I need to implement a static extension method supporting member constraints on some basic primitive types like integers, floats, etc. Here's my code for signed integers:
module MyOperators =
let inline foo (x : ^T) = (^T : (static member Foo : ^T -> int) (x))
type System.Int32 with
static member Foo(x : Int32) = 7 // static extension
Test code:
open MyOperators
let x = foo 5 // x should be 7
But compiler complains with error:
The type 'System.Int32' does not
support any operators named 'Foo'
What am I missing here? Thanks!
Static member constraints in F# never find 'extension methods', they can only see intrinsic methods on types (and a few special cases called out in the F# language spec).
Perhaps you can use method overloading instead? What is your ultimate goal?
F#'s static type constraints don't work with extension methods. Extension methods cannot statically be checked at compile time, and even so, you can have multiple definitions for Int32::Foo (depending on which namespace you imported).
Unfortunately, to solve your problem you might have to resort to using reflection.
Related
F# extension methods can be defined on types with the same name and type signature as existing instance and static methods to effectively override the default implementation of these methods, however I can't get this to work on static properties.
In particular, I'm trying to create an extension method for DateTime that returns a more precise time as follows:
#nowarn "51"
open System
module DateTimeExtensions =
open System.Runtime.InteropServices
[<DllImport("Kernel32.dll", CallingConvention = CallingConvention.Winapi)>]
extern void private GetSystemTimePreciseAsFileTime(int64*)
type System.DateTime with
// example showing that static methods can be overridden
static member IsLeapYear(_: DateTime) =
printfn "Using overridden IsLeapYear!"
true
// more accurate UtcNow method (note: not supported by older OS versions)
static member UtcNow =
printfn "Using overridden UtcNow!"
let mutable fileTime = 0L
GetSystemTimePreciseAsFileTime(&&fileTime)
DateTime.FromFileTimeUtc(fileTime)
However, the output when executing
open DateTimeExtensions
let _ = DateTime.IsLeapYear(DateTime.UtcNow)
is just
Using overridden IsLeapYear!
which shows that the static method 'override' is working, but not the static property.
(Note: I'm using F# 4.0)
This statement seems to be incorrect:
F# extension methods can be defined on types with the same name and
type signature as existing instance and static methods to effectively
override the default implementation of these methods, however I can't
get this to work on static properties.
No, they don't override.
You might be confused because in fact your signature of IsLeapYear is wrong, it should take an integer, that's why it works I mean you are not overriding anything, just adding a new (extension) method.
If you try it with the original signature you'll see that it doesn't work either:
type System.DateTime with
// example showing that static methods can NOT be overridden
static member IsLeapYear(_: int) =
printfn "Using overridden IsLeapYear!"
false
> DateTime.IsLeapYear(2000);;
val it : bool = true
Which is consistent with the behavior of the static property extension.
Anyway I'm not sure why it was decided not to override, if there was such decision at all when designing the language. I think it would be an interesting feature and if there is a good reason not to implement it at least it should emit a warning saying that since the method already exists it will never be called.
Maybe I will open an issue or a suggestion for the F# compiler.
I get an error on the definition of ToBar
type Foo = {foo: string}
type Bar = {bar: string}
[<AbstractClass>]
type AbstractType< ^T> (fn: ^T -> Foo) =
member inline this.ToFoo (x: ^T) = fn x
abstract ToBar: string -> Bar
This is the error message
This code is not sufficiently generic.
The type variable ^T could not be generalized
because it would escape its scope.
Apart from the fact that (even after reading all other SO questions on this) I don't get what this error tries to tell me ... but it is totally stunning that ToBar which does not even use that type parameter is getting an error
This happens because ToBar is not inline, which is a requisite for using statically resolved type constraints. But it seems that you don't really need them, simple generics would be enough. So just replace ^T with 'T, and it will work fine:
[<AbstractClass>]
type AbstractType<'T> (fn: 'T -> Foo) =
member inline this.ToFoo (x: 'T) = fn x
abstract ToBar: string -> Bar
If you think about it a bit more, it only makes sense: an abstract member can't really make use of SRTC, because it's dispatched at runtime, and SRTC-types need to be known at compile time.
On a related note, even if you get rid of ToBar in an effort to keep SRTC, you'll hit the next error:
error FS1113: The value 'ToFoo' was marked inline but its implementation makes use of an internal or private function which is not sufficiently accessible
This can be fixed by making the type itself private:
type private AbstractType< ^T> (fn: ^T -> Foo) =
member inline this.ToFoo (x: ^T) = fn x
This will work, because the type won't be accessible from outside assemblies, and therefore won't need to expose its SRTC parameters.
