PowerFx
             }
         }
 
+        // Get combined symbol values (extra, modules, etc)
+        public ReadOnlySymbolValues GetCombined()
+        {
+            // Combine with symbols from modules. 
+            var list = new List<ReadOnlySymbolTable>();
+
+            foreach (var module in _loadedModules.Values)
+            {
+                list.Add(module.Symbols);
+            }
+
+            if (this.ExtraSymbolValues != null)
+            {
+                list.Add(this.ExtraSymbolValues.SymbolTable);
+            }
+
+            var m1 = ReadOnlySymbolTable.Compose(list.ToArray());
+
+            var values = m1.CreateValues(this.ExtraSymbolValues);
+
+            return values;
+        }
+
+        internal bool TryResolveModule(string path, out Module module)
+        {
+            if (_loadedModules.TryGetValue(path, out module))
+            {
+                return true;
+            }
+
+            // can we resolve by short name?
+            List<Module> list = new List<Module>();
+
+            foreach (var m in _loadedModules.Values)
+            {
+                string shortName = System.IO.Path.GetFileName(m.FullPath);
+                if (string.Equals(shortName, path, StringComparison.OrdinalIgnoreCase))
+                {
+                    list.Add(m);                    
+                }
+            }
+
+            if (list.Count == 1)
+            {
+                module = list[0];
+                return true;
+            }
+
+            return false;
+        }
+
+        internal void DeleteModule(Module module)
+        {
+            _loadedModules.Remove(module.FullPath);
+        }
+
+        internal void AddModule(Module module)
+        {
+            string id = module.FullPath;
+            _loadedModules[id] = module;
+        }
+
         // Interpreter should normally not throw.
         // Exceptions should be caught and converted to ErrorResult.
         // Not called for OperationCanceledException since those are expected.
@@ -123,6 +197,8 @@ namespace Microsoft.PowerFx
             this.MetaFunctions.AddFunction(new Help1Function(this));
         }
 
+        private bool _finishInit = false;
+
         private bool _userEnabled = false;
 
         public void EnableSampleUserObject()
@@ -217,6 +293,22 @@ namespace Microsoft.PowerFx
             }
         }
 
+        // Property ctor is run before Init properties are set. 
+        // So apply final initialization after property initializers but before we execute commands. 
+        private void FinishInit()
+        {
+            if (!_finishInit)
+            {
+                _finishInit = true;
+                if (this.AllowImport)
+                {   
+                    this.MetaFunctions.AddFunction(new ImportFunction(this));
+                    this.MetaFunctions.AddFunction(new ListModulesFunction(this));
+                    this.MetaFunctions.AddFunction(new DeleteModuleFunction(this));
+                }
+            }
+        }
+
         /// <summary>
         /// Directly invoke a command. This skips multiline handling. 
         /// </summary>
@@ -239,6 +331,8 @@ namespace Microsoft.PowerFx
                 return new ReplResult();
             }
 
+            FinishInit();
+
             if (this.Echo)
             {
                 await this.WritePromptAsync(cancel);
@@ -247,9 +341,10 @@ namespace Microsoft.PowerFx
                 await this.Output.WriteLineAsync(expression.TrimEnd(), OutputKind.Repl, cancel);
             }
 
-            var extraSymbolTable = this.ExtraSymbolValues?.SymbolTable;
+            var extraValues = this.GetCombined();
+            var extraSymbolTable = extraValues.SymbolTable;
             
-            var runtimeConfig = new RuntimeConfig(this.ExtraSymbolValues)
+            var runtimeConfig = new RuntimeConfig(extraValues)
             {
                  ServiceProvider = new BasicServiceProvider(this.InnerServices)
             };
@@ -363,7 +458,7 @@ namespace Microsoft.PowerFx
                         // Get the type. 
                         var rhsExpr = declare._rhs.GetCompleteSpan().GetFragment(expression);
 
-                        var setCheck = this.Engine.Check(rhsExpr, ParserOptions, this.ExtraSymbolValues?.SymbolTable);
+                        var setCheck = this.Engine.Check(rhsExpr, ParserOptions, this.GetCombined().SymbolTable);
                         if (!setCheck.IsSuccess)
                         {
                             await this.Output.WriteLineAsync($"Error: Failed to initialize '{name}'.", OutputKind.Error, cancel)

