Nemerle

Nemerle
Paradigm	multi-paradigm: metaprogramming, functional, object-oriented, imperative
Designed by	Kamil Skalski, Michał Moskal, Prof. Leszek Pacholski and Paweł Olszta at Wrocław University
First appeared	2003
Stable release	1.0.0 / May 13, 2011
Typing discipline	static, strong, inferred, nominal
Platform	CLI (.Net & Mono)
Filename extensions	.n
Website	http://nemerle.org
Major implementations
	Nemerle
Influenced by
	C#, ML, Lisp

Nemerle is a general-purpose high-level statically typed programming language designed for platforms using the Common Language Infrastructure (.NET/Mono). It offers functional, object-oriented and imperative features. It has a simple C#-like syntax and a powerful metaprogramming system. In June of 2012, the core developers of Nemerle were hired by the Czech software development company JetBrains. The team is focusing on the development of a project known as "N2"; a framework for implementing new and existing programming languages. This framework will likely be used to create future versions of Nemerle. ^[1] ^[2] ^[3]

The language was named after the Archmage Nemmerle, a character in the fantasy novel A Wizard of Earthsea by Ursula K. Le Guin.

Features

Nemerle's most notable feature is the ability to mix object oriented and functional styles of programming. Programs may be structured using object oriented concepts such as classes and namespaces, while methods can (optionally) be written in a functional style. Other notable features include:

strong type inference
a flexible metaprogramming subsystem (using macros)
full support for OOP (in the style of C#, Java, and C++)
full support for functional programming (in the style of ML, OCaml, Haskell) with the following features:
- higher-order functions
- pattern matching
- algebraic types
- local functions
- tuples and anonymous types
- partial application of functions

The metaprogramming system allows for a great deal of compiler extensibility, embedding of domain specific languages, partial evaluation, and aspect-oriented programming, taking a high-level approach to lift as much of the burden from the programmer as possible. The language combines all CLI standard features, including parametric polymorphism, lambdas, extension methods etc. Accessing the libraries included in the .NET or Mono platforms is as easy as in C#.

Type inference

def x = 1;           // int
def myList = List(); // generic List[T], type T is deduced from the usage in the next line
myList.Add(x);       // compiler deduces type of T as int making myList type of List[int]

Everything is an expression

def x =
  { // similar to x = 3
    def y = 1;
    def z = 2;
    y + z   // this last statement is a block return value
  };

def x =
  if (DateTime.Now.DayOfWeek == DayOfWeek.Monday)  // if, using, try  are also expressions
    "Monday"
  else
    "other day";

def x = try int.Parse(someString)
        catch { | FormatException() => 0 };

def x = returnBlock : 
  {
    foreach (i in [1, 2, 3])
      when (i > 2)
        returnBlock(true); // exit block (x = true)

    false // x = false
  };

Tuples

def k = (1, "one"); // k : (int * string)
def (a, b) = k; // a = 1, b = "one"

Pattern matching

def result = match (number)
{
  | 0            => "zero"
  | 1            => "one"
  | x when x < 0 => "negative"
  | _            => "more than one"
}

Other pattern matching examples

Type matching with variable binding:

def check (o : object) {
  match (o) 
  {
    | i is int    => $"An int: $i"
    | s is string => $"A string: $(s.ToUpper())"
    | _           => "Object of another type"
  }
}

Tuple pattern matching:

match (tuple) 
{
  | ( 42, _ ) => "42 on first position"
  | ( _, 42 ) => "42 on second position"
  | ( x, y )  => $"( $x, $y )"
}

