I need clarity on how objects are declared and assigned a definition in F#.
What's happening in this code?
let service = {
new IService with
member this.Translate(_) = raise error }
My guess is we're creating an object that will implement some interface on the fly even though there is no actual class that's backing this object. Hence, we're removing the ceremony involved with creating an object by not having to declare a separate class to use it. In this case, we're minimizing the ceremony involved for implementing a mock object that could be used within a unit test.
Is my understanding accurate?
I tried to research my question and found the specification for F# 3.0 (Section - 6.3.8 Object Expressions)
6.3.8 Object Expressions An expression of the following form is an object expression: { new ty0 args-expropt object-members interface
ty1 object-members1 … interface tyn object-membersn } In the case
of the interface declarations, the object-members are optional and are
considered empty if absent. Each set of object-members has the form:
with member-defns endopt Lexical filtering inserts simulated $end
tokens when lightweight syntax is used. Each member of an object
expression members can use the keyword member, override, or default.
The keyword member can be used even when overriding a member or
implementing an interface.
For example:
let obj1 =
{ new System.Collections.Generic.IComparer<int> with
member x.Compare(a,b) = compare (a % 7) (b % 7) }
You can get a pretty good picture of what is happening behind the scenes if you look at the generated IL using a decompiler like ILSpy. For the example involving IComparer, it generates a hidden class, which implements the interface:
internal sealed class obj1#2 : IComparer<int> {
public obj1#2() : this() { }
int IComparer<int>.System-Collections-Generic-IComparer(int x, int y) {
int num = x % 7;
int num2 = y % 7;
if (num < num2) { return -1; }
return (num > num2) ? 1 : 0;
}
}
Inside the body of the method, it then creates a new instance:
IComparer<int> obj1 = new obj1#2();
Related
class X extends Y {
X(int a, int b) : super(a,b);
}
Can someone give me an explanation about the syntax meaning of the colon :?
This feature in Dart is called "initializer list".
It allows you to initialize fields of your class, make assertions and call the super constructor.
This means that it is not the same as the constructor body. As I said, you can only initialize variables and only access static members. You cannot call any (non-static) methods.
The benefit is that you can also initialize final variables, which you cannot do in the constructor body. You also have access to all parameters that are passed to the constructor, which you do not have when initializing the parameters directly in the parentheses.
Additionally, you can use class fields on the left-hand of an assignment with the same name as a parameter on the right-hand side that refers to a parameter. Dart will automatically use the class field on the left-hand side.
Here is an example:
class X {
final int number;
X(number) : number = number ?? 0;
}
The code above assigns the parameter named number to the final field this.number if it is non-null and otherwise it assigns 0. This means that the left-hand number of the assignment actually refers to this.number. Now, you can even make an assertion that will never fail (and is redundant because of that, but I want to explain how everything works together):
class X {
final int number;
X(number): number = number ?? 0, assert(number != null);
}
Learn more.
It's ok to access non static member in initializer list.
class Point {
num x, y;
Point(this.x, this.y);
Point.origin(): this.x = 10, this.y = 10;
}
main() {
Point p = Point.origin();
print(p.x); // 10
}
I have a protocol:
protocol MasterGame {}
and a class that produces a singleton
class MasterGameImp : MasterGame {
static let sharedInstance = MasterGameImp()
}
Now I have another object that has a dependency on the protocol and has the property injected by it's instantiator.
class MyGameObject {
var masterGame: MasterGame?
}
I want to write a unit test to test that the singleton is injected properly into an instance of MyGameObject. What is the right way to do this? === does not accept arguments of type MasterGame and MasterGameImp. So apparently you can't check sameness that way between a protocol and a class. So I need another way to check sameness between the singleton and the stored property. Thanks!