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

@@ -0,0 +1,44 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using System.Linq;
+using Microsoft.PowerFx.Core.Errors;
+using Microsoft.PowerFx.Core.Localization;
+using Microsoft.PowerFx.Syntax;
+using Xunit;
+
+namespace Microsoft.PowerFx.Core.Tests
+{
+    public class FileLocationTests
+    {
+        [Theory]
+        [InlineData(0, 200, 300)]
+        [InlineData(1, 200, 301)]
+        [InlineData(4, 201, 300)]
+        [InlineData(9, 202, 301)]
+        public void Apply(int min, int expectedLine, int expectedCol)
+        {
+            string file =
+
+ // 012 345 6 789
+   "ABC\nDE\r\nFG";
+
+            var loc = new FileLocation
+            {
+                Filename = "test",
+                LineStart = 200,
+                ColStart = 300
+            };
+
+            var span = new Span(min, min + 1);
+            var loc2 = loc.Apply(file, span);
+
+            Assert.Same(loc.Filename, loc2.Filename);
+            Assert.Equal(expectedLine, loc2.LineStart);
+            Assert.Equal(expectedCol, loc2.ColStart);
+        }
+    }
+}

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

@@ -68,6 +68,7 @@ namespace Microsoft.PowerFx.Core.Tests
                 "Microsoft.PowerFx.Syntax.IdentToken",
                 "Microsoft.PowerFx.Syntax.NumLitToken",
                 "Microsoft.PowerFx.Syntax.Span",
+                "Microsoft.PowerFx.Syntax.FileLocation",
                 "Microsoft.PowerFx.Syntax.StrLitToken",
                 "Microsoft.PowerFx.Syntax.Token",
                 "Microsoft.PowerFx.Syntax.TokKind",

src/tests/Microsoft.PowerFx.Repl.Tests.Shared/Microsoft.PowerFx.Repl.Tests.Shared.projitems

@@ -10,8 +10,20 @@
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="$(MSBuildThisFileDirectory)AssemblyProperties2.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Mocks\TempFileHolder.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)ModuleIdentityTests.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)ModuleTests.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)MultilineProcessorTests.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)ReplTests.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Mocks\TestReplOutput.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)StringWithSourceTests.cs" />
+  </ItemGroup>
+  <ItemGroup>
+    <Folder Include="$(MSBuildThisFileDirectory)Modules\" />
+  </ItemGroup>
+  <ItemGroup>
+    <None Include="$(MSBuildThisFileDirectory)modules\*.fx.yml">
+      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
+    </None>
   </ItemGroup>
 </Project>

src/tests/Microsoft.PowerFx.Repl.Tests.Shared/Mocks/TempFileHolder.cs

@@ -0,0 +1,27 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+using System;
+using System.IO;
+
+namespace Microsoft.PowerFx.Repl.Tests
+{
+    // Helper for creating temp files and cleaning up.
+    internal class TempFileHolder : IDisposable
+    {
+        public string FullPath { get; } = Path.GetTempFileName();
+
+        public void Dispose()
+        {
+            // Cleanup the file. 
+            try
+            {
+                File.Delete(this.FullPath);
+            }
+            catch
+            {
+                // Ignore
+            }
+        }
+    }
+}

