CSharp Similarities and Differences

This document lists some basic differences between Nemerle and C# in a terse form. If you know Java or C++ it should still be fairly helpful.

Changes In Expressions

C#	Nemerle	Remarks
const int x = 3; const string y = "foo"; readonly Object obj = getObject();	def x : int = 3; def y : string = "foo"; def obj : Object = getObject();	Variables defined with `def` cannot be changed once defined. This is similar to `readonly`or `const` in C# or`final` in Java. Most variables in Nemerle aren't explicitly typed like this.
int x = 3; string y = "foo"; Object obj = getObject();	mutable x : int = 3; mutable y : string = "foo"; mutable obj : Object = getObject();	Variables defined with `mutable` can be changed once defined. Most variables in Nemerle aren't explicitly typed like this.
var = 3; //Will compile. var y; y = "foo";//Won't compile.	def = 3;//Will compile! mutable y; y = "foo";//Will compile!	Nemerle's type inference is lightyears ahead of C#'s. If there is clear evidence of a variable's type, there's a 99% chance Nemerle will infer it.
int a = b = c;	def a = c; def b = c;	The type of the assignment operator is void.
value = cond ? var1 : var2;	value = if(cond) var1 else var2	No ternary operator is needed as everything is an expression in Nemerle. The 'else' branch is mandatory here! (Don't panic! if-without-else has its own keyword.)
Class myClass = new Class(parms);	def myClass = Class(parms);	Nemerle doesn't require `new` when calling a constructor.
Book[] books = new Book[size];	def books = array(size) : array[Book];	Often the array type can be inferred and this is simplified; as in the next example.
Book[] books = new Book[size]; books[0] = new Book();	def books = array(size); books[0] = Book();	When the type can be inferred from context or later use (which is most of the time), you can drop the type declaration
int[] numbers = {1, 2, 3};	def numbers = array[1, 2, 3];	Initializing an array. Without the `array`keyword this would create a list.
int[,] numbers = new int[2,3];	def numbers = array(2,3) : array.[2][int];	Multidimensional array constructor. The type can usually be inferred from use and not declared.
int[,] numbers = { {1,2,3}, {1,4,9} };	def numbers = [ [1,2,3], [1,4,9] ];	Multidimensional array initialization.
new {Prop1 = 1; Prop2 = "string"}	using Nemerle.Extensions; new (Prop1 = 1, Prop2 = "string")	Nemerle anonymous typesare a bit more flexible (e. g. can be generic or returned from a method). They must be imported from Nemerle.Extensions however.
new Class { Property1 = 1; Property2 = "string" }	Class()	The Nemerle Object Modifier macro is more powerful.
if(cond) answer = 42; ...	when(cond) answer = 42; ...	`if` without `else` is called `when`. Nemerle requires`if` statements to be paired with `else`for clarity.
if(!cond) answer = 42; ...	unless(cond) answer = 42; ...	In Nemerle,`if(!cond)` can use the clearer`unless(cond)`syntax. Of course,`when(!cond)` can also always be used.
if (cond) return foo; do_something (); return bar;	match(cond){ \| true => foo \| _ => {doSomething(); foo} }	Pattern Matchingprovides a clearer way of delegating control flow.
if (cond) return foo; do_something (); return bar;	using Nemerle.Imperative; when(cond) return foo do_something (); return bar;	Alternately the Imperative namespace may be imported. This isdiscouraged however.
try {...} catch (FooException e) { ... } catch (BarException e) { ... }	try {...} catch { \| e is FooException => ... \| e is BarException => ... }	Nemerle's somewhat different`try ... catch`syntax is consistent with its pattern matching syntax.
(type) expr	expr :> type	Runtime type cast, allows for downcasts and upcasts.
(type) expr	expr : type	Static cast, only upcasts are allowed.
using System; using SWF = System.Windows.Forms; using System.Xml; ... Console.WriteLine ("foo"); SWF.Form x = new SWF.Form(); XmlDocument doc = new XmlDocument();	using System; using System.Console; using SWF = System.Windows.Forms; ... WriteLine("foo"); def x = SWF.Form(); def doc = Xml.XmlDocument();	In Nemerle, you can apply the `using`directive to classes as well as namespaces. Opened namespaces allow you to drop the prefix of other namespaces, like`System` in`System.Xml`. More info.
using System.Windows.Forms; Button button = control as Button; if (button != null) ... else ...	match (control) { \| button is Button => ... \| listv is ListView => ... \| _ => ...//something else }	`as` can be simulated with`match`. It is a bit more to type up in simple cases, but in general Nemerle's construct is more powerful.
int y = x++; ++x;	def y = x; x++; ++x;	The ++ and -- operators return void, just like assignment. So, both prefix and postfix versions are equivalent.

