Syntax for implicit conversion from F# function to C# Func<> - f#

This compiles
let a (t2: Type) = obj()
let c: Func<Type,obj> = a
but this
let a (t2: Type) = obj()
let b (t:Type) = a
let c: Func<Type,(Type->obj)> = b
says This expression was expected to have type
'Func<Type,(Type -> obj)>' but here has type
''a -> 'b -> obj'
I know it is something simple, but my brain has thrown the towel in.

The normal delegate conversion would be the following:
let c : System.Func<Type, Type, obj> = b
If you want to interpret b as a Func returning another function, you have to be explicit about it only having one input argument:
let a (t2: Type) = obj()
let b (t:Type) = a
let c = Func<_, _>(b)
The F# spec has this to say about delegate conversions:
[14.4]: If a formal parameter has delegate type D, an actual argument farg has known type
ty1 -> ... -> tyn -> rty, and the number of arguments of the Invoke method of delegate type
D is precisely n, interpret the formal parameter in the same way as the following:
new D(fun arg1 ... argn -> farg arg1 ... argn).
So in other words, the implicit conversion to the Func delegate of your b function is the same as
let c = System.Func<_, _, _>(fun a1 a2 -> b a1 a2)
I think the reason for this is for interop with other dotnet languages and to be consistent with how curried functions are compiled:
[8.13.4]: The compiled representation of a curried method member is a .NET method in which the arguments are concatenated into a single argument group.

Related

How do I assign a multi-parameter F# function to a C# variable?

I have the following F# function:
let myFSharpFunction : IO.TryTransform<IDatabaseService,EditForm,SyncType,ErrorDescription> =
fun _ _ -> Ok someValue
I would like to use this F# function as a value/delegate in my C# code:
FSharpFunc<IDatabaseService, FSharpFunc<EditForm, FSharpResult<SyncType, ErrorDescription>>> referenceToFSharpFunction = myFSharpFunction;
However, I get an error:
Error CS0428 Cannot convert method group 'myFSharpFunction' to
non-delegate type 'FSharpFunc>>'. Did you intend to invoke the
method?
I then tried:
public delegate FSharpResult<SyncType, ErrorDescription> myDelegate(IDatabaseService database, EditForm editForm);
...
myDelegate someDelegate = myFSharpFunction;
FSharpFunc<IDatabaseService, FSharpFunc<EditForm, FSharpResult<SyncType, ErrorDescription>>> FSharpFuncToInvoke = someDelegate;
However, this attempt also gave me an error:
type 'Sevan.Android.FormsSyncFragment.myDelegate' to
'Microsoft.FSharp.Core.FSharpFunc>>'
Update:
Attempt 1:
I then tried adding the following function to an F# module:
let toFunc2 f = Func<_, _, _> f // f : 'a -> 'b -> 'c
I next updated the C# code to the following:
var referenceToFSharpFunction = toFunc2<IDatabaseService, EditForm, FSharpResult<SyncType, ErrorDescription>>(myFSharpFunction);
But I then received this error:
Cannot convert method group 'FSharpFuncToInvoke' to non-delegate type
'FSharpFunc>>'. Did you intend to invoke
the method?
Attempt 2:
I also tried setting the F# code to the following:
let toFunc2 f = Func<IDatabaseService,EditForm, Result<SyncType,ErrorDescription>> f // f : 'a -> 'b -> 'c
Then within my C#, I attempted this:
var referenceToFSharpFunction = toFunc2<IDatabaseService, EditForm, FSharpResult<SyncType, ErrorDescription>>(myFSharpFunctione);
But that attempt gave me this error:
Error CS0308 The non-generic method
'Mock.toFunc2(FSharpFunc>>)' cannot be used with type arguments
In conclusion, how do I assign a multi-parameter F# function to a C# variable?
If you are defining some F# functionality that should be used by a C# client, then I would strongly recommend exposing all the functionality in a C#-friendly way - that means, doing all the wrapping on the F# side, so that the C# consumer does not have to worry about things like FSharpFunc at all.
It's a bit hard to say how this should work in your case - because your example does not actually show any realistic piece of code - only some kind of adapter with a fairly complex interface - but you could do something along the following lines:
// Your F# function that is used elsewhere in F# code
let myFsharpFunction =
fun _ _ -> Ok someValue
// Your F# logic, exposed as a C#-friendly `System.Func` delegate
let myCsharpFunction =
System.Func<_, _, _>(myFsharpFunction)
The F# function type FSharpFunc is something completely separate from the Func/Action used in C#, so you need to "convert" it by creating the respective type that C# understands, which is easy, but needs to be done for each number of arguments individually.
let toFunc1 f = Func<_, _> f // f : 'a -> 'b
let toFunc2 f = Func<_, _, _> f // f : 'a -> 'b -> 'c
// etc.
let toAction0 f = Action f // f : unit -> unit
let toAction1 f = Action<_> f // f : 'a -> unit
// etc.

Partially apply type constructor

I have a type with a constructor taking two arguments:
type Foo = Foo of int * int
And I can call it with a tuple of (int, int).
let foo = Foo(1,2)
But in my case I want to partially apply the constructor to use it in a pipe
let someFunc param =
calculateSomeInt param
|> Foo 42 //error
But this is not possible because the constructor cannot be called in curried form.
Is there some possibility to tell F# I want the constructor in curried form without having to define a new function let fooCtr a b = Foo (a,b)?
No, there isn't. The let fooCtr a b = Foo (a,b) approach you mentioned in your question is the only way to do this. (I would probably call it makeFoo or better yet mkFoo rather than fooCtr myself, since I feel that function names should be verbs, but that's personal style preference).
rmunn's answer is correct, but I'd like to add a detail.
You can add the constructor as a parameter to the make function, so you can use make to partially apply any constructor which takes a pair (a 2-tuple) as parameter:
type TypeA =
| A of int*int
| B of int*string
type TypeB =
| C of int*float
// make : (('a * 'b -> 'c) -> 'a -> 'b -> 'c)
let make t a b = t(a,b)
// x : int -> TypeA
let x = make A 1
// y : string -> TypeA
let y = make B 2
// z : float -> TypeB
let z = make C 3
In fact, it works with any function which takes a pair as parameter, so the name make is slightly misleading in this case.