The issue very much simplified is the following:
protocol P {
}
class X : P {
}
let x = X()
let p : P = x
print(x === p)
Binary operator === cannot be applied to operands of type 'X' and 'P'
print((x as P) === p)
Binary operator === cannot be applied to two 'P' operands
print(x === (p as! X))
true -> Working
What you have to do is upcast the protocol to its implementation, which es really not pretty at all :/
I did not find a better solution, it has something to do how swift handles protocols different than regular class inheritance. Changing to class P will result in all three above statements to succeed.
As #matt noted the code will of course crash if you incorrectly provide a wrong p which is not really a X. To solve that issue you should wrap the check into a if-condition:
if let xp = p as? X {
print(x === xp)
} else {
print("p is not of type X")
}
When I extend a type with a new function, I usually want it to be available from both dot-notation and free form. Either can be more readable depending on the situation, and the former helps with IntelliSense while the latter helps with currying.
In C#/VB.net, extension methods do this (although I cannot restrict the function to a static method of the extended static class, as in F#). I can write the function once and then invoke it both ways:
<Extension>
public function bounded(s as string, min as UShort, max as UShort) as string
if min > max then throw new ArgumentOutOfRangeException
if string.IsNullOrEmpty(s) then return new string(" ", min)
if s.Length < min then return s.PadRight(min, " ")
if s.Length > max then return s.Substring(0, max)
return s
end function
' usage
dim b1 = bounded("foo", 10, 15)
dim b2 = "foo".bounded(0, 2)
(That's not quite perfect yet, as I'd like bounded to be a static method of String, but C#/VB.Net can't do that. Point to F# in that regard.)
In F#, on the other side, I have to declare the function separatedly from the method:
// works fine
[<AutoOpen>]
module Utilities =
type List<'T> with
member this.tryHead = if this.IsEmpty then None else Some this.Head
module List =
let tryHead (l : List<'T>) = l.tryHead
Question: Is there a more elegant way to declare both methods at once?
I tried to use:
// doesn't quite work
type List<'T> with
member this.tryHead = if this.IsEmpty then None else Some this.Head
static member tryHead(l : List<'T>) = l.tryHead
which at least would let me skip the module declaration, but while the definition compiles, it doesn't quite work - someList.tryHead is OK, but List.tryHead someList results in a Property tryHead is not static error.
Bonus question: As you can see, the static member definition requires a type annotation. However, no other type could have access to the method that was just defined. Why, then, can't the type be inferred?
I don't know of a way to declare both APIs in a single line of code, but you can get rid of the type annotations by making the function the implementation, and then defining the method it terms of the function:
[<AutoOpen>]
module Utilities =
module List =
let tryHead l = if List.isEmpty l then None else Some (List.head l)
type List<'a> with
member this.tryHead = List.tryHead this
this is example code in C# :
class exampleClass
{
struct exampleStruct
{
public int number;
}
private exampleStruct[,] e;
private enum exampleMove { Up, Down, Right, Left, Enter, Escape };
Stack<int> returnPath;
bool returntoBeg;
int numRandomMoves;
public exampleClass()
{
e = new exampleStruct[5, 5];
exampleStruct ex;
returntoBeg = false;
returnPath = new Stack<int>();
numRandomMoves = 0;
for (int y = 0; y < 5; y++)
{
for (int x = 0; x < 5; x++)
{
ex = new exampleStruct();
ex.number = 0
e[x, y] = ex;
}
}
}
}
I have an example code like above, i want to translate it into F#. But the problem is, when i make a class using F# and define struct in it, it shows errors and pointing that i can't declare type inside class type. Any help?
I think the following is a good workaround for nested types.
namespace MyNamespace
module private PrivateTypes =
[<Struct>]
type ExampleStruct(number: int) =
member __.Number = number
open PrivateTypes
type ExampleClass() =
let e = Array2D.init 5 5 (fun y x -> ExampleStruct(0))
//other members
ExampleStruct is nested under PrivateTypes, which is only visible in the same file.
While you cannot nest types, you can use intrinsic complex types that F# provides. Tuples are often a good data structure for data that has not very wide, observable scope, such as it is in your case.