Regexp matching:

using Nemerle.Text;
regexp match (str) {
  | "a+.*"                          => printf("a\n");
  | @"(?<num : int>\d+)-\w+"        => printf("%d\n", num + 3);
  | "(?<name>(Ala|Kasia))? ma kota" =>
     match (name) 
     {
       | Some (n) => printf("%s\n", n)
       | None     => printf("noname?\n")
     }

  | _                               => printf("default\n");
}

Functional types and local functions

using System.Console; // classes and modules (static classes) can be put in namespaces
def next(x) { x + 1 }; // the type of x argument and other function arguments can be deduced from usage

def mult(x, y) { x * y };

def fibonacci(i)
{
  | 0     => 0
  | 1     => 1
  | other => fibonacci(i - 1) + fibonacci(i - 2)
};

WriteLine(next(9));        // 10  similar to "Console.WriteLine(next(9));" 
WriteLine(mult(2, 2));     // 4
WriteLine(fibonacci(10)); // 55

Variants

Variants (called data types or sum types in SML and OCaml) are forms of expressing data of several different kinds:

 variant RgbColor{
   | Red
   | Yellow
   | Green
   | Different {
       red : float;
       green : float;
       blue : float;
     }
 }

Metaprogramming

Nemerle's macro system allows for the creation, analysis, and modification of a program's code during the compilation process. Macros can be used in the form of a method call or as a new language construct. Many constructs within the language were implemented using macros (if, for, foreach, while, using etc.).

"if" macro example:

macro @if (cond, e1, e2)
syntax ("if", "(", cond, ")", e1, Optional (";"), "else", e2)
{
  /*
    <[ ]> defines an area of quasi-quotation, the Nemerle compiler transforms a code in a such block
    to an AST tree, such transformations are somewhat similar to an Expression compiling in C#
  */
  <[
    match ($cond : bool)
    {
      | true => $e1
      | _ => $e2
    }
  ]>
}

// using this macro in code:
def max = if (a > b) a else b;
// during a compile time the upper line will be transformed to the following:
def max = match (a > b)
{
  | true => a
  | _    => b
}

IDE

Nemerle can be integrated into Visual Studio 2008. Also it has a completely free IDE based on Visual Studio 2008 Shell^[4] (like Visual Studio Express Editions) and SharpDevelop (link to plugin source code).

Nemerle can be also integrated into Visual Studio 2010 using the add-in.

Examples

Hello, World!

The traditional "Hello World!" can be implemented in a more C#-like fashion:

class Hello
{
  static Main () : void
  {
    System.Console.WriteLine ("Hello, world!");
  }
}

or more simply:

System.Console.WriteLine("Hello, world!");

Examples of macros

Macros allow generation of boilerplate code with additional static checks performed by the compiler. They reduce the amount of code that must be written by hand, make code generation safer, and allow for programmatic generation of code with compiler checks, while keeping source code relatively small and readable.

String formatting

The string formatting macro simplifies variables to string manipulations using $ notation:

def s = $"The number is $i"; //insert the value of the variable i where $i is placed 
def s = $"$x + $y = $(x+y)"; // $(...) can be used to make calculations or access members

Declarative code generation

StructuralEquality, Memoize, json, and with are macros which generate code in compile time. Although some of them (StructuralEquality, Memoize) can look like C# attributes, during compile time they will be examined by the compiler and transformed to appropriate code using logic predefined by their macros.

[StructuralEquality] // Implement IEquatable[Sample] .Net interface using by element comparison equality.
class Sample
{
   [Memoize]  // remember first evaluation result 
   public static SomeLongEvaluations() : int  
   {
       MathLib.CalculateNthPrime(10000000)
   }

   [DependencyProperty] // WPF dependency property
   public DependencyPropertySample { get; set; }
   
   public static Main() : void
   {
/* syntax macro "json" generates code: 
JObject.Object([("a", JValue.Number(SomeLongEvaluations())), ("b", JValue.Number(SomeLongEvaluations() + 1))])
*/
       def jObject = json { a: SomeLongEvaluations(); b: (SomeLongEvaluations() + 1)} 
// object initialization macro "<-" is development of C# curly brackets object initialization
       def k = Diagnostics.Process() <- 
       {
          StartInfo <- // can init inner objects properties without ctor call
          {
              FileName = "calc.exe";
              UseShellExecute = true;
          }   
          Exited += () => WriteLine("Calc done"); // events and delegates
       }

