I am new to F#, and I am trying to execute a static C# function that accepts multiple parameters from the F# file/code.
I have a single solution which contains both, C# project as well as F# project.
C# project
Code from the C# file:
using ...
namespace Factories
{
public static class FruitFactory
{
public static string GiveMe(int count, string fruitname)
{
...
...
return ... (string) ...
}
}
}
F# project
Code from the F# file:
open System
open Factories
[<EntryPoint>]
let main argv =
let result = FruitFactory.GiveMe 2 "Apples"
printfn "%s" result
printfn "Closing Fruit Factory!"
0
From the above piece of code, I am getting the below errors for the code let result = FruitFactory.GiveMe 2 "Apples"
Error 1:
Program.fs(6, 37): [FS0001] This expression was expected to have type
'int * string'
but here has type
'int'
Error 2:
Program.fs(6, 18): [FS0003] This value is not a function and cannot be applied.
C# functions are uncurried, so you have to call it as though it takes a tuple, like this: FruitFactory.GiveMe(2, "Apples").
If you really want to create a C# function that can be called with curried arguments from F#, you have process each argument separately. It's not pretty, but it can be done like this:
C# project
using Microsoft.FSharp.Core;
public static class FruitFactory
{
/// <summary>
/// Curried version takes the first argument and returns a lambda
/// that takes the second argument and returns the result.
/// </summary>
public static FSharpFunc<string, string> GiveMe(int count)
{
return FuncConvert.FromFunc(
(string fruitname) => $"{count} {fruitname}s");
}
}
F# project
You can then call it the way you want from F#:
let result = FruitFactory.GiveMe 2 "Apple"
printfn "%s" result
Multiparameter .NET methods show up in F# as methods with tupled parameters:
let result = FruitFactory.GiveMe(2, "Apples")
Related
I've read the-f-equivalent-of-cs-out but still I can't make it work for my case (the simplest solution/syntax).
I have this method in a C# project:
//<Project Sdk="Microsoft.NET.Sdk">
//<TargetFrameworks>netcoreapp3.1;netstandard2.0</TargetFrameworks>
public static void ABC(out byte[] a, out byte[] b, byte[] c)
{
var aaa = new byte[10];
var bbb = new byte[10];
a = aaa;
b = bbb;
}
Now, I want to use it in a F# project:
I'm using FSharp.Core 4.7.2
(* <Project Sdk="Microsoft.NET.Sdk">
<TargetFrameworks>netcoreapp3.1;netstandard2.0</TargetFrameworks> *)
let a,b = ABC(c)
I'm imitating the syntax of TryParse and this compiles without errors:
let success, number = System.Int32.TryParse("0")
The compiler on my ABC(c) call complains about the fact the signature asks for 3 parameters, not 1.
Compared to the TryParse I see 2 differences:
It does not return void
It uses Array objects
The compiler accepts this syntax:
let a = Array.empty<byte>
let b = Array.empty<byte>
ABC(ref a, ref b, c)
but:
I think it is not correct to use ref here, not in this way (because a and b are not mutable)
I'd like to use the clean syntax similar to TryParse and I WANT to know why it does not work here
I can change the C# project code, but replacing all the out parameters in that proejct will be a second step and maybe a new qeustion if I have difficulties or doubt.
[Update: parameter position]
I played a little with this and seems like I found when the "simple" syntax (without passing ref parameters) is broken.
public static void TryParseArray(string input, out int[] result) {
result = new int[0];
}
public static void TryParseArray_2(out int[] result, string input) {
result = new int[0];
}
let arr = csharp.TryParseArray("a") // OK
let arr = csharp.TryParseArray_2("a") // ERROR
It seems like the out parameter must be at the end (= not followed by normal parameters) in the C# methods, to make possible for F# to use them as returned tuple.
You correctly noticed that the "simplified" F# syntax for turning out parameters into returned tuples only works in very limited situations - only when you have one parameter and it is the last one. In other words, this feature helps with some common patterns, but it does not fully replace out parameters.