In practice, I usually define implementation types in a module called e. g. Internal, and do not allow them to escape from the library. You may also define separate module per logical group of classes or even per complex class implementation.
I know virtually nothing about F#. I don’t even know the syntax, so I can’t give examples.
It was mentioned in a comment thread that F# can declare functions that can take parameters of multiple possible types, for example a string or an integer. This would be similar to method overloads in C#:
public void Method(string str) { /* ... */ }
public void Method(int integer) { /* ... */ }
However, in CIL you cannot declare a delegate of this form. Each delegate must have a single, specific list of parameter types. Since functions in F# are first-class citizens, however, it would seem that you should be able to pass such a function around, and the only way to compile that into CIL is to use delegates.
So how does F# compile this into CIL?
This question is a little ambiguous, so I'll just ramble about what's true of F#.
In F#, methods can be overloaded, just like C#. Methods are always accessed by a qualified name of the form someObj.MethodName or someType.MethodName. There must be context which can statically resolve the overload at compile-time, just as in C#. Examples:
type T() =
member this.M(x:int) = ()
member this.M(x:string) = ()
let t = new T()
// these are all ok, just like C#
t.M(3)
t.M("foo")
let f : int -> unit = t.M
let g : string-> unit = t.M
// this fails, just like C#
let h = t.M // A unique overload for method 'M' could not be determined
// based on type information prior to this program point.
In F#, let-bound function values cannot be overloaded. So:
let foo(x:int) = ()
let foo(x:string) = () // Duplicate definition of value 'foo'
This means you can never have an "unqualified" identifier foo that has overloaded meaning. Each such name has a single unambiguous type.
Finally, the crazy case which is probably the one that prompts the question. F# can define inline functions which have "static member constraints" which can be bound to e.g. "all types T that have a member property named Bar" or whatnot. This kind of genericity cannot be encoded into CIL. Which is why the functions that leverage this feature must be inline, so that at each call site, the code specific-to-the-type-used-at-that-callsite is generated inline.
let inline crazy(x) = x.Qux(3) // elided: type syntax to constrain x to
// require a Qux member that can take an int
// suppose unrelated types U and V have such a Qux method
let u = new U()
crazy(u) // is expanded here into "u.Qux(3)" and then compiled
let v = new V()
crazy(v) // is expanded here into "v.Qux(3)" and then compiled
So this stuff is all handled by the compiler, and by the time we need to generate code, once again, we've statically resolved which specific type we're using at this callsite. The "type" of crazy is not a type that can be expressed in CIL, the F# type system just checks each callsite to ensure the necessary conditions are met and inlines the code into that callsite, a lot like how C++ templates work.
(The main purpose/justification for the crazy stuff is for overloaded math operators. Without the inline feature, the + operator, for instance, being a let-bound function type, could either "only work on ints" or "only work on floats" or whatnot. Some ML flavors (F# is a relative of OCaml) do exactly that, where e.g. the + operator only works on ints, and a separate operator, usually named +., works on floats. Whereas in F#, + is an inline function defined in the F# library that works on any type with a + operator member or any of the primitive numeric types. Inlining can also have some potential run-time performance benefits, which is also appealing for some math-y/computational domains.)
When you're writing C# and you need a function that can take multiple different parameter sets, you just create method overloads:
string f(int x)
{
return "int " + x;
}
string f(string x)
{
return "string " + x;
}
void callF()
{
Console.WriteLine(f(12));
Console.WriteLine(f("12"));
}
// there's no way to write a function like this:
void call(Func<int|string, string> func)
{
Console.WriteLine(func(12));
Console.WriteLine(func("12"));
}
The callF function is trivial, but my made-up syntax for the call function doesn't work.
