Lisp Debugger


I’m still working on my Lisp language clone.
Today I started to add debugging support to it.
I made the AST aware of the original source code, so each node can reference back to the place it was parsed from, and I also added a bit of call stack features.

I’m pretty satisfied with it so far, it almost feels like a real language now ;-)

Anyway, here is a screenshot of the not so well designed GUI.

 Debugger
Click for full size.

Downloads are available at: 

Source: http://www.puzzleframework.com/roger/mylisp.zip
Binaries: http://www.puzzleframework.com/roger/mylispbin.zip

(The code is written in C#3, VS.NET 2008 solution)

Lisp weekend


I’ve been reading up a bit on functional programming the last few week, the reason is just to comprehend the new features and possibilities in .NET 3.5 as much as possible.

Anyway, I got a bit carried away and started to read about Lisp, and decided to learn what it’s all about.
So what better way to learn a language than to make your own parser for it is there? ;-)

I started to hammer away on a simple parser, and once the parser was done, I couldn’t stop, so I began writing an engine too.
So after a few hours of Aha moments, I finally got my very own Lisp(ish) code executor and a bit more understanding for the language. ;-)

Well, enough blabbering, here are a few samples of whats currently possible in my still un-named language.

Hello world:

(print 'Hello world!')

Simple function and call:

(defun Mul (x y) 
 (* x y))       

(print (Mul 2 3))

Variables:

(let my-var 'hello lisp') 
(let my-int 123) 
(let my-double 123.456) 
(let half-pi (/ pi 2)) 
(let my-arr (arr 1 2 3 4 5 6 7))

Loops:

(foreach item my-arr 
 (print item))      

(for i 1 20 
 (print i))      

(let i 0) 
(while (< i 20) 
 ((print i) (++ i)))

Lambdas:

(let my-lambda (lambda (x y) (* x y))) 
(my-lambda 2 3)

Delegates:

(let my-delegate Mul) // delegate to Mul 
(let print other-print-func) //redirect the print function to "other-print-func"

Objects:

(let form (new Form)) 
(set form Text 'hello windows forms') 
(let button (new Button)) 
(set button Text 'my button') 
(set-event button Click MyButtonClick) 
(list-add (get form Controls) button) 
(call form Show)

List comprehensions:

(foreach item (select (lambda (concat 'transformed: ' item '!')) 
              (where (lambda (> (get item Length) 3)) 
              (list 'foo' 'bar' 'roger' '.net' 'lisp')))       

      (print item))

The next step will be to make it possible to define your own classes.
Im thinking of emitting true .NET classes and let the methods redirect the calls to the engine.
Thus making it possible to redefine the behaviour of a method in runtime.

That, and find some reason to use it :-P

Making a custom Linq engine


You can download the complete source here
NOTE: The download contains much more than this post.

Today I’m going to show how to make a custom linq engine.

So, why would you want to create your own engine?

Well replicating the standard in mem engine is pretty pointless except for educational purposes.
But you might need custom engines for your own frameworks where you want to query your own special DB or media of some sort.
We use this in NPerist, we transform the linq query into our own NPath DSL and then run the NPath query through NPersist into the database.

Anyway, back to the sample.
Lets start by creating a simple console application and add the following code in the Main method:

using System.Linq; 
... 
... 
... 
//setup test data 
List<string> source = new List<string>() { 
    "public", 
    "protected internal", 
    "void", 
    "static", 
    "for", 
    "string", 
    "int", 
    "long", 
    "List<T>" 
};                  

//run a simple query 
var result = from item in source 
             where item.Length > 1 
             select item + " hello linq"; //display the result                

Console.WriteLine(); 
Console.WriteLine("Result:------------------------"); 
foreach (var item in result) 
{ 
    Console.WriteLine("{0}", item); 
}

Nothing fancy, we simply init a list with a few strings, and then we run a standard linq query on it.

