Python says hello to Ruby

Today we’re going to finish our REPL that we started a couple of posts ago. We’ll add support for switching among available DLR languages and for execution of multi-line snippets of code. We’ll then use the REPL to show how IronPython and IronRuby can interact with each other.

Let’s get right to the final REPL code:

# github: repl.rb
load_assembly 'Microsoft.Scripting'
Hosting = Microsoft::Scripting::Hosting
Scripting = Microsoft::Scripting

class REPL
  def initialize
    @engine = IronRuby.create_engine
    @scope = @engine.create_scope
    @exception_service = @engine.method(:get_service).of(Hosting::ExceptionOperations).call
    @language = "rb"

  def run
    while true  
      print "#@language> "
      line = gets
      break if line.nil?

      if line[0] == ?#
        execute_command line[1..-1].rstrip 
        execute_code read_code(line)
  # Reads lines from standard input until a complete or invalid code snippet is entered.
  # Returns ScriptSource that represents an interactive code.
  def read_code first_line
    code = first_line
    while true
      interactive_code = @engine.create_script_source_from_string(code, Scripting::SourceCodeKind.InteractiveCode)
      case interactive_code.get_code_properties
        when Scripting::ScriptCodeParseResult.Complete, Scripting::ScriptCodeParseResult.Invalid:
          return interactive_code
          print "#@language| "
          next_line = gets
          return interactive_code if next_line.nil? or next_line.strip.size == 0 
          code += next_line

  # Executes given ScriptSource and prints any exceptions that it might raise.
  def execute_code source
  rescue Exception => e
    message, name = @exception_service.get_exception_message(e)
    puts "#{name}: #{message}"

  def execute_command command
    case command
      when 'exit': exit
      when 'ls?': display_languages
      else puts "Unknown command '#{command}'" unless switch_language command

  def display_languages
    @engine.runtime.setup.language_setups.each { |ls| puts "#{ls.display_name}: #{ls.names.inspect}" }

  def switch_language name
    has_engine, engine = @engine.runtime.try_get_engine(name)
    @language, @engine = name, engine if has_engine

As you can see, we’ve encapsulated the code from previous posts into a class called “REPL”. We’ve added a support for meta commands, i. e. commands processed by the REPL itself. Any string starting with “#” chracter is treated as a meta command. The commands recognized are “#exit” and “#ls?”. The former terminates the REPL and the latter displays the available languages. If the command you enter is not one of these two we try to get an engine of that name and change the current language of the REPL. For example, “#py” makes Python the current language, which is indicated by the prompt “py>”.

More of DLR Hosting API

We’ve used a couple of Hosting API concepts that we’ve not explained yet. They are, in order of appearance:

  • Engine Services

    ScriptEngine represents a language in Hosting API. We already used it for code execution (ScriptEngine.Execute) and performing dynamic object operations (ScriptEngine.Operations). Apart from these basic functionality an engine can also provide various services that might or might not be language specific. The idea here is that if languages and hosts agree on a new dynamic language service API the languages can implement it and the hosts can consume it without modifications to DLR hosting API. It’s up to each language whether it provides a particular service, so the host should be prepared for the case that some of the languages it hosts might not respond to a service request.

    Services are identified by the CLR type that provides the service API. The service we use in our REPL is ExceptionOperations from Microsoft.Scripting.Hosting namespace. It provides API for handling exceptions, formatting stack traces in a language specific way, etc. We use it in execute_code method to retrieve message and name from an exception raised by the executed script. A default exception operations are provided by DLR if the language doesn’t provide implementation.

  • Script Source

    So far we’ve used ScriptEngine.Execute for execution of code stored in a string. ScriptEngine also provides a set of methods prefixed CreateScriptFrom- that create ScriptSource objects representing source code stored in a string, file, stream, an editor buffer, etc. ScriptSource is mainly designed for hosts that need to provide a custom storage for the code, handle encoded code, analyze code, or pre-compile the code so that multiple executions are faster.