src/tests/Microsoft.PowerFx.Repl.Tests.Shared/ModuleIdentityTests.cs

@@ -0,0 +1,81 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+using System;
+using System.Collections.Generic;
+using Xunit;
+
+namespace Microsoft.PowerFx.Repl.Tests
+{
+    public class ModuleIdentityTests
+    {
+        [Fact]
+        public void Collections()
+        {
+            var id1a = ModuleIdentity.FromFile(@"z:\path1.txt");
+            var id1b = ModuleIdentity.FromFile(@"z:\Path1.txt");
+
+            var id2 = ModuleIdentity.FromFile(@"z:\path2.txt");
+
+            Assert.Equal(id1a, id1a); // identity 
+            Assert.Equal(id1a.GetHashCode(), id1a.GetHashCode()); 
+
+            Assert.NotEqual(id1a, id2);
+
+            // paths should be normalized 
+            Assert.Equal(id1a, id1b);
+
+            // Work with collections.
+            var set = new HashSet<ModuleIdentity>();
+
+            var added = set.Add(id1a);
+            Assert.True(added);
+
+            added = set.Add(id1a);
+            Assert.False(added);
+
+            added = set.Add(id1b);
+            Assert.False(added);
+
+            Assert.Contains(id1a, set);
+            Assert.Contains(id1b, set);
+
+            Assert.DoesNotContain(id2, set);
+
+            added = set.Add(id2);
+            Assert.True(added);
+        }
+
+        // Ensure we normalize file paths, particularly when using ".."
+        [Fact]
+        public void Canonical()
+        {
+            var id1a = ModuleIdentity.FromFile(@"z:\foo\path1.txt");
+            var id1b = ModuleIdentity.FromFile(@"z:\foo\bar\..\Path1.txt");
+
+            Assert.Equal(id1a, id1a); // identity 
+        }
+
+        [Fact]
+        public void NoToString()
+        {
+            var id1 = ModuleIdentity.FromFile(@"z:\foo\path1.txt");
+
+            // *Don't implement ToString* -  we want to treat identity as opauque.
+            var str = id1.ToString();
+
+            Assert.Equal("Microsoft.PowerFx.Repl.ModuleIdentity", str);
+        }
+
+        [Fact]
+        public void MustBeFullPath()
+        {
+            Assert.Throws<ArgumentNullException>(() => ModuleIdentity.FromFile(null));
+                        
+            // Must be a full path 
+            Assert.Throws<ArgumentException>(() => ModuleIdentity.FromFile("path1.txt"));
+            Assert.Throws<ArgumentException>(() => ModuleIdentity.FromFile(@".\path1.txt"));
+            Assert.Throws<ArgumentException>(() => ModuleIdentity.FromFile(string.Empty));
+        }
+    }
+}

