public interface IShape
{
float Area { get; }
float Perimeter { get; }
}
public Circle : IShape
{
readonly float r ;
public Circle(float r)
{
this.r = r;
}
public float Area { get { return Math.PI * r * r; } }
public float Perimeter { get { return 2 * Math.PI * r; } }
}
public Square: IShape
{
readonly float a ;
public Square(float a)
{
this.r = r;
}
public float Area { get { return a * a; } }
public float Perimeter { get { return 4 * a; } }
}
Abstract classes have disjunctive semantics (logical OR) whereas interfaces have conjunctive semantics (logical AND).
The actual important thing that I wanted to capture is that an instance of Shape can only be one kind of shape at a time. With an interface a class could be more than one kind of Shape.
interface IFoo : IShape
{
//some stuff
}
public class SomeShape : IShape, IFoo
{
//I am a circle AND a square!
}Extension methods don't extend the type. They extend the functionality attached to the type.
F# has these too [1] (actually more powerful versions):
The thing about using sum-types (the original Shape example) is that they're not supposed to be extended. You're defining the range of the type, which allows for powerful inference of completeness when using them. If you need extensible classes then F# can do those too, it can represent interfaces, abstract classes and concrete classes [2][3][4], and can even generate ad-hoc classes from an interface by using Object Expressions [5].
I personally try to stick to the pure functional approach, but it can be very handy when interoperating with the .NET framework, or other C# based libraries.
[1] http://fsharpforfunandprofit.com/posts/type-extensions/
[2] http://fsharpforfunandprofit.com/posts/classes/
[3] http://fsharpforfunandprofit.com/posts/inheritance/
[4] http://fsharpforfunandprofit.com/posts/interfaces/
[5] http://fsharpforfunandprofit.com/posts/object-expressions/
type Shape =
| Circle of float
| Square of float
let area = function
| Circle r -> Math.PI * r * r
| Square sd -> sd * sd
let perimeter = function
| Circle r -> 2.0 * Math.PI * r
| Square sd -> sd * 4.0Also would be interesting to see how long would C#6 code be (twice as short?).
Apart from the sample, I also don't understand why F#'s naming patterns are not consistent with the rest of .NET (in terms of starting letter case). Feels alien.
Also how come this was considered a sane syntax for arrays?
[|1; 2; 3|]
instead of
[1, 2, 3]
Even Haskell's syntax is less alien.
Well, programming languages do have a prescriptive aspect. Arrays in C# 1.0 were more fundamental than List whereas the situation in F# 1.0 was the reverse, and F# doesn't really want you to use arrays as much as it wants you to use list.
...
Personally, I always mark my classes as either sealed or abstract. In OO terms, I only care about extending the Shape class, not the individual subclasses because those themselves are are already extensions... of the Shape class. If I did want to extend, say, the Circle class, I would probably make a new abstract class called Ellipse that derived from Shape and then derive Circle from Ellipse.
But, I don't really think in strict OO terms any more. C# has a decent type system (despite what my fellow ML programmers might say), but one of the mistakes of the language (i.e. the spec) is that it conflates types with classes. Those are different things. You may see three classes, but I see one type. In this case it's a sum type called Shape that ranges over Circle and Square. A sum type , as you may know, is just the type-theoretic version of logical OR.
Indeed, it wasn't until I learned about sum types that I finally understood why you would use an abstract class as opposed to an interface: abstract classes are for when you want to say that a Foo is a Bar OR a Baz OR a Biz whereas interfaces are for when you want to say that Foo is a Bar AND a Baz, AND a Biz.
How do I extend Shape to include Triangle without modifying the current code (e.g. in a separate file)?
Subclassing is a good and useful tool that simply works better in C# than F#. But, just as importantly, class hierarchies are often a poor way to model a problem.
It just occurred to me that you may have been asking about extending Shape in the C# example. In that case, you'd just subclass Shape as normal[1]. Some people argue that sum types shouldn't be extended, but I never really understood that. Basically, C# lets you define unbounded sum types (in the form of subclassing) whereas idiomatic F# more or less demands that you know up front how many variants you're going to need.
[1] Although, sometimes I do define my classes to prevent this by marking the constructor(s) of the abstract base class as private, and then writing the derived class definitions within the body of the base class.
let tuple1 = (1, 3) //tuple (int, int)
let tuple2 = 1, 3 //same as before, parens are optional
let list = [ 1; 2; 3 ] //list of int
let array = [| 1; 2; 3; |] //array of int
let listOfTuple1 = [ (0,0); (1,2); (4,5) ] //list of tuple (int, int)
let listOfTuple2 = [ 0,0; 1,2; 4,5 ] //same as before
nice and fast syntax guide http://fsharpforfunandprofit.com/posts/fsharp-in-60-seconds/
type Shape =
| Circle of float
| Square of float
| Rectangle of float * float
Also, even though F# can also subclass, that facility is a bit weak since it can't define protected members.
[1]http://c2.com/cgi/wiki?ExpressionProblem - funny thing is that the linked page coincidentally uses the exact same example code.
F# is to C# as C is to Assembler (I regularly use both C# and F# and don't quite agree with this, but it is defensible).
Short answer:
Programming, especially OOP, has a lot of prescriptions: things like coding against interfaces rather than implementations, using abstractions, the law of Demeter, avoiding mutations, prefer composition to inheritance, etc.
Basically, you can think of F# as a language that compiles down to C#, but follows all of the best practices without fuss.
Consider this example of F# code:
Here is the moral equivalent in C#: A couple of things to note:* The C# code needs 5 times as many line as the F# code (adding a new shape to F# will require about 3 lines, but the corresponding C# class definition will be at least 15 lines).
* The F# code is idiomatic, but hardly any C# programmer seals classes or defines them as immutable by default, even though it is a good practice.
That said, there are some advantages that C# enjoys. Consider our example code: Even though C# requires more lines of code to define a derivative of Shape, at a certain point it will always be easier to add new classes to C# than the corresponding variant to F# (see "The Expression Problem" - on the other hand, adding new functions that apply to Shape will be easier in F# than adding the corresponding new methods in C#). Also, F# is supposed to be a hardcore statically-typed language, but there are some advanced concepts (e.g. type classes) that are easy to realize in C# yet not in F#.