    Each ScriptSource instance is also tagged by a value of SourceCodeKind enumeration. This value tells the language parser how to parse the code: whether it should be parsed as an expression, a single statement, multiple statements, like it was a file, or an interactive code snippet. The default value is AutoDetect meaning that the parser should parse any piece of code and decide from its structure what kind of code it is.

    Internally, ScriptEngine.Execute creates a string based ScriptSource with code kind auto-detection and executes it.

    In our REPL, read_code method creates an interactive ScriptSource from a string that user entered. It then queries the ScriptSource for its code properties. ScriptSource asks the parser of the language is it bound to to parse the code and determine whether it is correct and complete, incorrect (has a syntax error), correct but its last token is incomplete (for example, an unterminated string literal), correct but the last statement is incomplete (for example, “1+”). These properties are captured by ScriptCodeParseResult enumeration in Microsoft.Scripting namespace. We use this value in read_code to determine if we should continue reading lines from the input. We continue reading until we get a complete or invalid code.

.NET interop

We have just explained what Hosting APIs we use in the REPL, yet there is still some magic going on in how we call these APIs from Ruby. Let’s look at .NET interop involved here.

  • Generic method call

    The first piece of .NET interop magic is used in REPL#initialize method. We have an instance of ScriptEngine in @engine variable and we want to get its ExceptionOperations service. ScriptEngine.GetService<TService> is a generic method. To invoke a generic method you need to provide the generic parameters somehow. The way it works in IronRuby right now is not ideal but one gets the job done nevertheless. Calling Kernel#method on an object gives us a Ruby Method object that represents the (generic) method. IronRuby monkey-patches this class adding Method#of method that takes a list of classes/modules that represent .NET types and returns a new instance of Method class with the method’s generic parameters bound to the given types. Once the generic parameters are bound the method is callable like any other Ruby method (i.e. via Method#call).

  • Out parameters

    The call to try_get_engine in switch_language doesn’t seem to be any special at the first glance. However, if you look at the C# implementation of ScriptRuntime.TryGetEngine method, you’ll see that it has two parameters, second of which is an out-parameter:

    bool TryGetEngine(string languageName, out ScriptEngine engine)

    When IronRuby encounters a call to such a method it basically takes all the out parameters in the order they appear in the signature and returns them in a CLR array along with the return value. Like if the method’s implementation was as follows:

    object[] TryGetEngine(string languageName) {
      return new object[] { return_value, engine }

    IronRuby is able to splat CLR vector arrays. In fact, any class implementing IList interface can be splatted. Hence the parallel assignment we use in switch_language assigns the return value (a Boolean) to has_engine and the value of the out parameter to engine local variable.

Python interop preview

Finally, we can enter Ruby multi-line snippets and also some Python code!

rb> class C
rb|   def say_hello caller
rb|     puts "#{caller} says hello to Ruby"
rb|   end
rb| end
=> nil
rb> #py
py> import C
py> c = C()
py> c.say_hello("Python")
Python says hello to Ruby

This is a nice little example of Ruby-Python interop that gives you a taste of DLR potential. What’s going on here? First we declared a class C in Ruby with a method say_hello. Then we switched over to Python (using “#py” meta command) and executed some Python code. We imported “C” global variable to the local Python scope. IronRuby maps all constants defined on Object class to DLR global variables. IronPython maps its global variables to the DLR globals as well. Hence IronRuby and IronPython see each other’s variables and can exchange them.

Now that we successfully imported Ruby class object “C” into Python we can call it, right? Wait a second! You can’t call a Ruby class! Well, that’s right, but we are in Python – a call to a class object in Python makes an instance of the class. Python has no new method like Ruby has or new keyword like C# has. When IronPython sees an invocation of a dynamic object that is not its own it sends it an DLR interop message “Invoke”. IronRuby’s class objects don’t respond to this message. After all you can’t call a Ruby class. IronPython is smart and knows that if the object doesn’t respond to “Invoke” message it might respond to “CreateInstance” message. And so it sends the “CreateInstance” message to which the Ruby class responds by sending over a new instance of itself. And after a short chat between IronPython and IronRuby mediated by DLR, we store an instance of class C in Python’s local variable c.

