I am writing an F# interop class to be used from C#. I thought that F# had an implicit conversion from .NET Tuple<> type (similar to IEnumerable treated as seq), so I wrote the following code:
type MyClass() =
member this.MyMethod (t: Tuple<int, int>) =
let meaningOfLife (t : int * int) = 42
meaningOfLife t
This code fails to compile with the following error:
error FS0001: This expression was expected to have type int * int but here has type Tuple
Then how to convert tuples between C# and F# (and back)?
If you're targeting .NET 4.0 and above, you don't need to specify the object Tuple. F# tuples get automatically compiled to Tuple. You can use:
type MyClass() =
member this.MyMethod (t: int * int) =
let meaningOfLife (t : int * int) = 42
meaningOfLife t
And it should work fine from C#.
Related
I'm pretty sure this isn't possible but thought I'd double check. I guess I'm trying to mimic typescript's union types.
I have a type
type StringOrInt =
| String of string
| Int of int
And then a function
let returnSelf (x: StringOrInt) = x
At the moment the function has to be called like
returnSelf (String "hello")
Is it possible to do
returnSelf "hello"
and infer that it is a valid StringOrInt?
At the moment it's not supported out of the box, but fairly simple generic casting method can be implemented.
Given an example function:
let call v = (* v has inferred type StringOrInt *)
match v with
| String s -> printfn "called a string %s" s
| Int i -> printfn "called an int %i" i
We could eventually call it like so:
(* % is a unary prefix operator *)
call %"abc"
call %1
We need to provide some way to tell how to convert ordinary string/int into StringOrInt type. This can be used by exemplar convention call:
type StringOrInt =
| String of string
| Int of int
static member inline From(i: int) = Int i
static member inline From(s: string) = String s
Here our discriminated union provides two static methods which are responsible for casting. Since they correspond to the same naming convention, we can use them together with F# statically resolved generic type parameters - these are generics that are resolved at compile time unlike the .NET generics which exists also at runtime - to create a casting function (or operator):
(* casting operator *)
let inline (~%) (x: ^A) : ^B =
(^B : (static member From: ^A -> ^B) x)
This way you can use % prefix operator over any type that implements a casting mechanism in form of static From method.
This question already has an answer here:
How to use overloaded explicit conversion operators?
(1 answer)
Closed 4 years ago.
Let's say that I have a class in C# with overloaded implicit and explicit operators:
public static implicit operator CSClass(int a) => ...;
public static explicit operator int(CSClass a) => ...;
I compile this project as class library.
In F# now I can add my operator for implicit conversions and use it:
#r #"C:\path\to.dll"
open Some.Namespace.ToMyClass
let inline (!>) (x:^a) : ^b = ((^a or ^b) : (static member op_Implicit : ^a -> ^b) x)
let a : CSClass = !> 5
But how can I do an explicit overloaded conversion in F#? (CSClass to int)
It is my understanding that F# does not usually do explicit conversions. Instead, you would just use a function. For example, if you have a char and want to convert that explicitly into an int, in C# you write:
char theChar = 'A';
int convertedChar = (int)theChar;
In F#, the int operator (function) is used for the same purpose:
let theChar = 'A'
let convertedChar = int theChar;
Therefore the idiomatic way to do the conversion would be something like this:
module Some.Namespace.MyClass
let toInt (x : MyClass) = [...]
You would use it like so:
let convertedMyClass = MyClass.toInt myClass
It can also be piped:
funcReturningMyClass x y
|> MyClass.toInt
|> printfn "%d"
I have a native C library and I want do some F# coding with it. The thing is I get exception:
System.TypeLoadException: Cannot marshal field 'log' of type
'LoggingModel': There is no marshaling support for this type.