Now lets start writing our own Linq engine.
To do this we need a few delegates:

// T = item type 
// IN = item type IN 
// OUT = item type OUT  
  
//delegate to handle the where clause 
public delegate bool WhereFunc<T>(T type);                

//delegate to handle the select clause 
public delegate OUT SelectFunc<IN, OUT>(IN item);

We also need two extension methods:

namespace MyLinqEngine; 
{ 
public static class Sequence 
{ 
    //extension method that will handle where clauses 
    public static List<T> Where<T> 
    (this List<T> source, WhereFunc<T> matchDelegate) 
    { 
        //create a result list 
        List<T> result = new List<T>();         
        //scan all items in the source 
        foreach (T item in source) 
        { 
            //check for match 
            bool match = matchDelegate(item);             
            if (match) 
            { 
                //add match to result 
                result.Add(item); 
            } 
        } 
   
        return result; 
    }     
    
    //extension method that will handle select clauses 
    public static List<OUT> Select<IN, OUT> 
    (this List<IN> source, SelectFunc<IN, OUT> selectorDelegate ) 
    { 
        //create a result list 
        //since the result will have the exact same size as 
        //the source we can init it with a fixed size 
        List<OUT> result = new List<OUT>(source.Count);                  
        //scan each item in the source 
        foreach (IN item in source) 
        { 
            //transform the items in the source to an 
            //item of the given out type 
            OUT resultItem = selectorDelegate(item); 
            result.Add(resultItem); 
        }         
        return result; 
    } 
} 
}

And that’s it!
That’s all you need in order to create the simplest form of custom Linq engine.

The default Linq extensions are activated by the “using System.Linq;” at the top of your program.cs class file.
We can now remove that line and replace it with “using MyLinqEngine;” in order to activate our own linq extensions.

You’ve now got your very own debuggable Linq engine.

Happy coding.

//Roger

You can download the complete source here
NOTE: The download contains much more than this post.

ORM and C#3 anonymous types


Here goes my first blog post in 1+ years.

Nowadays I’m playing around with VS 2008 trying to abuse all the new features as much as possible.

Earlier today I was talking to Mats Helander about making a NPersist v3, for .NET 3.5.
We were discussing new features such as supporting anonymous types instead of our old “Tabular” concept.

After a while I started to think about the possibilities to use anonymous types to create read only views for stored procs,
and I came up with a quite slick solution ( in my own opinion OFC ;-) )

Anyway, here is what I came up with:

//create a prototype for our result 
var prototype = new { FirstName = "" , Age = 0 , Email = "" };      

//fill a list with items of the above type with data from an sproc 
var res = MyDal.ExecuteStoredProc(prototype, "sp_MyStoredProc", p1, p2, p3);      

//present the data in our top notch UI 
foreach (var item in res) 
{ 
    Console.WriteLine("FirstName = {0}", item.FirstName); 
}

What I do here is:
I create an object of an anonymous type, which will act as a prototype for our result.
The anonymous type contains our properties and the type for each property.

Then I pass the prototype object into my DAL and ask my DAL to execute an stored procedure with some args.
The DAL executes procedure and fill a generic list of my anonymous type with the result.
And then return the filled list the consumer.

This allows the consumer to call a stored procedure and get the result in an objectified way.
Making it possible to work with his data in a typed way w/o adding typed datasets or entity classes.
OK, the anonymous type is a sort of entity class too.
But since it can be designed per use case, you do not have to clutter the project with loads of view specific classes.

Well that’s it for now.
Full sample code will be provided soon.