We call say_hello method on the next line. Since we are in Python there are actually two operations involved here: “GetMember” and “Invoke”. Python first asks the object for its member say_hello (by sending “GetMember” message to the object). The Ruby object knows how to get a member – it returns an instance of Method class that represents say_hello method. Python now asks to invoke the returned object and indeed Ruby’s Method knows how to invoke itself.



Multilingual REPL

DLR Hosting API is a common API for all DLR languages: IronRuby, IronPython, and others. It abstracts away language specific details and provides API that could be used in a language independent manner. The API is primarily designed to ease usage of dynamic languages from strongly typed .NET languages and to make it easier to write applications with a scriptable object model. Nonetheless we can take advantage of the API in a scripting language as well. For that purpose the most interesting features are management of isolated scripting environments (script runtimes) and cross language code execution. I presented the former in my last post. Let’s check out the latter today.

To run examples below you’ll need to build the latest IronRuby, IronPython and DLR from sources. You can do so using the latest sources from IronRuby GIT repository or DLR CodePlex site:

  • Building from IronRuby GIT repo (mapped locally to C:\Git\ironruby):

    C:\Git\ironruby\Merlin\Main > msbuild Languages\Ruby\Ruby.sln
    C:\Git\ironruby\Merlin\Main > msbuild Languages\IronPython\IronPython.sln

    We’ve recently included IronPython projects to the repository. If you already have the repo set up just pull the latest commit. You can also use rake to build IronRuby, we don’t provide rake file for IronPython though.

  • Building from DLR CodePlex (downloaded and extracted to C:\CodePlex\DLR_Main):

    C:\Codeplex\DLR_Main > msbuild Codeplex-DLR.sln

Both builds build unsigned debug assemblies and place the binaries into bin\Debug directory.

Available Languages

The DLR runtime can host multiple languages at once. Listing all languages that are available is easy:

# github: languages.rb
# load IronRuby library that provides Ruby programs an easy access to the Hosting API:
require 'IronRuby'

# create a new DLR runtime:
runtime = IronRuby.create_runtime
# list all available languages:
runtime.setup.language_setups.each { |ls| puts "#{ls.display_name}: #{ls.names.inspect}" }

The last line enumerates all instances of LanguageSetup class (defined by Hosting API) that are stored in the setup of the runtime. For each such language setup its display name and a list of short names are printed. The short names uniquely identify languages within the runtime. A language can be identified by multiple short names.

C:\Codeplex\DLR_Main\Bin\Debug>ir languages.rb
IronPython 2.6 Alpha: ["IronPython", "Python", "py"]
IronRuby 1.0 Alpha: ["IronRuby", "Ruby", "rb"]
ToyScript: ["ToyScript", "ts"]

Where does this list come from? We have said nothing about IronPython in our script. Does IronRuby know about IronPython? Making IronRuby dependent on all languages it could possibly interact with wouldn’t be a good design. Instead, Hosting API creates and sets up a script runtime according to the current .NET application’s configuration. It is told to do so in IronRuby#create_runtime. The .NET application is ir.exe hence the configuration is loaded from ir.exe.config file that lives next to the ir.exe. Indeed, the list of languages can be found in this file:

  <language names="IronPython;Python;py" extensions=".py" displayName="IronPython 2.6 Alpha"
    type="IronPython.Runtime.PythonContext, IronPython, Version=, Culture=neutral, PublicKeyToken=null" />
  <language names="IronRuby;Ruby;rb" extensions=".rb" displayName="IronRuby 1.0 Alpha"     
    type="IronRuby.Runtime.RubyContext, IronRuby, Version=, Culture=neutral, PublicKeyToken=null" />
  <language names="ToyScript;ts" extensions=".ts"    
    type="ToyScript.ToyLanguageContext, ToyScript, Version=, Culture=neutral, PublicKeyToken=null" />

Whenever you find a DLR language you want to use for scripting your app and don’t have yet you can just add it to the list and you’re all set.

