How do I implement an F# high order function in C#?
public ICommand RequestAccount =
new DelegateCommand(FuncConvert.ToFSharpFunc( _ => Debug.WriteLine() ),
FuncConvert.ToFSharpFunc( _ => return true )
);
Error CS0411 The type arguments for method
'FuncConvert.ToFSharpFunc(Action)' cannot be inferred from the
usage. Try specifying the type arguments explicitly.
Based on the error, I'm not aware of how to express the type parameters explicitly. Hence, I don't think C# understands what a unit is that's to be returned on the first lambda.
DelegateCommand
type DelegateCommand (action:(obj -> unit), canExecute:(obj -> bool)) =
let event = new DelegateEvent<EventHandler>()
interface ICommand with
[<CLIEvent>]
member this.CanExecuteChanged = event.Publish
member this.CanExecute arg = canExecute(arg)
member this.Execute arg = action(arg
If you are in control of both the C# and the F# part of the code, then I would not try to create F# functions explicitly from C# - that will just make your C# code ugly. You can easily add a static method that will take Func and Action delegates and provide a C#-friendly interface:
type DelegateCommand (action:(obj -> unit), canExecute:(obj -> bool)) =
let event = new DelegateEvent<EventHandler>()
interface ICommand with
[<CLIEvent>]
member this.CanExecuteChanged = event.Publish
member this.CanExecute arg = canExecute(arg)
member this.Execute arg = action(arg)
static member Create(action:Action<obj>, canExecute:Func<obj, bool>) =
DelegateCommand(action.Invoke, canExecute.Invoke)
Now you can use DelegateCommand.Create from C# in a nice way:
DelegateCommand.Create(
(o => Console.WriteLine(o)),
(o => true) )
For the record, I also do not quite see the value of defining DelegateCommand in F# and using that from C# if you are not doing anything else on the F# side - it seems like a simple type that could as well be defined in C# (i.e. you are not gaining much by doing that in F#).
Try to explicitly specify the arguments type
public ICommand RequestAccount =
new DelegateCommand(FuncConvert.ToFSharpFunc<object>(obj => Debug.WriteLine(obj)),
FuncConvert.ToFSharpFunc<object, bool>(_ => true));
Related
In a c# dll I have a method, that takes func parameters:
public static AttrDiffRule Create<T>(string a_attr, string b_attr, Func<IAttrProxy,IAttrProxy,T,bool> parametricRule, T ruleParam, string desc = null)
and some predefined default methods intended for it:
public static bool NumberEqualsWithTolerance(IAttrProxy a, IAttrProxy b, double tolerance)
Now when using this in C#, I can write the following and it works:
var tmp = DefaultRules.Create("fds", "fds", DefaultRules.NumberEqualsWithTolerance, 10.0);
But, in F# this:
let attrRule = DefaultRules.Create("fd","fdsa", DefaultRules.NumberEqualsWithTolerance, 89.)
gives syntax error: "Error FS0002 This function takes too many arguments, or is used in a context where a function is not expected"
What would be the correct way to pass a C# static method into a parameter expecting a Func<> in F#?
It is important to actually pass in the function, and not a lambda wrapper, because the Create method's job is to use the argument function's MethodInfo, which gets hidden by the lambda wrapper's one.
The passed in function does not have overloads, also tried with specifying the type in place like
(DefaultRules.NumberEqualsWithTolerance : Func<IAttrProxy,IAttrProxy,float,bool>)
This is a case of F# being very thoughtful on your behalf - by helping you write more idiomatic F#.
In .NET, you are not actually passing in the function, as if it's a member reference, rather you are passing in a delegate object of type Func<>. The construction of the delegate object is done implicitly by C# when it has the necessary type information.
We can see this more clearly if we refactor this into an actual delegate type:
public delegate bool ParametricRule<T>(IAttrProxy a, IAttrProxy b, T value);
public static AttrDiffRule Create<T>(string a_attr, string b_attr, ParametricRule<T> parametricRule, T ruleParam, string desc = null)
{
return default;
}
If you try to construct a ParametricRule in F#, you'll see that its type is:
ParametricRule(IAttrProxy -> IAttrProxy -> 'a -> bool)
The rationale is that this way you can use regular F# functions, instead of some un-F#ish tupled input function. And this why it doesn't work in your case.
Because you're trying to throw the tupled version from C# right back at it.
So if you refactor your C# implementation to:
protected static bool NumberEqualsWithToleranceImpl(IAttrProxy a, IAttrProxy b, float tolerance)
{
return default;
}
public static ParametricRule<float> NumberEqualsWithTolerance => NumberEqualsWithToleranceImpl;
you'll see that it works like you'd expect it to, both from F# and C#.
let attrRule = DefaultRules.Create("fd","fdsa", DefaultRules.NumberEqualsWithTolerance, 89.0f) //compiles, yay!
