F# – Functional Approach

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!