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
Related
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
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));
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 have the following interface in C#
public interface IDog
{
String Bark();
}
I want to create an implementation in F#. I had no problem with this:
type GermanShepherd() =
interface IDog with
member this.Bark() = "Woof"
but when I added a supporting function that is not part of the interface like this:
type GermanShepherd() =
interface IDog with
member this.Bark() = "Woof"
member this.Eat() = "Yummy"
the compiler complained:
Error 1 No abstract or interface member was found that corresponds to this override
Error 2 This value is not a function and cannot be applied*
Is there a way of implementing private/internal functions with a type that implements an interface without those functions being part of the interface? I can't change the interface in my 'real' application b/c there are other projects that implement the interface. None of the examples on MSDN that I found have this particular scenario.
The interface block should only contain functions that are part of the interface, but you can place other functions before the block:
type GermanShepherd() =
member this.Eat() = "Yummy"
interface IDog with
member this.Bark() = "Woof"
If you do not need a public member, then you can go with a private let-bound function:
type GermanShepherd() =
let eat() = "Yummy"
interface IDog with
member this.Bark() = "Woof"
Also note that F# currently only allows explicit interface implementations, which means that you can treat GermanShepherd as IDog, but you won't see IDog members explicitly:
let g = GermanShepherd()
g.Eat() // OK
g.Bark() // Error, interface members are implemented explicitly
let d = g :> IDog // To 'Bark', we need to get 'IDog' first
d.Bark() // OK
One workaround for this is to add the Bark method explicitly as a separate member outside of the interface block. Although there is F# language issue for this too!
type SQLConn =
val mutable private connection : string option
member this.Connection
with get() : string = this.connection.Value
and set(v) = this.connection <- Some v
new (connection : string) = {connection = Some connection;}
new() = SQLConn #"Data Source=D:\Projects\AL\Service\ncFlow\dbase\dbflow.db3; Version=3;Password=432432434324"
I want to use "let x = 5+5" there or something like that, so how can I use private functions in my type (class) (record) , I know that I can use them if I do SQLConn() , but then I can't use val, I want to use both : val and let ...
thank you
As Tim explains, you can only use local let bindings with the implicit constructor syntax. I would definitely follow this approach as it makes F# code more readable.
Do you have any particular reason why you also want to use val in your code? You can still use them with the implicit constructor syntax, but they have to be mutable and initialized using mutation:
type SQLConn(connection:string) as x =
let mutable connection = connection
// Declare field using 'val' declaration (has to be mutable)
[<DefaultValue>]
val mutable a : int
// Initialize the value imperatively in constructor
do x.a <- 10
member this.Connection
with get() = connection and set(v) = connection <- v
new() = SQLConn #"Data Source=.."
As far as I can tell val is only needed to create fields that are not private (which may be required by some code-gen based tools like ASP.NET, but is otherwise not really useful).
The error message explains the problem:
error FS0963: 'let' and 'do' bindings are not permitted in class definitions unless an implicit construction sequence is used. You can use an implicit construction sequence by modifying the type declaration to include arguments, e.g. 'type X(args) = ...'.
The error message is suggesting that you declare your class as type SQLConn(connection) =. If you do this, you probably ought to remove the member this.Connection property, since you'll no longer have a mutable field.
A more likely workaround would be to declare x as val x : int, then put the x = 5 + 5; initializer inside your constructor.
What about the following?
type SQLConn(conn:string) =
// could put some other let bindings here...
// ex: 'let y = 5 + 5' or whatever
let mutable conn = conn
new() = SQLConn(#"some default string")
member __.Connection
with get () = conn and set v = conn <- v