If you want to use out parameter in F#, you can either pass a reference to a local mutable variable using the &var syntax, or you can specify a reference cell of type int ref as an argument. The following shows the two options using the standard TryParse method:
// Using a local mutable variable
let mutable n = 0
Int32.TryParse("42", &n)
printfn "Got: %d" n
// Using a reference cell initialized to 0
let n = ref 0
Int32.TryParse("42", n)
printfn "Got: %d" n.Value
In a c# dll I have a method, that takes func parameters:
public static AttrDiffRule Create<T>(string a_attr, string b_attr, Func<IAttrProxy,IAttrProxy,T,bool> parametricRule, T ruleParam, string desc = null)
and some predefined default methods intended for it:
public static bool NumberEqualsWithTolerance(IAttrProxy a, IAttrProxy b, double tolerance)
Now when using this in C#, I can write the following and it works:
var tmp = DefaultRules.Create("fds", "fds", DefaultRules.NumberEqualsWithTolerance, 10.0);
But, in F# this:
let attrRule = DefaultRules.Create("fd","fdsa", DefaultRules.NumberEqualsWithTolerance, 89.)
gives syntax error: "Error FS0002 This function takes too many arguments, or is used in a context where a function is not expected"
What would be the correct way to pass a C# static method into a parameter expecting a Func<> in F#?
It is important to actually pass in the function, and not a lambda wrapper, because the Create method's job is to use the argument function's MethodInfo, which gets hidden by the lambda wrapper's one.
The passed in function does not have overloads, also tried with specifying the type in place like
(DefaultRules.NumberEqualsWithTolerance : Func<IAttrProxy,IAttrProxy,float,bool>)
This is a case of F# being very thoughtful on your behalf - by helping you write more idiomatic F#.
In .NET, you are not actually passing in the function, as if it's a member reference, rather you are passing in a delegate object of type Func<>. The construction of the delegate object is done implicitly by C# when it has the necessary type information.
We can see this more clearly if we refactor this into an actual delegate type:
public delegate bool ParametricRule<T>(IAttrProxy a, IAttrProxy b, T value);
public static AttrDiffRule Create<T>(string a_attr, string b_attr, ParametricRule<T> parametricRule, T ruleParam, string desc = null)
{
return default;
}
If you try to construct a ParametricRule in F#, you'll see that its type is:
ParametricRule(IAttrProxy -> IAttrProxy -> 'a -> bool)
The rationale is that this way you can use regular F# functions, instead of some un-F#ish tupled input function. And this why it doesn't work in your case.
Because you're trying to throw the tupled version from C# right back at it.
So if you refactor your C# implementation to:
protected static bool NumberEqualsWithToleranceImpl(IAttrProxy a, IAttrProxy b, float tolerance)
{
return default;
}
public static ParametricRule<float> NumberEqualsWithTolerance => NumberEqualsWithToleranceImpl;
you'll see that it works like you'd expect it to, both from F# and C#.
let attrRule = DefaultRules.Create("fd","fdsa", DefaultRules.NumberEqualsWithTolerance, 89.0f) //compiles, yay!
Sometimes the type resolution has trouble when passing a method as a function parameter, because there can be overloads on the method that make the signature ambiguous. You can just wrap the function in a lambda that passes the parameters.
let attrRule =
DefaultRules.Create(
"fd",
"fdsa",
(fun a b tolerance -> DefaultRules.NumberEqualsWithTolerance(a, b, tolerance)),
89.0)
Why is an argument not required for the following statement?
viewModel.Submit.Execute()
Is the argument an implicit unit type?
The signature for ICommand.Execute is the following:
/// <summary>
/// Defines the method that should be executed when the command is executed.
/// </summary>
/// <param name="parameter">A parameter that may be used in executing the command. This parameter may be ignored by some implementations.</param>
void Execute(object parameter);
Note the last comment on the summary:
This parameter may be ignored by some implementations.