src/tests/Microsoft.PowerFx.Repl.Tests.Shared/ModuleTests.cs

@@ -0,0 +1,306 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.IO;
+using System.Linq;
+using System.Text;
+using System.Text.RegularExpressions;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.CodeAnalysis.VisualBasic.Syntax;
+using Microsoft.PowerFx.Types;
+using Xunit;
+
+#pragma warning disable CS0618 // Type or member is obsolete
+#pragma warning disable SA1118 // Parameter should not span multiple lines
+
+namespace Microsoft.PowerFx.Repl.Tests
+{
+    // Thesde tests should target public Module Import() function 
+    // (and not internal services)
+    public class ModuleTests
+    {
+        private PowerFxREPL _repl;
+        private readonly TestReplOutput _output = new TestReplOutput();
+
+        public ModuleTests()
+        {
+            var config = new PowerFxConfig();
+            config.SymbolTable.EnableMutationFunctions();
+
+            // config.EnableSetFunction();
+            var engine = new RecalcEngine(config);
+
+            _repl = new PowerFxREPL
+            {
+                Engine = engine,
+                Output = _output,
+                AllowSetDefinitions = true,
+                AllowUserDefinedFunctions = true,
+                AllowImport = true,
+                ParserOptions = new ParserOptions() { AllowsSideEffects = true }
+            };
+        }
+        
+        // Run line, return the normal output. 
+        private string HandleLine(string fx, bool expectErrors = false)
+        {
+            Debugger.Log(0, string.Empty, ">> " + fx);
+            _repl.HandleLine(fx);
+
+            if (!expectErrors)
+            {
+                AssertNoErrors();
+            }
+
+            var log = _output.Get(OutputKind.Repl);
+
+            Debugger.Log(0, string.Empty, log);
+            return log;
+        }
+
+        private void AssertNoErrors()
+        {
+            var errors = _output.Get(OutputKind.Error);
+            Assert.Empty(errors);
+        }
+
+        private string GetModuleFullPath(string path)
+        {
+            string fullpath = Path.Combine(
+                Environment.CurrentDirectory,
+                "Modules",
+                path);
+
+            return fullpath;
+        }
+
+        private void Import(string path, bool expectErrors = false)
+        {
+            string fullpath = GetModuleFullPath(path);
+
+            var expr = $"Import(\"{fullpath}\")";
+            HandleLine(expr, expectErrors);
+
+            if (expectErrors)
+            {
+                var modules = _repl.Modules.ToArray();
+                Assert.Empty(modules); // nothing actually loaded. 
+            }
+        }
+
+        // Ensure that we only can access modules if they're enabled. 
+        [Fact]
+        public void MustEnable()
+        {
+            var engine = new RecalcEngine();
+
+            _repl = new PowerFxREPL
+            {
+                Engine = engine,
+                Output = _output            
+            };
+
+            // Defaults to false
+            Assert.False(_repl.AllowUserDefinedFunctions);
+
+            Import("basic1.fx.yml", expectErrors: true);
+            
+            // Failed to import. 
+            var msg = _output.Get(OutputKind.Error);
+            Assert.Contains("'Import' is an unknown or unsupported function.", msg);
+        }
+
+        [Fact]
+        public void Test1()
+        {
+            Import("basic1.fx.yml");
+
+            // Call method defined in moduel.
+            var log = HandleLine("DoubleIt(3)");
+
+            Assert.Equal("6", log);
+        }
+
+        // Load multiple modules
+        [Fact]
+        public void Test2()
+        {
+            Import("basic1.fx.yml");
+            Import("basic2.fx.yml");
+
+            // Call method defined in moduel.
+            var log = HandleLine("DoubleIt(Add1(3))");
+
+            Assert.Equal("8", log);
+        }
+
+        // Load with imports
+        [Fact]
+        public void TestImport()
+        {
+            Import("Depend2.fx.yml");
+
+            // Call method defined in moduel.
+            var log = HandleLine("Func2(3)");
+
+            Assert.Equal("8", log);
+        }
+
+        // Load with imports
+        [Fact]
+        public void TestImport2()
+        {
+            Import("Depend2.fx.yml");
+
+            // Call method defined in moduel.
+            var log = HandleLine("Func2(3)");
+            Assert.Equal("8", log);
+
+            // Inner dependencies are not exported 
+            log = HandleLine("Add1(3)", expectErrors: true);
+            var msg = _output.Get(OutputKind.Error);
+            Assert.Contains("Add1' is an unknown or unsupported function.", msg);
+
+            // But we can import the inner module and call.
+            Import("basic2.fx.yml");
+            log = HandleLine("Add1(3)");
+            Assert.Equal("4", log);
+        }
+
+        // Load with imports
+        [Fact]
+        public void Recursion()
+        {
+            Import("recursion.fx.yml");
+
+            // 0,1, 1, 2, 3, 5, 8,13
+            var log = HandleLine("Fib(7)"); 
+            Assert.Equal("13", log);
+        }
+
+        // Load with imports
+        [Fact]
+        public void Shadow()
+        {
+            Import("shadow.fx.yml");
+
+            var log = HandleLine("Foo()");
+            Assert.Equal("-2", log);
+
+            log = HandleLine("Abs(-7)"); // calls new one from module
+        }
+
+        // Diamond inheritence.
+        // 1 --> {2a, 2b}. 2a-->3. 2b-->3.
+        // This is interesting since module 3 gets used multiple times, but it's not a cycle. 
+        [Fact]
+        public void Diamond()
+        {
+            Import("diamond_1.fx.yml");
+
+            var log = HandleLine("Func1(5)");
+            Assert.Equal("\"2A(3(5)),2B(3(5))\"", log);
+        }
+
+        // Conflict if we import 2 modules that define the same symbols. 
+        [Fact]
+        public void DuplicateSymbolsConflict()
+        {
+            Import("conflict1.fx.yml", expectErrors: true);
+            var errorMsg = _output.Get(OutputKind.Error);
+
+            // message contains useful information.
+            Assert.Contains("DoubleIt", errorMsg); // conflicting symbol
+            Assert.Contains("basic1.fx.yml", errorMsg); // module 1
+            Assert.Contains("basic1_dup.fx.yml", errorMsg); // module 2
+        }
+
+        // Import() is a meta function and can only be called 
+        // at top-levle repl and not within a module itself.
+        [Fact]
+        public void ImportIsAMetafunction()
+        {
+            Import("ErrorImport.fx.yml", expectErrors: true);
+            var errorMsg = _output.Get(OutputKind.Error);
+
+            Assert.Contains("'Import' is an unknown or unsupported function", errorMsg);
+        }
+
+        // When we import a file twice, we get the updated contents.
+        [Fact]
+        public void GetsLatest()
+        {
+            using var temp = new TempFileHolder();
+            string fullpath = temp.FullPath;
+
+            // First version
+            File.WriteAllText(fullpath, @"
+Formulas: |
+  Func(x: Number) : Number = x * 10;
+");
+            HandleLine($"Import(\"{fullpath}\")");
+
+            // Call method defined in moduel.
+            var before = HandleLine("Func(3)");
+            Assert.Equal("30", before);
+
+            // Update the file, 2nd version
+            File.WriteAllText(fullpath, @"
+Formulas: |
+  Func(x: Number) : Number = x * 20;
+");
+
+            HandleLine($"Import(\"{fullpath}\")");
+
+            var after = HandleLine("Func(3)");
+            Assert.Equal("60", after);
+        }
+
+        // Error gets spans. 
+        [Fact]
+        public void ErrorShowsFileRange()
+        {
+            Import("Error1.fx.yml", expectErrors: true);
+
+            var errorMsg = _output.Get(OutputKind.Error);
+                                    
+            // Key elements here are that:
+            // - the message has the filename
+            // - the location (5,9) is relative into the file, not just the expression.
+            Assert.Equal("Error: Error1.fx.yml (5,9): Name isn't valid. 'missing' isn't recognized.", errorMsg);
+        }
+
+        // Circular references are detected 
+        [Fact]
+        public void Cycles()
+        {
+            Import("cycle1.fx.yml", expectErrors: true);
+
+            var errorMsg = _output.Get(OutputKind.Error);
+            Assert.Contains("Circular reference", errorMsg);
+        }
+
+        // Loading a missing file 
+        [Fact]
+        public void Missing()
+        {
+            Import("missing_file.fx.yml", expectErrors: true);
+
+            var errorMsg = _output.Get(OutputKind.Error);
+            Assert.Contains("missing_file.fx.yml", errorMsg);
+        }
+
+        // Loading a file with yaml parse errors.
+        [Fact]
+        public void YamlParseErrors()
+        {
+            Import("yaml_parse_errors.fx.yml", expectErrors: true);
+
+            var errorMsg = _output.Get(OutputKind.Error);
+        }
+    }
+}