       ReadLine();
   }
}

Database accessibility

Using Nemerle macros for SQL you can write:

ExecuteReaderLoop("SELECT firstname, lastname FROM employee WHERE firstname = $myparm", dbcon,
  {
    WriteLine ($"Name: $firstname $lastname")
  });

instead of

string sql = "SELECT firstname, lastname FROM employee WHERE firstname = :a";
using (NpgsqlCommand dbcmd = new NpgsqlCommand (sql, dbcon, dbtran))
{
  dbcmd.Parameters.Add("a", myparm);

  using (NpgsqlReader reader = dbcmd.ExecuteReader())
  {
     while(reader.Read()) 
     {
        var firstname = reader.GetString (0);
        var lastname = reader.GetString (1);
        Console.WriteLine ("Name: {0} {1}", firstname, lastname)
     }
  }
}

and this is not just hiding some operations in a library, but additional work performed by the compiler to understand the query string, the variables used there, and the columns returned from the database. The ExecuteReaderLoop macro will generate code roughly equivalent to what you would have to type manually. Moreover, it connects to the database at compilation time to check that your SQL query really makes sense.

New language constructs

With Nemerle macros you can also introduce some new syntax into the language:

macro ReverseFor (i, begin, body)
syntax ("ford", "(", i, ";", begin, ")", body)
{
  <[ for ($i = $begin; $i >= 0; $i--) $body ]>
}

defines a macro introducing the ford (EXPR ; EXPR) EXPR syntax and can be used like

ford (i ; n) print (i);

Nemerle with ASP.NET

Nemerle can be either embedded directly into ASP.NET:

<%@ Page Language="Nemerle" %>
<script runat="server">

    Page_Load(_ : object, _ : EventArgs) : void {
        Message{{Not a typo|.}}Text = $"You last accessed this page at: $(DateTime.Now)";
    }

    EnterBtn_Click(_ : object, _ : EventArgs) : void {
        Message{{Not a typo|.}}Text = $"Hi $(Name.Text), welcome to ASP.NET!";
    }

</script>

<html>
    <body>
        <form runat="server">
            Please enter your name: <asp:TextBox ID="Name" runat="server" />
            <asp:Button OnClick="EnterBtn_Click" Text="Enter" runat="server" />

            <p><asp:Label ID="Message" runat="server" /></p>
        </form>
    </body>
</html>

...Or stored in a separate file and entered with a single line:

<%@ Page Language="Nemerle" Src="test.n" Inherits="Test" %>

PInvoke

Nemerle can take advantage of native platform libraries. The syntax is very similar to C#'s and other .NET languages. Here is the simplest example:

using System;
using System.Runtime.InteropServices;

class PlatformInvokeTest
{
    [DllImport("msvcrt.dll")]
    public extern static puts(c : string) : int;

    [DllImport("msvcrt.dll")]
    internal extern static _flushall() : int;

    public static Main() : void
    {
        _ = puts("Test");
        _ = _flushall();
    }
}

References

^ https://twitter.com/orangy/status/216127804482404352
^ http://blogs.jetbrains.com/dotnet/2012/06/jetbrains-and-nemerle/
^ https://groups.google.com/forum/#!topic/nemerle-en/LKkEcftHF9I
^ Nemerle Studio Microsoft Setup Installer can be installed after installation of Visual Studio Shell 2008 Isolated

External links

[1] ttps://twitter.com/orangy/status/216127804482404352

[2] ttp://blogs.jetbrains.com/dotnet/2012/06/jetbrains-and-nemerle/

[3] ttps://groups.google.com/forum/#!topic/nemerle-en/LKkEcftHF9I

[4] Nemerle Studio Microsoft Setup Installer can be installed after installation of Visual Studio Shell 2008 Isolated

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