Hosting IronPython in IronRuby

Now that we know how IronPython identifies itself to the runtime, we can load its engine and execute some Python code. Let’s take the simple REPL implementation from the previous post and change it to execute Python code instead of Ruby:

# github: python_repl.rb
require 'IronRuby'
runtime = IronRuby.create_runtime

# ask the runtime for Python engine using one of its simple names:
engine = runtime.get_engine("IronPython")

scope = engine.create_scope

while true  
  print "> "
  code = gets
  break if code.nil?
    result = engine.execute(code, scope)

    # print the Ruby's view of the result:
    p result

    # print the result as Python would display it:
    puts engine.operations.format(result)
  rescue Exception
    puts $!

Let’s play now:

C:\Codeplex\DLR_Main\Bin\Debug>ir python_repl.rb
> def add(a,b): return a + b
> add
<function add at 0x000000000000002B>
> add(1,1)
> add((1, 2), (3, 4))
(1, 2, 3, 4)
> ^Z

As you can see, each language might have a different view on the same object.

To make the REPL really useful we need to support multi-line statements (i.e. wait with code execution until a complete statement is entered). Let’s keep that and more for the next post.

Isolated Code Execution

Every Ruby programmer is familiar with irban interactive Ruby shell, aka REPL (Read-Eval-Print Loop). It is a handy tool for discovering how Ruby language features or libraries work. It has some limitations though. It is entirely written in Ruby and so you might bump into a problem like this:

C:\Program Files\Ruby\bin> irb
irb(main):001:0> class Fixnum
irb(main):002:1>   remove_method :+
irb(main):003:1* end
=> Fixnum
irb(main):004:0> 1
C:/M1/Merlin/External/Languages/Ruby/ruby-1.8.6/lib/ruby/1.8/irb/input-method.rb:99:in `gets': undefined method `+' for
3:Fixnum (NoMethodError)
        from C:/M1/Merlin/External/Languages/Ruby/ruby-1.8.6/lib/ruby/1.8/irb.rb:132:in `eval_input'
        from C:/M1/Merlin/External/Languages/Ruby/ruby-1.8.6/lib/ruby/1.8/irb.rb:259:in `signal_status'
        from C:/M1/Merlin/External/Languages/Ruby/ruby-1.8.6/lib/ruby/1.8/irb.rb:131:in `eval_input'
        from C:/M1/Merlin/External/Languages/Ruby/ruby-1.8.6/lib/ruby/1.8/irb/ruby-lex.rb:189:in `call'
        from C:/M1/Merlin/External/Languages/Ruby/ruby-1.8.6/lib/ruby/1.8/irb/ruby-lex.rb:189:in `buf_input'
        from C:/M1/Merlin/External/Languages/Ruby/ruby-1.8.6/lib/ruby/1.8/irb/ruby-lex.rb:104:in `getc'
        from C:/M1/Merlin/External/Languages/Ruby/ruby-1.8.6/lib/ruby/1.8/irb/slex.rb:206:in `match_io'
        from C:/M1/Merlin/External/Languages/Ruby/ruby-1.8.6/lib/ruby/1.8/irb/slex.rb:76:in `match'
         ... 8 levels...
        from C:/M1/Merlin/External/Languages/Ruby/ruby-1.8.6/lib/ruby/1.8/irb.rb:70:in `start'
        from C:/M1/Merlin/External/Languages/Ruby/ruby-1.8.6/lib/ruby/1.8/irb.rb:69:in `catch'
        from C:/M1/Merlin/External/Languages/Ruby/ruby-1.8.6/lib/ruby/1.8/irb.rb:69:in `start'
        from C:/M1/Merlin/External/Languages/Ruby/ruby-1.8.6/bin/irb:13 
C:\Program Files\Ruby\bin> 

Oops. What happened? We removed method “+” from Fixnum class and, accidentally, irb itself uses that method somewhere in the REPL implementation. The solution is obvious. Run any user code typed in an isolated environment (or “virtual machine”) different from the one that drives the REPL. Let’s take a look how to do this in IronRuby.