src/tests/Microsoft.PowerFx.Repl.Tests.Shared/Modules/Depend2.fx.yml

@@ -0,0 +1,7 @@
+Imports:
+- File: Basic1.fx.yml
+- File: Basic2.fx.yml
+
+# Depends on Basic1
+Formulas: |
+  Func2(x: Number) : Number = DoubleIt(Add1(x));

src/tests/Microsoft.PowerFx.Repl.Tests.Shared/Modules/Error1.fx.yml

@@ -0,0 +1,6 @@
+# Has an error in expression. 
+# Test that error reporting gets right line. 
+Formulas: |
+  DoubleIt(x: Number) : Number = 
+        missing  // <-- error is on line 5
+    * 2; 

src/tests/Microsoft.PowerFx.Repl.Tests.Shared/Modules/ErrorImport.fx.yml

@@ -0,0 +1,7 @@
+# Error - we can't call Import() within a function.
+# must be a top-level yaml construct. 
+
+Formulas: |
+  Func2(x: Number) : Void = { 
+    Import("Basic1.fx.yml")
+  };

src/tests/Microsoft.PowerFx.Repl.Tests.Shared/Modules/basic1.fx.yml

@@ -0,0 +1,3 @@
+# Most basic Power Fx module. 
+Formulas: |
+  DoubleIt(x: Number) : Number = x * 2;

