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.
Related
Given a function defined as let get<'T> var1 var2 : 'T option what type signature should the given to a record field that function will be assigned to?
I've tried various permutations of type MyType = {AFunc<'T> : obj -> obj -> 'T option} but but can't find any variant that lets me introduce the type argument.
I can do this type MyType = {AFunc: obj -> obj -> obj option} and that will let me create the record {AFunc = get} but then can't apply the function because the type argument is missing.
There's a bit of ambiguity in your question. Do you want to be able to store get<'t> in a record for one particular 't per record, or do you want to have the record itself store a "generic" function like get<_>?
If the former, then TeaDrivenDev's answer will work.
If the latter, then there's no completely straightforward way to do it with F#'s type system: record fields cannot be generic values.
However, there's a reasonably clean workaround, which is to declare an interface type with a generic method and store an instance of the interface in your record, like this:
type OptionGetter = abstract Get<'t> : obj->obj->'t option
type MyType = { AFunc: OptionGetter }
let get<'t> var1 var2 : 't option = None // your real implementation here
let myRecord = { AFunc = { new OptionGetter with member this.Get v1 v2 = get v1 v2} }
let test : int Option = myRecord.AFunc.Get "test" 23.5
You have to make the record type itself generic; only then will 'T be defined and usable.
type MyType<'T> = { AFunc : obj -> obj -> 'T option }
I have defined an interface in F# with an overloaded method. As per compiler request, the overload uses tupled arguments instead of curried ones:
type IInterface =
abstract member Do : (int * string) -> unit
abstract member Do : int -> unit
I then create a class which implements the interface:
type ImplementingClass =
interface IInterface with
member this.Do (i, s) = ()
member this.Do i = ()
However, doing so yields the compiler error for the first of both methods: "This override takes a different number of arguments to the corresponding abstract member"
What am I doing wrong here?
There is a subtle difference between the following two:
abstract member Do : int * string -> unit
abstract member Do : (int * string) -> unit
If you add the parentheses, you're saying that the parameter is a tuple and the compiler should produce a method taking Tuple<int, string>. Without parentheses, the method will be compiled as taking two parameters. Most of the time, this is hidden and you can ignore it - but sadly, not always.
So, you can either change your interface definition to use ordinary "two-parameter" method (this would be my preferred method - you can still call the method with tuple as an argument and looks nicer in the .NET/C# view):
type IInterface =
abstract member Do : int * string -> unit
abstract member Do : int -> unit
type ImplementingClass =
interface IInterface with
member this.Do (i, s) = ()
member this.Do i = ()
Or you can implement the interface as it is:
type ImplementingClass =
interface IInterface with
member this.Do((i:int, s:string)) = ()
member this.Do(i:int) = ()
Sadly, this is a bit ugly - you need the type annotations so that the compiler can unambiguously decide which method you're implementing.
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> () = ...
When I look the definition of event class in f#
type Event<'T> =
class
new Event : unit -> Event<'T>
member this.Trigger : 'T -> unit
member this.Publish : IEvent<'T>
end
I can pass only one type on event class like
let nameChanged = new Event<unit>()
But I saw some sample too, that pass two variables like
let propertyChanged = Event<PropertyChangedEventHandler, PropertyChangedEventArgs>()
How could be that be possible?
And when I implement an interface like
module SapHandler
open SAP.Middleware.Connector
type Connector() =
let configurationChanged = Event<RfcDestinationManager.ConfigurationChangeHandler, RfcConfigurationEventArgs>()
interface IDestinationConfiguration with
member self.ChangeEventsSupported() =
false
[<CLIEvent>]
member self.ConfigurationChanged = configurationChanged.Publish
The compiler complain:
The type 'RfcDestinationManager.ConfigurationChangeHandler' has a non-standard delegate type
Why?
The definition of delegate type is:
It's possible because there's another class too:
type Event<'Delegate,'Args (requires delegate)> =
class
new Event : unit -> Event<'Delegate,'Args>
member this.Trigger : obj * 'Args -> unit
member this.Publish : IEvent<'Delegate,'Args>
end
You're getting that error because your delegate for the event is probably missing the first object sender argument which is standard in .net. You can use Control.DelegateEvent<'Delegate> to get around this
let propertyChanged = DelegateEvent<RfcDestinationManager.ConfigurationChangeHandler>()
I have an type that is implementing IEnumerable<T> interface, all is ok:
open System
type Bar() =
interface Collections.IEnumerable with
member x.GetEnumerator () = null
interface Collections.Generic.IEnumerable<int> with
member x.GetEnumerator () = null
But things goes wrong if type inherits IEnumerable interface implementation via the base type:
open System
type Foo() =
interface Collections.IEnumerable with
member x.GetEnumerator () = null
type Bar() =
inherit Foo()
interface Collections.Generic.IEnumerable<int> with
member x.GetEnumerator () = null
Code above produces the type inference errors:
The member 'GetEnumerator<'a0 when 'a0 : null> : unit -> 'a0 when 'a0 : null' does not have the correct type to override any given virtual method
The member 'GetEnumerator<'a0 when 'a0 : null> : unit -> 'a0 when 'a0 : null' does not have the correct number of method type parameters. The required signature is 'GetEnumerator : unit -> Collections.Generic.IEnumerator<int>'.
Am I doing something wrong or this is an F# compiler bug?
Microsoft (R) F# 2.0 Interactive build 4.0.30319.1
Update more canonical example:
type IFoo = abstract Bar : obj list
type IFoo<'a> = abstract Bar : 'a list
inherit IFoo
/* ok */
type Foo = interface IFoo with member x.Bar = []
interface IFoo<Foo> with member x.Bar = []
/* fail */
type FooBase = interface IFoo with member x.Bar = []
type FooDerived = interface IFoo<Foo> with member x.Bar = [] // <---
inherit FooBase
/*
error FS0017: The member 'get_Bar : unit -> 'a list' does not
have the correct type to override any given virtual method.
*/
The compiler cannot infer the correct type from your "null"-implementation. Try
open System
type Foo() =
interface Collections.IEnumerable with
member x.GetEnumerator () = null
type Bar() =
inherit Foo()
interface Collections.Generic.IEnumerable<int> with
member x.GetEnumerator () : Collections.Generic.IEnumerator<int> = null
UPDATE:
The reason is, that the type of the GetEnumerator method implemented by the Bar type is ambigous as IEnumerable<'a> implements/inherits the non-generic IEnumerable which also specifies a (non-generic) GetEnumerator method. So, how should the compiler infer, which method exactly you are trying to implement if all he gets is null? Therefore we need a type annotation in this case.
This is not a bug, this is just an type inference fail because of F# may implement inherited interface members in the derived interface implementation declaration:
type IA = abstract A : int
type IB = inherit IA
type IC = inherit IB
type Baz =
interface IC with
member x.A = 1
So in my example I should specify the correct return type explicitly because member x.GetEnumerator() in derived Bar type may match both IEnumerable.GetEnumerator() and IEnumerable<T>.GetEnumerator().