//Roger

Code mangling AOP vs. Runtime Proxy AOP


 THIS IS AN OLD POST FROM MY OLD BLOG (2006) 

AOP is gaining momentum in .NET and there are starting to pop up quite a few AOP frameworks.
A comparison of (some of) those can be found here:
List of existing approaches

As you can see there are a few different approaches to accomplish AOP in .net
the ones that I know are:

  • ContextBoundObject – Inherit ContextBound
  • Source transformation – Extra compile step
  • IL instrumentation – Extra compile step
  • .NET profiling API – Modify the Jitted IL
  • Runtime Subclass proxies – Reflection emit subclasses
  • Runtime Interface proxies – Reflection emit proxies with interfaces

These approaches can be divided into two categories:

  • Inheritance based (Runtime proxies)
  • Code mangling (all the others)

(Ok ContextBound might be a separate category but since its too limited Ill just ignore it ;-) )

So what is the difference between these two approaches?

Code mangling can be very powerful and can modify your code in ways that is not supported by inheritance.
e.g. you could add interception to non virtual members, you could intercept field access and you can change base class of other classes.

Runtime proxies can only intercept ctors, virtual members or interface members.
So runtime proxies are in most cases considered to be the less of the two.
But here is my take on it:

If we take a look at good’ol OO, we have constructs like private, sealed/final, virtual, override etc.
Those constructs all have a purpose, to let the author of a class specify things like:

“It’s all OK to alter the behaviour of this method and things should work all OK if you do because its virtual”

Or:

“You may not alter this method because nasty things might happen”

(e.g. sealed or private etc)

No one has ever questioned those constructs in OOP.
So why should different rules apply to AOP?

the goal is the same as in inheritance, to alter or extend behaviour / functionality of a class.

So in my opinion inheritance based AOP is the way to go because its 100% OO compatible.
It doesn’t break the above rules.

While you might be able to do cool hacks with code mangling, you are also opening up for a whole lot of problems.
e.g., it requires much more of a consumer of a virtual method than a consumer of a non virtual method.

Why?

Because when you consume a non virtual method you know exactly what might happen, you know what exceptions that might occur and you know what the expected behaviour is.
When consuming a virtual method, you don’t know what might happen because you might call the method on your base class or on a subclass and the subclass could have been provided from someone else.
so when you consume the virtual method you might have to add some extra exception handling or make your code a bit more generic because you can’t be certain that things behave exactly as when you call the base implementation.

So if AOP would allow you to break the fundamental OO rules, you could alter the behaviour of a non virtual method and introduce behaviours that is not expected by the consumer.
and I think that is VERY bad.
If the author of the consuming code wrote his code knowing that it was a non virtual method he called, he did not prepare his code for anything else, and the AOP’ed variant of the provider method might screw things up big time.

And since the entire framework is designed w/o AOP in mind, I bet there is plenty of code in both your applications and in the framework itself that simply expects those non virtual methods to behave as they did when the code was written.

So in short, I don’t think AOP should be allowed to break the fundamental OO rules because most of the code you use is designed for those rules.
(However I think its a completely different story with things like AspectJ where the language itself is designed for AOP and those who use it know that every kind of method might be intercepted.)
The next thing is debugging.

Code mangling can accomplish lots of weird stuff like changing the base class of a type or introduce fields.
That might seem like a cool feature but since we all write our code in .NET languages that is NOT designed for AOP, e.g. C# or VB.NET
How are we supposed to debug such code?

The code executing is not the same code you have in your source files.
For me, that is just madness.

In runtime proxies we do not have those problems because the proxies are just dumb objects redirecting your calls to your interceptors, mixins and base methods.
so you will never ever need to debug the proxies themselves since they just redirect the calls.
and your base class and interceptors are still normal .NET code which you can debug just fine.
However there is a scenario where I do agree it is nice to be able to intercept non virtuals.
Debug aspects.
e.g. tracing, code coverage, interaction tests and various mock aspects.
Such aspects is a special case for me since they do pretty much the same as your IDE debugger.
in the debugger you can step into a private method, you can view all private variables etc.
Debug aspects does the same but in a different way.
Well that’s it :-)

And as stated in the beginning, this is just my take on it.
I’m not a fanatic so I could convert if someone just provides me some valid arguments why breaking OO rules is good in AOP :-)

//Roger