The following DelegateCommand implementation is as follows:
module UILogic.Interaction
open System
open System.Windows
open System.Windows.Input
open System.ComponentModel
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)
The client to this command is as follows:
// Setup
let viewModel = ViewModel()
viewModel.FirstName <- "Scott"
viewModel.LastName <- "Nimrod"
// Test
viewModel.Submit.Execute()
In conclusion, I just don't understand how an argument NOT marked optional can still be ignored in F#.
Any explanation on why this can occur?
() is a value in (of type unit) in F# too ... it will be translated into null
you can try this if you want:
> ();;
val it : unit = ()
> box ();;
val it : obj
= null
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 know virtually nothing about F#. I don’t even know the syntax, so I can’t give examples.
It was mentioned in a comment thread that F# can declare functions that can take parameters of multiple possible types, for example a string or an integer. This would be similar to method overloads in C#:
public void Method(string str) { /* ... */ }
public void Method(int integer) { /* ... */ }
However, in CIL you cannot declare a delegate of this form. Each delegate must have a single, specific list of parameter types. Since functions in F# are first-class citizens, however, it would seem that you should be able to pass such a function around, and the only way to compile that into CIL is to use delegates.
So how does F# compile this into CIL?
This question is a little ambiguous, so I'll just ramble about what's true of F#.
In F#, methods can be overloaded, just like C#. Methods are always accessed by a qualified name of the form someObj.MethodName or someType.MethodName. There must be context which can statically resolve the overload at compile-time, just as in C#. Examples:
type T() =
member this.M(x:int) = ()
member this.M(x:string) = ()
let t = new T()
// these are all ok, just like C#
t.M(3)
t.M("foo")
let f : int -> unit = t.M
let g : string-> unit = t.M
// this fails, just like C#
let h = t.M // A unique overload for method 'M' could not be determined
// based on type information prior to this program point.
In F#, let-bound function values cannot be overloaded. So:
let foo(x:int) = ()
let foo(x:string) = () // Duplicate definition of value 'foo'
This means you can never have an "unqualified" identifier foo that has overloaded meaning. Each such name has a single unambiguous type.
Finally, the crazy case which is probably the one that prompts the question. F# can define inline functions which have "static member constraints" which can be bound to e.g. "all types T that have a member property named Bar" or whatnot. This kind of genericity cannot be encoded into CIL. Which is why the functions that leverage this feature must be inline, so that at each call site, the code specific-to-the-type-used-at-that-callsite is generated inline.
let inline crazy(x) = x.Qux(3) // elided: type syntax to constrain x to
// require a Qux member that can take an int
// suppose unrelated types U and V have such a Qux method
let u = new U()
crazy(u) // is expanded here into "u.Qux(3)" and then compiled
let v = new V()
crazy(v) // is expanded here into "v.Qux(3)" and then compiled
So this stuff is all handled by the compiler, and by the time we need to generate code, once again, we've statically resolved which specific type we're using at this callsite. The "type" of crazy is not a type that can be expressed in CIL, the F# type system just checks each callsite to ensure the necessary conditions are met and inlines the code into that callsite, a lot like how C++ templates work.
(The main purpose/justification for the crazy stuff is for overloaded math operators. Without the inline feature, the + operator, for instance, being a let-bound function type, could either "only work on ints" or "only work on floats" or whatnot. Some ML flavors (F# is a relative of OCaml) do exactly that, where e.g. the + operator only works on ints, and a separate operator, usually named +., works on floats. Whereas in F#, + is an inline function defined in the F# library that works on any type with a + operator member or any of the primitive numeric types. Inlining can also have some potential run-time performance benefits, which is also appealing for some math-y/computational domains.)
When you're writing C# and you need a function that can take multiple different parameter sets, you just create method overloads:
string f(int x)
{
return "int " + x;
}
string f(string x)
{
return "string " + x;
}
void callF()
{
Console.WriteLine(f(12));
Console.WriteLine(f("12"));
}
// there's no way to write a function like this:
void call(Func<int|string, string> func)
{
Console.WriteLine(func(12));
Console.WriteLine(func("12"));
}
The callF function is trivial, but my made-up syntax for the call function doesn't work.