Hosting API

The Dynamic Language Runtime (DLR), which IronRuby is built on top of, provides API for hosting dynamic languages in an arbitrary .NET application (see Hosting API specification). You only need to know a few concepts to use this API in a powerful way. The first is script runtime. Each instance of ScriptRuntime class provides an environment for script execution. The runtime hosts scripting language engines. A single runtime can host engines of multiple DLR languages (IronRuby, IronPython, etc.), at most one engine for each language. Each engine is represented by an instance of ScriptEngine class. It holds on a global state that the particular language defines. IronRuby engine, for example, maintains a dictionary of global variables, objects that represent Ruby modules and classes, the list of loaded Ruby files, etc. An application can create any number of ScriptRuntimes and thus any number of IronRuby or IronPython “virtual machines” in a single process.

Hosting API is implemented in C# in Microsoft.Scripting.dll assembly distributed with IronRuby. IronRuby has a rich support for .NET interop, so we could load this assembly and use the Hosting API classes directly from IronRuby scripts. Although it would be simple we made it even easier: IronRuby comes with IronRuby module that provides basic hosting API specialized for Ruby.

Ruby REPL Based on Hosting API

A very simple REPL in Ruby might read as follows.

# load IronRuby library
require 'IronRuby'

# create a new IronRuby engine and a new runtime:
engine = IronRuby.create_engine

while true  
  print "> "
  code = gets
  break if code.nil?
    p engine.execute(code)
  rescue Exception
    puts $!

The REPL script itself runs in a script runtime created by ir.exe host. The code typed in the loop is evaluated in the runtime created by IronRuby#create_engine. The result of ScriptEngine#execute is sent to the calling runtime and printed out via Kernel#p method.

C:\IronRuby\Merlin\Main\Bin\Debug\ir.exe repl.rb
> 1+1
> Fixnum.send :remove_method, :+
> 1+1
undefined method `+' for 1:Fixnum
> 2.times { |i| puts i }
> ^Z

Note that there are as many class objects for each class as there are runtimes in which the class is used. IronRuby appends “@n” to the name of a class that comes from a different runtime, whose id is n, to differentiate it from the class of the same name in the current runtime. Hence Module#remove_method, which returns the class the method has been removed from, returns “Fixnum@2”.

There are many ways in which we can make our REPL better. The most serious deficiency can be easily demonstrated:

> x = 1
> puts x
undefined method `x' for main:Object

Why we got an exception? We defined a local variable x in the first line, so why is it not available in the second line? We are missing some kind of variable dictionary shared among multiple snippet executions. If we used Kernel#eval in Ruby we would pass an instance of Binding to it. The Hosting API uses a similar concept: script scope. I’m going to explain DLR scopes in a separate blog post in detail. Let’s just say for now that IronRuby associates a top-level Ruby binding with each ScriptScope class instance against which it executes code. We need to make a very simple change in our REPL script to use DLR scopes:

# github: repl.rb
require 'IronRuby'
engine = IronRuby.create_engine
scope = engine.create_scope

while true  
  print "> "
  code = gets
  break if code.nil?
    p engine.execute(code, scope) 
  rescue Exception
    puts $!

Local variables work as expected now (actually, the fix in IronRuby source code that makes this work is not yet committed to the public repo, it’s coming soon though):

> x = 1
> puts x

Another feature our simple REPL lacks is handling of multi-line expressions and statements. Let’s keep that and more for the next blog post.