src/tests/Microsoft.PowerFx.Repl.Tests.Shared/Modules/basic1_dup.fx.yml

@@ -0,0 +1,4 @@
+# test symbol conflicts. 
+# Same symbols as base1.fx.yml 
+Formulas: |
+  DoubleIt(x: Number) : Number = x * 200;

src/tests/Microsoft.PowerFx.Repl.Tests.Shared/Modules/basic2.fx.yml

@@ -0,0 +1,3 @@
+# Most basic Power Fx module. 
+Formulas: |
+  Add1(x: Number) : Number = x +1;

src/tests/Microsoft.PowerFx.Repl.Tests.Shared/Modules/conflict1.fx.yml

@@ -0,0 +1,6 @@
+Imports:
+- File: basic1.fx.yml
+- File: basic1_dup.fx.yml
+
+Formulas: |
+    Run() : Number = DoubleIt(5);    

src/tests/Microsoft.PowerFx.Repl.Tests.Shared/Modules/cycle1.fx.yml

@@ -0,0 +1,6 @@
+# Test circular dependency 
+Imports:
+- File: cycle2.fx.yml
+
+Formulas: |
+  Func1(x: Number) : Number = 1;

src/tests/Microsoft.PowerFx.Repl.Tests.Shared/Modules/cycle2.fx.yml

@@ -0,0 +1,6 @@
+# Test circular dependency 
+Imports:
+- File: cycle1.fx.yml
+
+Formulas: |
+  Func2(x: Number) : Number = 1;

src/tests/Microsoft.PowerFx.Repl.Tests.Shared/Modules/diamond_1.fx.yml

@@ -0,0 +1,7 @@
+Imports:
+- File: diamond_2a.fx.yml
+- File: diamond_2b.fx.yml
+
+# Depends on 2a,2b. Both depend on 3. 
+Formulas: |
+  Func1(x: Text) : Text = Func2a(x) & "," & Func2b(x);

src/tests/Microsoft.PowerFx.Repl.Tests.Shared/Modules/diamond_2a.fx.yml

@@ -0,0 +1,8 @@
+Imports:
+- File: diamond_3.fx.yml
+
+
+# Depends on Basic1
+Formulas: |
+  Func2a(x: Text) : Text = $"2A({Func3(x)})";
+

src/tests/Microsoft.PowerFx.Repl.Tests.Shared/Modules/diamond_2b.fx.yml

@@ -0,0 +1,8 @@
+Imports:
+- File: diamond_3.fx.yml
+
+
+# Depends on Basic1
+Formulas: |
+  Func2b(x: Text) : Text = $"2B({Func3(x)})";
+

src/tests/Microsoft.PowerFx.Repl.Tests.Shared/Modules/diamond_3.fx.yml

@@ -0,0 +1,3 @@
+
+Formulas: |
+  Func3(x: Text) : Text = $"3({x})";

src/tests/Microsoft.PowerFx.Repl.Tests.Shared/Modules/recursion.fx.yml

@@ -0,0 +1,4 @@
+# Define a basic recursive function. 
+Formulas: |
+    Fib(n: Number) : Number = 
+        If(n <= 1, n, Fib(n - 1) + Fib(n - 2));

src/tests/Microsoft.PowerFx.Repl.Tests.Shared/Modules/shadow.fx.yml

@@ -0,0 +1,8 @@
+
+Formulas: |
+    // Redefine a builtin. 
+    // Give a different (wrong) implementation so we know which is called. 
+    Abs(x: Number) : Number = x  *2;
+
+    // Binds to the one in this scope.
+    Foo() : Number = Abs(-1);