Error FS0001, this function was expected to have type byref<int>

I have a made a simple function that wraps common F# functions of signature 'a -> 'b -> 'c option to more "C# compliant" function as : 'a -> b -> byref<'c> -> bool. But somehow when I try to wrap such a method in a class I am getting error FS0001 and I can't locate the error.
Code below
open System
open System.Runtime.InteropServices
// Given a function, f: 'a -> 'b -> 'c option returns
// A new function g: 'a -> 'b -> byref<'c> -> bool
let wrapOptionF f a b (g:byref<'c>) =
match f a b with
| Some v ->
do g <- v
true
| None ->
false
let tryDivide (a:int) (b:int) =
match Math.DivRem(a,b) with
| v, 0 -> Some v
| _ -> None
type TryDivideWrapper() =
static member TryDivide(a, b, [<Out>]cRef:byref<int>) : bool =
let f = wrapOptionF tryDivide a b
f cRef
The offending line is f cRef.
This post contains a more in-depth explanation, but in short you can replace your final type definition with the following:
type TryDivideWrapper() =
static member TryDivide(a, b, [<Out>]cRef:byref<int>) : bool =
wrapOptionF tryDivide a b &cRef
The reason for this is that your wrapOptionF takes a byref parameter. However, byref<int> isn't really a type like int or int ref - it's just an indication to the compiler that your parameter should be passed by reference (like out in C#). Once inside your function, however, what you have is a regular int.
Edit: Note that Intellisense will show cRef as having type byRef<int>. However, if you bind another variable to cRef, you'll see that the type you get is a regular int. You can put this line inside TryDivide and then hover your mouse over a to see it:
let a = cRef
Using the & operator tells the compiler that you're passing cRef into f by reference - which is exactly what f needs - making the type system happy. I've tested this using a C# project, and TryDivideWrapper.TryDivide(a, b, out c) works as expected. Add in #MarkSeemann's tryDivide and you should be good to go.
I'm not exactly sure I understand the reason, but this seems to work:
type TryDivideWrapper() =
static member TryDivide(a, b, [<Out>]cRef:byref<int>) : bool =
wrapOptionF tryDivide a b &cRef
BTW, the OP tryDivide implementation throws an exception on tryDivide 1 0. Here's an alternative implementation that works:
let tryDivide (a:int) (b:int) =
if b = 0
then None
else Some (a / b)
FSI:
> tryDivide 1 0;;
val it : int option = None
> tryDivide 10 5;;
val it : int option = Some 2

Resolve operator conflicts on similar generic types

I'm using a third party library (Sauve.IO) which defines the standard bind operator >>= :
val inline (>>=) : first:('T -> Async<'U option>) -> second:('U -> Async<'V option>) -> input:'T -> Async<'V option>
I also have an internal library that would also like to define the operator over the type signature
Async<Response<'a>> -> ('a -> Async<Response<'b>>) -> Async<Response<'b>>
What is the cleanest way to use these two functions in the same namespace/module without running afoul of the restriction on overloading inline functions ?
How about renaming Suave's operator to some other infix operator?
let (>=>) = Suave.(>>=)
Judging from the signature, it looks like it is not bind anyway,
but actually Kleisli composition.
You may be able to leverage F#'s statically resolved ad-hoc polymorphism: hide the qualified operator invocation behind a separate, overloaded operator; then define yet another, inlined operator for actual use.
type Foo = Foo with
static member ($) (_ : Foo, first : _ -> Async<_ option>) =
fun second value -> Module1.(>>=) first second value
static member ($) (_ : Foo, arg1 : Async<Response<_>>) =
Module2.(>>=) arg1
let inline (>>=) arg1 arg2 = (Foo $ arg1) arg2
Do both operators have the same name (>>=) ?
If so I assume that Suave's operator came with it's own module or namespace?
In this case you can qualify their operator appending their module/namespace name like this Suave.(>>=) but in this case you have to call it as usual function. Here is simplified example:
module Suave =
let inline (>>=) a b = a + 2 * b
module Mine =
open Suave
let (>>=) a b = a - 3 * b
let r1 = Suave.(>>=) 1 3
let r2 = 1 >>= 3
Result:
val r1 : int = 7
val r2 : int = -8

In F#, How can I cast a function to a different delegate?

I need to cast a function with the signature Thing -> Thing -> int to a Comparison<Thing>.
For example, when I have:
Array.Sort(values, mySort)
I get an error stating "This expression was expected to have type Comparison but here has type Thing -> Thing -> int"
When I try this:
Array.Sort(values, (fun (a, b) -> mySort a b))
(actually, this is by way of a wrapper object)
I get an error stating "Type constraint mismatch. The type ''a -> 'b' is not compatible with the type 'Comparison'"
How am I supposed to provide a Comparison<T>?
FYI: Comparison<T> is a delegate with the signature 'T * 'T -> int
This works for me:
let values = [|1;2;3|]
let mySort a b = 0
Array.Sort(values, mySort)
But have you tried:
Array.Sort(values, Comparison(mySort))
Oddly, despite the fact that delegate signatures are expressed in tupled form (e.g. 'T * 'T -> int), you need to provide your function in curried form (e.g. fun a b -> mySort a b). See section 6.4.2 of the spec.

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