Sometimes the type resolution has trouble when passing a method as a function parameter, because there can be overloads on the method that make the signature ambiguous. You can just wrap the function in a lambda that passes the parameters.
let attrRule =
DefaultRules.Create(
"fd",
"fdsa",
(fun a b tolerance -> DefaultRules.NumberEqualsWithTolerance(a, b, tolerance)),
89.0)
I am trying to achieve the following:
I have an interface, called IBotCommand and a few classes that implement it. I want to find all these classes, through reflection, instantiate an instance of each and put the result in a dictionary.
the interface is the following:
type IBotCommands =
abstract member Name: unit -> string
abstract member Description: unit -> string
abstract member Help: unit -> string
abstract member Execute: MessageEventArgs -> string[] -> string
and the code:
let t = typeof<IBotCommands>
t.Assembly.GetTypes()
|> Seq.filter (fun x -> x.IsSubclassOf(t))
|> Seq.iter (fun x ->
(
let i = Activator.CreateInstance(x) :> IBotCommands
botCommands.[i.Name] <- i
)
)
the issue I have is with the CreateInstance line. CreateInstance returns an obj type that can't be cast to IBotCommands.
I have the same in C# and it works properly but the C# version is using dynamics:
public static IEnumerable<Type> FindClassSubclassOfType<T>()
{
var a = typeof(T)
.Assembly.GetTypes()
.Where(t => t.IsSubclassOf(typeof(T)))
.Select(t => t);
return a.ToList();
}
var types = ReflectionHelper.FindClassSubclassOfType<BotCommand>();
foreach (var t in types)
{
dynamic c = Activator.CreateInstance(t);
BotCommands[c.Name] = c;
}
how can I get this behavior to work in F#?
can you cast an object to an interface in F#? it's my first time using interfaces in F#
In F#, there is a difference between upcast a :> T and downacst a :?> T.
Upcast is used when the compiler statically knows that a implements an interface T. This is useful if you have a value of a concrete class and want to get a value that has a type of an interface.
Downcast is used when the compiler does not statically know whether a implements an interface. In other words, this means that the cast can fail.
In your case, you need a downcast, because the compiler does not know whether obj implements IBotInterface. All you need to do is to add ?:
let i = Activator.CreateInstance(x) :?> IBotCommands
botCommands.[i.Name] <- i
I have written a class in F# implementing an interface in order to build a C#-friendly interface for my F#-assembly.
I have written some of the properties as indexed properties. However, when I try to use the type from C#, I only get the synthetic get_PropertyName methods in intellisense and the compiler likewise complains in case I want to use the indexed properties like I would do for C# ones.
Code for reference:
type IMyInterfaceType =
abstract MyProperty : MyType1 with get
abstract MyIndexedProperty : MyType2 -> MyType3 with get
abstract MyTwoDimensionalIndexedProperty : (MyType4 * MyType5) -> MyType6 with get
type MyInterfaceType =
new () = { }
interface IMyInterfaceType with
member this.MyProperty with get () = new MyType1 ()
member this.MyIndexedProperty with get parameter = new MyType3 ()
member this.MyTwoDimensionalIndexedProperty with get pair = new MyType6 ()
When trying to access this class from C#, I only get methods
get_MyIndexedProperty(MyType2 parameter)
get_MyTwoDimensionalIndexedProperty(Tuple<MyType4, MyType5>)
instead of the indexed properties I had hoped for.
Am I doing something wrong or is this a known issue?
cheers
--Mathias.
Response to the original question:
Indexer properties in C# have special name Item thus to create indexer accessible from C# you must name your indexer property "Item", e.g.:
type X () =
member this.Item with get key = ....
Now it can be accessed both in F# using (x : X).[key] or in C# using x[key] .
Response to the updated question:
C# does not support indexed properties the way F# does. Instead using additional type is advised: https://msdn.microsoft.com/en-us/library/aa288464%28v=vs.71%29.aspx
So you can try to use something like this:
[<AbstractClass>]
type Indexer<'index, 'result> () =
abstract Get : 'index -> 'result
member this.Item with get key = this.Get key
type IMyInterfaceType =
abstract MyProperty : MyType1 with get
// F# indexed propetties
abstract MyIndexedProperty : MyType2 -> MyType3 with get
// C# workaround
abstract MyCSharpIndexedProperty : Indexer<MyType2, MyType3> with get
type MyInterfaceType () as this =
let proxy =
{ new Indexer<MyType2, MyType3> () with
member __.Get key = (this :> IMyInterfaceType).MyIndexedProperty key }
interface IMyInterfaceType with
member __.MyProperty with get () = new MyType1 ()
member __.MyIndexedProperty with get key = new MyType3 ()
member __.MyCSharpIndexedProperty with get () = proxy
And two dimensional property similarly by creating Indexer<'index1, 'index2, 'result> () = ...