When you're writing F# and you need a function that can take multiple different parameter sets, you create a discriminated union that can contain all the different parameter sets and you make a single function that takes that union:
type Either = Int of int
| String of string
let f = function Int x -> "int " + string x
| String x -> "string " + x
let callF =
printfn "%s" (f (Int 12))
printfn "%s" (f (String "12"))
let call func =
printfn "%s" (func (Int 12))
printfn "%s" (func (String "12"))
Being a single function, f can be used like any other value, so in F# we can write callF and call f, and both do the same thing.
So how does F# implement the Either type I created above? Essentially like this:
public abstract class Either
{
public class Int : Test.Either
{
internal readonly int item;
internal Int(int item);
public int Item { get; }
}
public class String : Test.Either
{
internal readonly string item;
internal String(string item);
public string Item { get; }
}
}
The signature of the call function is:
public static void call(FSharpFunc<Either, string> f);
And f looks something like this:
public static string f(Either _arg1)
{
if (_arg1 is Either.Int)
return "int " + ((Either.Int)_arg1).Item;
return "string " + ((Either.String)_arg1).Item;
}
Of course you could implement the same Either type in C# (duh!), but it's not idiomatic, which is why it wasn't the obvious answer to the previous question.
Assuming I understand the question, in F# you can define expressions which depend on the availability of members with particular signatures. For instance
let inline f x a = (^t : (member Method : ^a -> unit)(x,a))
This defines a function f which takes a value x of type ^t and a value a of type ^a where ^t has a method Method taking an ^a to unit (void in C#), and which calls that method. Because this function is defined as inline, the definition is inlined at the point of use, which is the only reason that it can be given such a type. Thus, although you can pass f as a first class function, you can only do so when the types ^t and ^a are statically known so that the method call can be statically resolved and inserted in place (and this is why the type parameters have the funny ^ sigil instead of the normal ' sigil).
Here's an example of passing f as a first-class function:
type T() =
member x.Method(i) = printfn "Method called with int: %i" i
List.iter (f (new T())) [1; 2; 3]
This runs the method Method against the three values in the list. Because f is inlined, this is basically equivalent to
List.iter ((fun (x:T) a -> x.Method(a)) (new T())) [1; 2; 3]
EDIT
Given the context that seems to have led to this question (C# - How can I “overload” a delegate?), I appear not to have addressed your real question at all. Instead, what Gabe appears to be talking about is the ease with which one can define and use discriminated unions. So the question posed on that other thread might be answered like this using F#:
type FunctionType =
| NoArgument of (unit -> unit)
| ArrayArgument of (obj[] -> unit)
let doNothing (arr:obj[]) = ()
let doSomething () = printfn "'doSomething' was called"
let mutable someFunction = ArrayArgument doNothing
someFunction <- NoArgument doSomething
//now call someFunction, regardless of what type of argument it's supposed to take
match someFunction with
| NoArgument f -> f()
| ArrayArgument f -> f [| |] // pass in empty array
At a low level, there's no CIL magic going on here; it's just that NoArgument and ArrayArgument are subclasses of FunctionType which are easy to construct and to deconstruct via pattern matching. The branches of the pattern matching expression are morally equivalent to a type test followed by property accesses, but the compiler makes sure that the cases have 100% coverage and don't overlap. You could encode the exact same operations in C# without any problem, but it would be much more verbose and the compiler wouldn't help you out with exhaustiveness checking, etc.
Also, there is nothing here which is particular to functions; F# discriminated unions make it easy to define types which have a fixed number of named alternatives, each one of which can have data of whatever type you'd like.
I'm not quite sure that understand your question correctly... F# compiler uses FSharpFunc type to represent functions. Usually in F# code you don't deal with this type directly, using fancy syntactic representation instead, but if you expose any members that returns or accepts function and use them from another language, line C# - you will see it.
So instead of using delegates - F# utilizes its special type with concrete or generic parameters.
If your question was about things like add something-i-don't-know-what-exactly-but-it-has-addition-operator then you need to use inline keyword and compiler will emit function body in the call site. #kvb's answer was describing exactly this case.