I am wondering why F# compiler allows the following
type MyMath() =
member this.Add a b = a + b
What would be the type of Add method and its arguments ? If I compiled this into the Library and try to use it in C# what types of arguments it is going to expect ?
Shouldn't the F# require you to explicitly specify types when it comes to Methods of classes ?
You can enter the code in F# interactive and see the inferred type yourself:
> type MyMath() =
member this.Add a b = a + b;;
type MyMath =
class
new : unit -> MyMath
member Add : a:int -> b:int -> int
end
Here, the compiler uses default type for the + operator which is int. The operator can be used with other types, but the inference uses int as the default. You can use type annotations, but you are not required to do that if you are happy with the inferred type.
In general, you can use type annotations in F# to specify types if you want to, but in many cases, the inferred type will be exactly what you want, so you do not have to make the code more verbose, if the inference behaves as expected.
Of course, if you were writing some library and wanted to be super careful about changing the API, then you might want to use type annotations (or you can add F# Interface file .fsi)
F# Interactive is your friend:
type MyMath =
class
new : unit -> MyMath
member Add : a:int -> b:int -> int
end
I want to implement IEnumerable<KeyValuePair<DateTime, 'T>> in my own class and add math operators to that class so that the operators could work like inline function on any numeric types of 'T - automatically add constraints.
I just cannot make the following piece of code work. It doesn't work neither with nor without 'inline' keyword at the member declaration.
Also, if I define a function
let inline add l r = l + r
before the type and use it instead of addition l.Value + r.Value, it also doesn't work.
Could someone please show me what I am doing wrong?
Probably the whole approach is wrong and there is a way to achieve the same goal the other way?
namespace Test
open System
open System.Linq
open System.Collections.Generic
[<SerializableAttribute>]
type TimeSeries<'T>(dictionary : IDictionary<DateTime, 'T>) =
let internalList = new SortedList<DateTime, 'T>(dictionary)
interface IEnumerable<KeyValuePair<DateTime, 'T>> with
member this.GetEnumerator() = internalList.GetEnumerator()
member this.GetEnumerator() : Collections.IEnumerator
= internalList.GetEnumerator() :> Collections.IEnumerator
member private this.sl = internalList
static member inline (+) (left : TimeSeries<'T>, right : TimeSeries<'T>) =
let res =
query {
for l in left do
join r in right on
(l.Key = r.Key)
select (l.Key, l.Value + r.Value)
}
new TimeSeries<'T>(res |> dict)
Your approach seems correct to me.
The reason why your code doesn't compile is because F# type inference is inferring a static constraint (compile-time) for the type variable 'T which is the same used for the type definition.
A generic parameter of a type definition can't be statically resolved (no "hat" types) but nothing stops you from defining a function or a member which uses these compile-time constraints.
Just change your type variable 'T to 'U in the static member (+) definition and it will be fine.
Still you'll be allowed to create a TimeSeries instance of a type which does not support (+) (ie: TimeSeries<obj>) but you will not be able to use (+) for those instances, anyway if you do it you'll get a nice error message at compile-time.
I've been struggling to get this to compile for about an hour. It must be something stupid. Can you spot it?
in my lib project:
namespace TravelerStuff
open System
type Traveler =
abstract GetData : unit -> unit
type public DeltaTraveler() =
interface Traveler with
member v.GetData () =
printf "hello"
and in my console test app:
[<EntryPoint>] let main _ =
let traveler = new TravelerStuff.DeltaTraveler()
traveler.GetData // this line won't compile: (The field, constructor or member 'GetData' is not defined)
As gradbot says, F# doesn't currently implicitly convert values to interfaces when searching for members. Also, F# only uses explicit interface implementation (as known from C#) and not implicit implementation where members are not only compiled as implementation of an interface, but also as ordinary (directly visible) members of the type.
Aside from casting, you can duplicate the member in the type definition:
type DeltaTraveler() =
member v.GetData () = printf "hello"
interface Traveler with
member v.GetData () = v.GetData()
Also, if you just want to implement an interface, but don't need to add any members, you can use F# object expressions (which are more lightweight):
let deltaTraveler() =
{ new Traveler with
member v.GetData () = printf "hello" }
// The function directly returns value of type 'Traveler'
let t = deltaTraveler()
t.GetData()
You need to upcast. F# currently won't do it for you in this situation.
(traveler :> TravelerStuff.Traveler).GetData()
// open the namespace to reduce typing.
open TravelerStuff
(traveler :> Traveler).GetData()
Snip from F# docs.
In many object-oriented languages,
upcasting is implicit; in F#, the
rules are slightly different.
Upcasting is applied automatically
when you pass arguments to methods on
an object type. However, for let-bound
functions in a module, upcasting is
not automatic, unless the parameter
type is declared as a flexible type.
For more information, see Flexible Types (F#).