at
System.StubHelpers.ValueClassMarshaler.ConvertToNative(IntPtr dst,
IntPtr src, IntPtr pMT, CleanupWorkList& pCleanupWorkList)
at
FSI_0009.Initialize(ComponentOverrideFlags flags, LoggingModel&
loggingModel, ThreadingModel& threadingModel, SchedulingModel&
schedulingModel, IntPtr memoryModel)
at
.$FSI_0011.main#()
in
D:\dev_p\f#\FunBindings\FunExample\Environment.fs:line 16 Stopped due
to error
Here the code:
module Interop
[<CLSCompliant(true); Flags>]
type LogTarget =
| None = 0
| Console = 1
| Trace = 2
| Custom = 4
[<UnmanagedFunctionPointer(CallingConvention.Cdecl)>]
type LogCallback = delegate of LogTarget * string * string * nativeint -> unit
[<UnmanagedFunctionPointer(CallingConvention.Cdecl)>]
type ReleaseCallback = delegate of nativeint -> unit
[<Struct>]
type LoggingModel =
val mutable targets : LogTarget
val mutable log : LogCallback
val mutable deleteModel : ReleaseCallback
val mutable userparam : IntPtr
[<DllImport("CLIBRARY.dll", CallingConvention = CallingConvention.Cdecl, EntryPoint = "txInitialize")>]
[<MethodImpl(MethodImplOptions.ForwardRef)>]
extern int Initialize(ComponentOverrideFlags flags, LoggingModel& loggingModel, ThreadingModel& threadingModel, SchedulingModel& schedulingModel, IntPtr memoryModel)
module Environment
let initialize =
let mutable loggingModel = new LoggingModel()
let mutable threadingModel = new ThreadingModel()
let mutable schedulingModel = new SchedulingModel()
Initialize(ComponentOverrideFlags.None, &loggingModel, &threadingModel, &schedulingModel, IntPtr.Zero)
Basically, I get the aforementioned error when I try to execute "initialize" function in interactive.
I would really appreciate any help.
Update: I've checked the code a bit more and noticed that outside of the interactive console it seems to be working, without failing with exceptions. I need to provide a bit more coverage for CLibrary to be sure. Meanwhile, if there anybody who knows what could cause this exception and how it could be prevented, I would really appreciate the answer.
I think the problem is that delegate of LogTarget * string * string * nativeint -> unit declares a delegate where the arguments are curried. (This doesn't really make sense to me either since a * b normally represents a tuple.)
The subtly different delegate of (LogTarget * string * string * nativeint) -> unit declares a delegate with tupled arguments which would be compatible with a native function.
You can see this difference if you try and assign a .NET method to two different delegate types:
type Curried = delegate of int * int -> int
type Tupled = delegate of (int * int) -> int
//let a = new Curried (Math.Max) // doesn't compile
let b = new Tupled (Math.Max) // works
Have you tried adding [<MarshalAsAttribute(UnmanagedType.FunctionPtr)>] to LoggingModel?
[<Struct>]
type LoggingModel =
val mutable targets : LogTarget
[<MarshalAsAttribute(UnmanagedType.FunctionPtr)>]
val mutable log : LogCallback
[<MarshalAsAttribute(UnmanagedType.FunctionPtr)>]
val mutable deleteModel : ReleaseCallback
val mutable userparam : IntPtr
IL code without this attribute is:
// Fields
.field public class Interop.LogCallback log
but with this attribute is:
// Fields
.field public marshal(Func) class Interop.LogCallback log
Without marshal(Func)/MarshalAs attribute the delegate cannot be marshalled even with the UnmanagedFunctionPointer attribute. Cannot test it with a native library though.
This doesn't work:
let increment(i: int byref) = i <- i + 1
let xxx = ref 0
increment(xxx) // this expression was expected to have type
// byref<int> but here has type int ref
But this works:
let incrementParam(a: int byref) = a <- a + 1
let mutable b = 30
incrementParam(&b)
as well as this:
type Incrementor =
static member Increment(i : int byref) =
i <- i + 1
let fff = ref 10
Incrementor.Increment(fff)
Because the spec says so. See Type Directed Conversions at Member Invocations (emphasis mine), especially the following:
Note: These type-directed conversions are primarily for interoperability with existing member-based .NET libraries and do not apply at invocations of functions defined in modules or bound locally in expressions.
To add some details to the reference that Daniel pointed out, the problem is that the type 'T ref is not the same as the type 'T byref and so the compiler needs to insert some conversion (to take the address).
I think this is only supported for members, because this is the main scenario (calling COM interop methods etc.) and because implicit conversions generally compilcate type inference. The type-directed conversions are an easier case where the compiler already knows the required type ('T byref). I suppose that, if this was allowed on functions, the compiler might infer that the type of argument should actually be 'T ref.
If you want to get the first sample to work, you have to explicitly construct 'T byref by taking the address of the contents field:
let increment(i: int byref) = i <- i + 1
let xxx = ref 0
increment(&xxx.contents)
How do you implement the equivalent of C#'s explicit operator in F#? Is it supported?
Just implement an op_Explicit static member like
type SomeType() =
static member op_Explicit(source: SomeType) : int =
1
and then you can use a corresponding F# explicit conversion operator like
SomeType() |> int
you can see a bit into how this works by noting the static member constraint on the type signature of int
^a -> int when ^a : (static member op_Explicit : ^a -> int)