When you're writing F# and you need a function that can take multiple different parameter sets, you create a discriminated union that can contain all the different parameter sets and you make a single function that takes that union:
type Either = Int of int
| String of string
let f = function Int x -> "int " + string x
| String x -> "string " + x
let callF =
printfn "%s" (f (Int 12))
printfn "%s" (f (String "12"))
let call func =
printfn "%s" (func (Int 12))
printfn "%s" (func (String "12"))
Being a single function, f can be used like any other value, so in F# we can write callF and call f, and both do the same thing.
So how does F# implement the Either type I created above? Essentially like this:
public abstract class Either
{
public class Int : Test.Either
{
internal readonly int item;
internal Int(int item);
public int Item { get; }
}
public class String : Test.Either
{
internal readonly string item;
internal String(string item);
public string Item { get; }
}
}
The signature of the call function is:
public static void call(FSharpFunc<Either, string> f);
And f looks something like this:
public static string f(Either _arg1)
{
if (_arg1 is Either.Int)
return "int " + ((Either.Int)_arg1).Item;
return "string " + ((Either.String)_arg1).Item;
}
Of course you could implement the same Either type in C# (duh!), but it's not idiomatic, which is why it wasn't the obvious answer to the previous question.
Assuming I understand the question, in F# you can define expressions which depend on the availability of members with particular signatures. For instance
let inline f x a = (^t : (member Method : ^a -> unit)(x,a))
This defines a function f which takes a value x of type ^t and a value a of type ^a where ^t has a method Method taking an ^a to unit (void in C#), and which calls that method. Because this function is defined as inline, the definition is inlined at the point of use, which is the only reason that it can be given such a type. Thus, although you can pass f as a first class function, you can only do so when the types ^t and ^a are statically known so that the method call can be statically resolved and inserted in place (and this is why the type parameters have the funny ^ sigil instead of the normal ' sigil).
Here's an example of passing f as a first-class function:
type T() =
member x.Method(i) = printfn "Method called with int: %i" i
List.iter (f (new T())) [1; 2; 3]
This runs the method Method against the three values in the list. Because f is inlined, this is basically equivalent to
List.iter ((fun (x:T) a -> x.Method(a)) (new T())) [1; 2; 3]
EDIT
Given the context that seems to have led to this question (C# - How can I “overload” a delegate?), I appear not to have addressed your real question at all. Instead, what Gabe appears to be talking about is the ease with which one can define and use discriminated unions. So the question posed on that other thread might be answered like this using F#:
type FunctionType =
| NoArgument of (unit -> unit)
| ArrayArgument of (obj[] -> unit)
let doNothing (arr:obj[]) = ()
let doSomething () = printfn "'doSomething' was called"
let mutable someFunction = ArrayArgument doNothing
someFunction <- NoArgument doSomething
//now call someFunction, regardless of what type of argument it's supposed to take
match someFunction with
| NoArgument f -> f()
| ArrayArgument f -> f [| |] // pass in empty array
At a low level, there's no CIL magic going on here; it's just that NoArgument and ArrayArgument are subclasses of FunctionType which are easy to construct and to deconstruct via pattern matching. The branches of the pattern matching expression are morally equivalent to a type test followed by property accesses, but the compiler makes sure that the cases have 100% coverage and don't overlap. You could encode the exact same operations in C# without any problem, but it would be much more verbose and the compiler wouldn't help you out with exhaustiveness checking, etc.
Also, there is nothing here which is particular to functions; F# discriminated unions make it easy to define types which have a fixed number of named alternatives, each one of which can have data of whatever type you'd like.
I'm not quite sure that understand your question correctly... F# compiler uses FSharpFunc type to represent functions. Usually in F# code you don't deal with this type directly, using fancy syntactic representation instead, but if you expose any members that returns or accepts function and use them from another language, line C# - you will see it.
So instead of using delegates - F# utilizes its special type with concrete or generic parameters.
If your question was about things like add something-i-don't-know-what-exactly-but-it-has-addition-operator then you need to use inline keyword and compiler will emit function body in the call site. #kvb's answer was describing exactly this case.