src/tests/Microsoft.PowerFx.Repl.Tests.Shared/Modules/yaml_parse_errors.fx.yml

@@ -0,0 +1,3 @@
+# not a valid yaml file
+Formulas::
+    

src/tests/Microsoft.PowerFx.Repl.Tests.Shared/StringWithSourceTests.cs

@@ -0,0 +1,176 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.IO;
+using System.Linq;
+using System.Text;
+using System.Text.RegularExpressions;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.CodeAnalysis.CodeFixes;
+using Microsoft.CodeAnalysis.VisualBasic.Syntax;
+using Microsoft.PowerFx.Types;
+using Xunit;
+using YamlDotNet.Core;
+using YamlDotNet.Serialization;
+
+namespace Microsoft.PowerFx.Repl.Tests
+{
+#pragma warning disable SA1117 // Parameters should be on same line or separate lines
+
+    public class StringWithSourceTests
+    {
+        private class Poco1
+        {
+            public StringWithSource Prop { get; set; }
+        }
+
+        private class Poco1Direct
+        {
+            public string Prop { get; set; }
+        }
+
+        // common fake filename used in locations. 
+        private const string Filename = "myfile1.txt";
+
+        private T Parse<T>(string contents)
+        {
+            // Deserialize.
+            var deserializer = new DeserializerBuilder()
+                .WithTypeConverter(new StringWithSourceConverter(Filename, contents))
+                .Build();
+
+            var poco = deserializer.Deserialize<T>(contents);
+
+            return poco;
+        }
+
+        private T ParseExpectError<T>(string contents, string expectedError)
+        {
+            try
+            {
+                // Deserialize.
+                var deserializer = new DeserializerBuilder()
+                    .WithTypeConverter(new StringWithSourceConverter(Filename, contents))
+                    .Build();
+
+                var poco = deserializer.Deserialize<T>(contents);
+
+                Assert.Fail($"Expected error: {expectedError}");
+                return poco;
+            }
+            catch (YamlException e)
+            {
+                // Exceptions from parser will be wrapped in YamlException.
+                var msg = e?.InnerException?.Message ?? e.Message;
+                Assert.Contains(expectedError, msg);
+            }
+
+            return default;
+        }
+
+        // In all cases, parse as 'string' should succeed. 
+        // But Parse with SourceLocaiton has more limitations.
+        //  expectedError specifies restrictions. 
+        private StringWithSource Test(string contents, string expectedError = null)
+        {
+            // Should always work with 'string'
+            var p1 = Parse<Poco1Direct>(contents);
+
+            if (expectedError != null)
+            {
+                ParseExpectError<Poco1>(contents, expectedError);
+                return null;
+            }
+            else
+            {
+                var p2 = Parse<Poco1>(contents);
+
+                Assert.Equal(p1.Prop, p2.Prop.Value);
+                Assert.Same(Filename, p2.Prop.Location.Filename);
+
+                if (p2.Prop.Value != null)
+                {
+                    p2.Prop.Value = p2.Prop.Value.Replace("\r", string.Empty);
+                }
+
+                return p2.Prop;
+            }
+        }
+
+        [Theory]
+        [InlineData(
+@"Prop: |
+  123
+", 2, 3, "123\n")]
+
+        // Leading space in front of property. 
+        [InlineData(
+@" Prop: |
+  123
+", 2, 3, "123\n")]
+
+        [InlineData(
+@"# another line 
+Prop: |
+     123
+     456", 3, 6, "123\n456")]
+
+        [InlineData(
+@"Prop: |-
+  123
+", 2, 3, "123")]
+
+        [InlineData(
+@"Prop: |-
+  true
+", 2, 3, "true")] // YDN will still return as string. 
+
+        // Other encodings (not | ) are not supported. 
+        [InlineData(
+@"Prop: >
+  123
+  456
+", 0, 0, "123456\n", "literal")] // Folding 
+
+        [InlineData(
+@"Prop: 123456
+", 0, 0, "123456", "literal")] // plain
+
+        [InlineData(
+@"Prop: ""123456""
+", 0, 0, "123456", "literal")] // quotes
+
+        [InlineData(
+@"Prop: ", 1, 6, null)] // empty 
+
+        public void Test1(string contents, int lineStart, int colStart, string value, string errorMessage = null)
+        {
+            var p1 = Test(contents, errorMessage);
+
+            if (errorMessage == null)
+            {
+                Assert.Equal(value, p1.Value);
+                Assert.Equal(colStart, p1.Location.ColStart);
+                Assert.Equal(lineStart, p1.Location.LineStart);
+            }
+            else
+            {
+                Assert.Null(p1);
+            }
+        }
+
+        // Error conditions. 
+        [Theory]
+        [InlineData(
+"Prop: { }")] // object , expecting string. 
+        public void TestError(string contents)
+        {
+            ParseExpectError<Poco1Direct>(contents, "Failed");
+            ParseExpectError<Poco1>(contents, "Expected 'Scalar'");
+        }
+    }
+}

