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> () = ...
Related
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 the below code:
type IQuery =
abstract List<'T> : unit -> IList<'T>
let create (str)=
let getList () : IList<'T> = upcast List<'T>()
{ new IQuery with
member this.List<'T>() = getList<'T>()
And for the last line it gives me a warning stating that:
The method or function 'getList' should not be given explicit type argument(s) because it does not declare its type parameters explicitly
However if I remove <'T> from getList call then I get a compilation error as :
The member 'List<'T> : unit -> IList<'a>' does not have the correct type to override the corresponding abstract method. The required signature is 'List<'T> : unit -> IList<'T>'.
What can I do ?
You can declare getList with an explicit type parameter:
let getList<'T> () : IList<'T> = upcast List<'T>()
You then get an error:
Explicit type parameters may only be used on module or member bindings
If you then move the let binding to the top-level at the same scope as the type, it all works:
type IQuery =
abstract List<'T> : unit -> IList<'T>
let getList<'T> () : IList<'T> = upcast List<'T>()
let create (str) =
{ new IQuery with
member this.List<'T>() = getList<'T>()
}
If your real code has getList using values only in scope in create, like str, you'll need to add them as explicit parameters to getList.
I'm trying to create an abstraction for a lightweight data storage module using an F# signature file. Here is my signature file code, let's say it's called repository.fsi
namespace DataStorage
/// <summary>Lightweight Repository Abstraction</summary>
module Repository =
/// <summary> Insert data into the repository </summary>
val put: 'a -> unit
/// <summary> Fetch data from the repository </summary>
val fetch: 'a -> 'b
/// <summary> Remove data from the repository </summary>
val remove: 'a -> unit
Here is the corresponding implementation, let's call it repository.fs
namespace DataStorage
module Repository =
(* Put a document into a database collection *)
let put entity = ()
(* Select a document from a database collection *)
let fetch key = ("key",5)
(* Remove a document from a database collection *)
let remove entity = ()
In my visual studio project file I have the signature file (repository.fsi) above
my implementation file (repository.fs). The put and remove functions are being parsed and verified correctly with no errors (in the implementation file) but the fetch function keeps giving me the red squiggly in visual studio with the following error message:
Module 'DataStorage.Repository' contains
val fetch: s:string -> string * int
but its signature specifies
val fetch<'a,'b> : 'a -> 'b
The respective type parameter counts differ
Can someone tell me what I'm doing wrong? Is my fetch function value defined wrong in
my signature file? I'm just trying to create a generic function ('a -> 'b) in my signature file and have the implementation take one type as input and return a different type as output.
I am not very strong in F# yet but I think here you are using signature files in wrong way.
First. Here is how you can fix compilation errors:
Replace:
let fetch key = ("key",5)
with:
let fetch key = ("key",5) :> obj :?> 'b
and you won't get compilation errors.
But this fix doesn't actually makes sense in many cases. For example if next works:
let a = Repository.fetch(56)
if you specify type explicity it will crash (in most cases):
let b = Repository.fetch<int, string>(56)
The case is that generic implementation should operate with generic types. If I understod correctly from what are you trying to do you should use OOP and polymophism when signature files are used to hide implementation aspects. For example:
namespace DataStorage
[<AbstractClass>]
type Repository<'TKey,'T>() =
abstract member put: 'T -> unit
abstract member fetch: 'TKey -> 'T
abstract member remove: 'T -> unit
type IntRepository() =
inherit Repository<int, int>()
override self.put item = ()
override self.fetch index = 5
override self.remove item = ()
type StringRepository() =
inherit Repository<int, string>()
override self.put item = ()
override self.fetch index = "Hello"
override self.remove item = ()
One (some what limiting) alternative that I've recently tried is sort of one step away from generics but seems to work for my scenario. Basically the signature file for the fetch function now looks like this.
'a -> RepositoryRecord
and the implementation of the RepositoryRecord is an algebraic datatype.
type RepositoryRecord = | SomeVal1 of int * string | SomeVal2 of string
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
Is it possible to create static member indexed properties in F#? MSDN show them only for instance members, however, I'm able to define the following class:
type ObjWithStaticProperty =
static member StaticProperty
with get () = 3
and set (value:int) = ()
static member StaticPropertyIndexed1
with get (x:int) = 3
and set (x:int) (value:int) = ()
static member StaticPropertyIndexed2
with get (x:int,y:int) = 3
and set (x:int,y:int) (value:int) = ()
//Type signature given by FSI:
type ObjWithStaticProperty =
class
static member StaticProperty : int
static member StaticPropertyIndexed1 : x:int -> int with get
static member StaticPropertyIndexed2 : x:int * y:int -> int with get
static member StaticProperty : int with set
static member StaticPropertyIndexed1 : x:int -> int with set
static member StaticPropertyIndexed2 : x:int * y:int -> int with set
end
But when I try to use one, I get an error:
> ObjWithStaticProperty.StaticPropertyIndexed2.[1,2] <- 3;;
ObjWithStaticProperty.StaticPropertyIndexed2.[1,2] <- 3;;
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
error FS1187: An indexer property must be given at least one argument
I tried a few different syntax variations and none worked. Also weird is that when I hover over set in VS2010 for one of the definitions in the type, I get info about ExtraTopLevelOperators.set.
If you wanted to recover the Type.Prop.[args] notation, then you can define a simple object to represent an indexable property with the Item property:
type IndexedProperty<'I, 'T>(getter, setter) =
member x.Item
with get (a:'I) : 'T = getter a
and set (a:'I) (v:'T) : unit = setter a v
type ObjWithStaticProperty =
static member StaticPropertyIndexed1 =
IndexedProperty((fun x -> 3), (fun x v -> ()))
ObjWithStaticProperty.StaticPropertyIndexed1.[0]
This returns a new instance of IndexedProperty every time, so it may be better to cache it. Anyway, I think this is quite nice trick and you can encapsulate some additional behavior into the property type.
A digression: I think that an elegant extension to F# would be to have first-class properties just like it has first-class events. (You could for example create properties that automatically support INotifyPropertyChange with just one line of code)
I believe that you call indexed properties using a different syntax (whether instance or static):
ObjWithStaticProperty.StaticPropertyIndexed2(1,2) <- 3
The only semi-exception to this is that an Item property on an instance x can be called via x.[...] (that is, Item is omitted and brackets are used around the arguments).