Changes In Type Definitions

C#	Nemerle	Remarks
static int foo (int x, string y) { ... }	static foo (x : int, y : string) : int { ... }	Types are written after variable names.
class Foo { public Foo (int x) { ... } }	class Foo { public this (x : int) { ... } }	The constructor's name is always`this`.
class Foo { ~Foo () { ... } }	class Foo { protected override Finalize () : void { ... } }	There is no special syntax for the destructor, you just override the`Finalize`method.
class Foo : Bar { public Foo (int x) : base (x) { ... } }	class Foo : Bar { public this (x : int) { base (x); ... } }	The base constructor is called in the constructor's function body.
class Foo { int x; }	class Foo { mutable x : int; }	Fields which will be changed outside of the constructor need to be marked as`mutable`.
class Foo { readonly int x; const int y = 10; }	class Foo { x : int; y : int = 10; }	Read-only/const are used by default.
class Foo { static int x = 1; }	class Foo { static mutable x : int = 1; }	Static variable.
class Foo { static readonly int x; static int method() { ... } }	module Foo { x : int; method() : int { ... } }	A module is a class in which all members are static.
using System.Runtime.CompilerServices.CSharp; class C { public object this [int i] { ... } [IndexerName("MyItem")] public int this [string name] { ... } }	class C { public Item [i : int] : object { ... } public MyItem [name : string] : int { ... } }	Indexers.

C#	Nemerle
When two interfaces use the same method to perform different functions, different names can be given to each method.
interface SpeaksEnglish{ void Speak(); } interface SpeaksGerman{ void Speak(); } class GermanTransfer : SpeaksEnglish, SpeaksGerman{ public void SpeaksEnglish.Speak() {} public void SpeaksGerman.Speak() {} }	interface SpeaksEnglish{ Speak() : void; } interface SpeaksGerman{ Speak() : void; } class GermanTransfer : SpeaksEnglish, SpeaksGerman{ public Speak() : void implements SpeaksEnglish.Speak{} public Sprechen() : void implements SpeaksGerman.Speak{} }

Nemerle

When two interfaces use the same method to perform different functions, different names can be given to each method.

interface SpeaksEnglish{

    void Speak();

}

interface SpeaksGerman{

    void Speak();

}

class GermanTransfer : SpeaksEnglish, SpeaksGerman{

    public void SpeaksEnglish.Speak() {}

    public void SpeaksGerman.Speak() {}

}

interface SpeaksEnglish{

    Speak() : void;

}

interface SpeaksGerman{

    Speak() : void;

}

class GermanTransfer : SpeaksEnglish, SpeaksGerman{

    public Speak() : void implements SpeaksEnglish.Speak{}

    public Sprechen() : void implements SpeaksGerman.Speak{}

}

Generics

C#	Nemerle	Remarks
class A { T x; }	class A [T] { x : T; }	Type parameters are written in square brackets [...].
typeof(A);	typeof(A[_,_]);	typeof expression

New Stuff

Nemerle contains many constructs which are not present in C#. Unfortunately, most of them don't really fit into a side-by-side comparison format:

Tuples -- a nameless, heterogeneous data structure.
Lists -- a special syntax for lists and list processing.
The Void Literal -- a useful construct for recursive functions.
Local functions -- defining functions within other functions.
Functional Values -- passing functions as arguments and returning them from other functions.
Anonymous Functions -- defining functions which don't need names.
Variants and Pattern Matching -- an alternative, and very useful, control flow construct.
Macros -- writing code that writes code.

Other Minor Differences

Ambiguity Isn't Tolerated

namespace YourAttributes{

    class Serializable : System.Attribute { }

}

namespace MyAttributes{

    using YourAttributes;

    class Serializable : System.Attribute { }

    [Serializable] class SomeClass { }

}

C# compilers will choose MyAttributes.Serializable or, if its definition is commented out, YourAttributes.Serializable. Nemerle will raise an error telling you to be more specific about which attribute you want to use.

Exclusion of Overridden Methods

 class BaseClass

 {

   public virtual AddItem (val : string) :  void { }

 }

 class TestClass : BaseClass

 {

   public AddItem (val : object) :  void { }

   public override AddItem (val : string) :  void { }

 }

 ...

   TestClass().AddItem ("a");  // C# will choose TestClass.AddItem (object)

                               // Nemerle will choose TestClass.AddItem (string)

This behaviour comes from section 7.6.5.1 of the C# specification, which states "...methods in a base class are not candidates [for overload resolution] if any method in a derived class is applicable (§7.6.5.1)." Unfortunately, this rule is patently absurd in situations like the above. The Nemerle compiler always chooses the method whose signature best matches the given arguments.

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