Entries from December 2009 ↓

The first release (R1.0) of HealthVault Java Library has been very successful and is being extensively used by HealthVault partners. This library provides basic capabilities to authenticate and exchange XML with HealthVault platform.

Over last few months, thanks to Rob, Siddhartha & Ali we have gained some momentum in developing this library. The current beta release (JAXB integration) includes beginnings of an object model for HealthVault methods and types.  Kudos to Rob for some great work here.

If you are already using the community supported HealthVault Java Library, I would encourage you to try  the new JAXB Integration release. It will be great to get your feedback before we make this work as our core development codebase. If you are new to working with HealthVault from Java please follow the getting started guide.

Looking forward to comments, feedback and contributions!

Functional programming is becoming more and more mainstream these days. C# 3.0, Python & Ruby have embodied many of the functional approaches. Microsoft even is releasing F# as first class language in Visual Studio 2010. F# is complaint in syntax with OCaml. Back in the day (at UC Santa Cruz) I wrote a language translator using OCaml and loved the symbolic computation capability a functional language provides.

In this version of interesting programming concepts, I would like to highlight type system based pattern matching available in F#/OCAML, its very unique and extremely useful if you are parsing a structured list or working on a symbol table:

type Expr =
  | Num of int
  | Add of Expr * Expr
  | Mul of Expr * Expr
  | Var of string

let rec Evaluate (env:Map<string,int>) exp =
    match exp with
    | Num n -> n
    | Add (x,y) -> Evaluate env x + Evaluate env y
    | Mul (x,y) -> Evaluate env x * Evaluate env y
    | Var id    -> env.[id]

In fact listed below is most of the code for code-generator main loop from my tool translating Berkeley Logic Interchange format (BLIF) to Reactive Modules :

let emit_atoms() =
  let vemit_atom a b = begin
    match b with
      Symb(Input,_,None) -> ()
    | Symb(_,_,None) -> emit_unmarked_atom a
    | Symb(_,_,TableAtom
         (Controls(p),Awaits(q),Relations(r))) ->
           begin
         emit_atom_start ();
         emit_table_io_stmts p q;
         emit_init_update ();
         emit_relations p q r;
         emit_atom_end ();
           end
    | Symb(_,_,ResetAtom
         (Controls(p),Awaits(q),Relations(r))) ->
           begin
         emit_atom_start ();
         emit_reset_io_stmts p q;
         emit_init_update ();
         emit_relations p q r;
         emit_atom_end ();
           end
...
    | Symb(_,_,SameAs(t)) -> ()
    | _ -> raise (Failure("Unknown Error"))
  end
  in
  Hashtbl.iter vemit_atom symTab;

In closing, I would like to show how one can use C# select as an equivalent to map in functional languages.

// Get elements in the store where filenames are GUIDs
public IEnumerable<Guid> GetKeys()
{
    string[] files = Directory.GetFiles(_StorePath);
    // functional equivalent: return files.map(|t| new Guid(t))
    return (files.Select( p => new Guid(
            Path.GetFileName(p))));
}

Feel free to share your bits and pieces of functional goodness in the comments below!

I have been meaning to write short posts about interesting programming concepts and alternative programming movements (like alt.net). Topic of this post is an interesting programming concept called Memoization and its implementation in C# as a method attribute.

Memoization is an optimization technique which speeds up a program by remembering the values returned by function calls for specific input values, its also known as tabling.

Many of the new languages (Ruby, Python, C#) provide a very neat way to implement Memoization and many functional languages (Lisp, OCaml) have it as a first class construct for the runtime. Here is a my cheap implementation showing how one can use an integer based Memo attribute in C#

class Test
{   [Memo]
    public int DoSomething(int i)
    {
        for (int k = 0; k < 10000; k++)
        {
            int j = 0;
            j = j + 500;
        }
        Console.WriteLine("DoSomething " + i);
        return i + 5;
    }
}

Here is a simple driver which implements the Memoization :

static void Main(string[] args)
 {
     Debug.WriteLine("Invoking Memoizable Code");

     Type t = typeof(Test);
     Test objT = new Test();

     // Implement Memoization
     Dictionary<int, int> memoize = new Dictionary<int, int>();
     MethodInfo[] mi = t.GetMethods();
     foreach (MethodInfo m in mi)
     {
         foreach (Attribute a in m.GetCustomAttributes(false))
         {
             if (a is MemoAttribute)
             {
                 for (int i = 0; i < 2; i++)
                 {
                     int j;
                     if (memoize.TryGetValue(i, out j))
                     {
                         Debug.WriteLine("Remembered " + i);
                         continue;
                     }
                     else
                     {
                         j = (int) m.Invoke(objT, new object[]{i});
                         memoize[i] = j;
                         Debug.WriteLine("Memorized " + i);
                     }
                 }
             }
         }
     }
     Console.ReadLine();
 }

May be in comments you can suggest ways to implement a more generic Memo attribute (similar to Python’s @memo).