I hit something new to me with the following piece of code when following the equivalent in C# here. The compiler gives multiple errors basically telling the IConnectableObservable created in source.Publish() does not match IObservable even though it derives from it (according to the MSDN article linked).
Is there something in F# that is different with regard to C# concerning inheritance in this case or can someone provider pointers as to what is going on? Have I just made a typo I can't see? What comes to the heading regarding covariance, it's just a wild guess as I'm at least temporarily out of ideas. And so, maybe writing somewhere may help me and others...
One example of the many error messages:
No overloads match for method 'Create'. The available overloads are shown below (or in the Error List window).
No overloads match for method 'Switch'. The available overloads are shown below (or in the Error List window).
Error Possible overload: '(extension) IObservable.Switch<'TSource>() :
IObservable<'TSource>'. Type constraint mismatch. The type
IObservable<IConnectableObservable<'b>> is not compatible with type
IObservable<IObservable<'a>> The type 'IObservable<'a>' does not match the type 'IConnectableObservable<'b>'.
open System.Reactive.Concurrency
open System.Reactive.Disposables
open System.Reactive.Subjects
open System.Reactive.Linq
type Observable with
static member inline Suspendable(source: IObservable<_>, suspend: IObservable<bool>, isSuspendedInitially: bool): IObservable<_> =
Observable.Create<_>(fun observer ->
let shared = source.Publish()
let pausable =
suspend.StartWith(isSuspendedInitially)
.TakeUntil(shared.LastOrDefaultAsync())
.DistinctUntilChanged()
.Select(fun p -> if p then shared else Observable.Empty<_>())
.Switch()
new CompositeDisposable(pausable.Subscribe(observer), shared.Connect()))
The corresponding C# code
public static class RxExtensions
{
public static IObservable<T> Suspendable<T>(this IObservable<T> stream, IObservable<bool> suspend, bool isSuspendedInitially)
{
return Observable.Create<T>(o =>
{
var shared = stream.Publish();
var pausable = suspend
.StartWith(isSuspendedInitially)
.TakeUntil(shared.LastOrDefaultAsync())
.DistinctUntilChanged()
.Select(p => p ? shared : Observable.Empty<T>())
.Switch();
return new CompositeDisposable(pausable.Subscribe(o), shared.Connect());
});
}
}
This was a bit tricky, but you need to add two upcasts: shared to IObservable<_>, and the result of the lambda function to IDisposable. These would be implicit in C#, but need to be explicit in F#:
type Observable with
static member inline Suspendable (source: IObservable<_>,
suspend: IObservable<bool>,
isSuspendedInitially: bool): IObservable<'a> =
Observable.Create<_>(fun observer ->
let shared = source.Publish()
let pausable =
suspend.StartWith(isSuspendedInitially)
.TakeUntil(shared.LastOrDefaultAsync())
.DistinctUntilChanged()
.Select(fun p -> if p then shared :> IObservable<_>
else Observable.Empty<_>())
.Switch()
new CompositeDisposable(pausable.Subscribe(observer),
shared.Connect()) :> IDisposable)
I'm using an object expression to implement two interfaces. One of the interfaces is IDisposable. I expected to be able to use the 'use' keyword with the results from this object expression, but I get an error:
Type constraint mismatch. The type
IConnMan is not compatible with type
IDisposable The type 'IConnMan' is not
compatible with the type
'IDisposable'
Why do I get this error?
let connectionstring = "context connection=true"
let connman () =
let conn = new SqlConnection(connectionstring)
conn.Open()
{ new IConnMan with
member x.Connect () = conn
member x.Disconnect c = ()
interface IDisposable with
member x.Dispose() =
conn.Close()
conn.Dispose()
}
...
let f() =
use cn = connman() // <-- Error!
An object expression can have just a single type. The type is the type of the first (main) implemented interface - in your case, that's the IconnMan type. F# doesn't allow you to use use, because it doesn't statically know that the result of connman function is IDisposable.
You could create interfaces in the object expression in the opposite order:
let connman () =
let conn = new SqlConnection(connectionstring)
conn.Open()
{ new IDisposable with
member x.Dispose() =
conn.Close()
conn.Dispose()
interface IConnMan with
member x.Connect () = conn
member x.Disconnect c = () }
Then you could write use cn = connman() but you couldn't use functions of IConnMan without casting (this is essentially the same as doing what Desco suggests). I don't think there is any nice solution to this problem.
Could your IConnMan interface inherit from IDisposable?
F# spec says:
Object expressions are statically checked as follows.
First, ty0 to tyn are checked and must all be named types. The overall type of the expression is ty0 and is asserted to be equal to the initial type of the expression. However, if ty0 is type equivalent to System.Object and where ty1 exists, then the overall type is instead ty1.
so you can use type tests and downcasts or change the sequence of interfaces being implemented so IDisposable will be first
let f() =
use cn = connman() :?> IDisposable