src/tools/Repl/AssertFunction.cs

@@ -0,0 +1,45 @@
+// <copyright file="AssertFunction.cs" company="PlaceholderCompany">
+// Copyright (c) PlaceholderCompany. All rights reserved.
+// </copyright>
+
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.PowerFx.Types;
+
+#pragma warning disable CS0618 // Type or member is obsolete
+
+namespace Microsoft.PowerFx.Repl.Functions
+{
+    /// <summary>
+    /// Assert Function.
+    /// </summary>
+    internal class AssertFunction : ReflectionFunction
+    {
+        public AssertFunction()
+            : base("Assert", FormulaType.Void, new[] { FormulaType.Boolean, FormulaType.String })
+        {
+            ConfigType = typeof(IReplOutput);            
+        }
+
+        public async Task<VoidValue> Execute(IReplOutput output, BooleanValue test, StringValue message, CancellationToken cancel)
+        {            
+            if (test.Value)
+            {
+                await output.WriteLineAsync($"PASSED: {message.Value}", OutputKind.Notify, cancel)
+                .ConfigureAwait(false);
+            } 
+            else
+            {
+                await output.WriteLineAsync($"FAILED: {message.Value}", OutputKind.Error, cancel)
+                .ConfigureAwait(false);
+            }
+
+            return FormulaValue.NewVoid();
+        }
+    }
+}

src/tools/Repl/Program.cs

@@ -18,6 +18,8 @@ using Microsoft.PowerFx.Types;
 
 namespace Microsoft.PowerFx
 {
+#pragma warning disable CS0618 // Type or member is obsolete
+
     public static class ConsoleRepl
     { 
         private const string OptionFormatTable = "FormatTable";
@@ -84,9 +86,9 @@ namespace Microsoft.PowerFx
 
             config.EnableSetFunction();
             config.EnableJsonFunctions();
-#pragma warning disable CS0618 // Type or member is obsolete
             config.EnableOptionSetInfo();
-#pragma warning restore CS0618 // Type or member is obsolete
+
+            config.AddFunction(new AssertFunction());
 
             config.AddFunction(new ResetFunction());
             config.AddFunction(new Option0Function());
@@ -97,9 +99,7 @@ namespace Microsoft.PowerFx
 
             var optionsSet = new OptionSet("Options", DisplayNameUtility.MakeUnique(options));
 
-#pragma warning disable CS0618 // Type or member is obsolete
             config.EnableRegExFunctions(new TimeSpan(0, 0, 5));
-#pragma warning restore CS0618 // Type or member is obsolete
 
             config.AddOptionSet(optionsSet);
 
@@ -124,9 +124,7 @@ namespace Microsoft.PowerFx
         }
 
         // Hook repl engine with customizations.
-#pragma warning disable CS0618 // Type or member is obsolete
         private class MyRepl : PowerFxREPL
-#pragma warning restore CS0618 // Type or member is obsolete
         {
             public MyRepl()
             {
@@ -138,6 +136,8 @@ namespace Microsoft.PowerFx
 
                 this.AllowSetDefinitions = true;
                 this.AllowUserDefinedFunctions = _enableUDFs;
+                this.AllowImport = true;
+
                 this.EnableSampleUserObject();
                 this.AddPseudoFunction(new IRPseudoFunction());
                 this.AddPseudoFunction(new SuggestionsPseudoFunction());
@@ -427,9 +427,7 @@ namespace Microsoft.PowerFx
 
         private class MyHelpProvider : HelpProvider
         {
-#pragma warning disable CS0618 // Type or member is obsolete
             public override async Task Execute(PowerFxREPL repl, CancellationToken cancel, string context = null)
-#pragma warning restore CS0618 // Type or member is obsolete
             {
                 if (context?.ToLowerInvariant() == "options" || context?.ToLowerInvariant() == "option")
                 {