I'm working with a library created in C#. I've been working on porting some code to F# but must use quite a few underlying types from the C# lib.
One piece of code needs to calculate a list of values and assign it to a public field/property in the class. The field is a C# class that contains two ICollection.
My F# code works fine and needs to return an F# Seq/List.
I tried the following code snippets which each produce errors.
Return type of F# member is a type called recoveryList with type Recoveries list
Public field in class that is a class itself containing two ICollection objects
this.field.Collection1 = recoveries
This gives the error Expected to have type ICollection but has type Recoveries list
this.field.Collection1 = new ResizeArray<Recoveries>()
Gives the error expected type ICollection but is ResizeArray
this.field.Collection1 = new System.Collections.Generic.List<Recoveries>()
Same error as above - expected ICollection but type is List
Any ideas? These operations seem valid from a C# point of view and List/ResizeArray implements ICollection so... I am confused how to assign the value.
I could change the type of the underlying C# library, but this might have other implications.
Thanks
F# doesn't do implicit casting like C#. So even though System.Collections.Generic.List<'T> implements the ICollection interface, you can't directly set some ICollection-typed property to an instance of System.Collections.Generic.List<'T>.
The fix is easy though -- all you need to do is add an explicit upcast to ICollection to your ResizeArray<'T> or System.Collections.Generic.List<'T> before assigning it:
// Make sure to add an 'open' declaration for System.Collections.Generic
this.field.Collection1 = (recoveries :> ICollection)
or
this.field.Collection1 = (ResizeArray<Recoveries>() :> ICollection)
Related
I have a seq of types
let rackStatusesPrototype : RackStatus seq
and now I want to assign it to a property which is from c# and is an ICollection type, but I am having a problem with it. I tried:
let test : ICollection<RackStatus> = rackStatusesPrototype :> ICollection<RackStatus>
But it says incompatible types. What can i do?
The simplest thing you can do is to open System.Linq and invoke ToList() on your rackStatusesPrototype to have it converted to C# list, which implement ICollection.
Also F# Array implements ICollection, so rackStatusesPrototype |> Array.ofSeq should work as well. This may be preferred way, if you are having no other interactions with C# world.
Note, that F# list/set/map currently doesn't implement ICollection interface, but adding it is planned.
I just started to study F# and accidentally wrote this binding
let List = 1
Now when I try to obtain List methods such as 'filter' I get this error
error FS0039: The field, constructor or member 'filter' is not defined.
Of course using method with full type name like Microsoft.FSharp.Collections.List.filter is still working.
I'm wondering why it is possible to use type name as identifier in F# and how I can set back name List to type List from Microsoft.FSharp.Collections.
When I tried to reassign like this
type List = Microsoft.FSharp.Collections.List<'T>
I get
Error FS0039: The type parameter 'T is not defined.
Thank you!
In F# you can redefine almost everything and shadow existing definitions. This applies to both types (well actually types have a different behavior regarding shadowing, they shadow their values as you open the namespaces) and values but not interchangeably since values and type (and also modules) can somehow coexist at the same time in the scope. The compiler will do his best to find out which one is.
You are not forced to, but it's a common good practice in F# not to use let bindings in uppercase.
Regarding your second question, you are using a type parameter in the right side which doesn't exist in the left side of the assignment, it should be:
type List<'T> = Microsoft.FSharp.Collections.List<'T>
But notice that filter doesn't belong to the type. It's rather defined in the List module.
You should just rename your let binding from List to something sensible - as Gustavo mentioned, your definition is shadowing the core List module from F# and there is no way to use List to refer both to your integer and to the module. Shadowing core functions will make your code pretty confusing. It's also a good idea to use camelCase for let bindings, but that's a matter of taste.
If you insist on shadowing List, then you won't be able to call List.filter using List.filter. If you wanted something shorter, you could define module alias:
module FsList = Microsoft.FSharp.Collections.List
Note that your attempt to do something similar with List<'T> does not do the same thing, because functions such as filter are in a module named List rather than being static members of the type. With this, you can call filter using FsList.filter.
I'd like to use a record with a data structure that requires its types to have a default constructor. Unfortunately this results in a compile time error.
Aside from changing the data structure (not possible here). Or changing the record to a type (would require adding in type annotations ever where) what options do you have in this type of scenario?
I suppose one could wrap the record inside a struct, but that feels like hack.
Try adding the CLIMutable attribute to your record.
This appears to have been fixed in F# 3.1. Consider this code:
[<CLIMutable>]
type R = {a : int}
let f<'a when 'a : (new : unit -> 'a)>() = new 'a()
let r = f<R>()
In F# 3.0 (VS 2012), you get an error on the last line - error FS0001: A generic construct requires that the type 'R' have a public default constructor
But it works in F# 3.1 (not RTM yet, but public pre-release was earlier this week)
I'm trying to see if inline can be applied to an implemented method so that the specific type coming in doesn't have to be spelled out. I've done this with one off (Not inherited/implemented) methods, but trying to also do using an interface.
type public IBookInteraction =
abstract inline CreateBook : 'a -> MethodResult<BasicBookModel>
type public BookInteraction(?userInteraction) =
interface IBookInteraction with
member inline x.CreateBook(bookModel) =
let userId = (^a : (member UserId : Int32 with get) (bookModel))
MethodResult<BasicBookModel>()
I'm guessing there's a way to do this, but it doesn't work with a generic operator(?) in the interface method signature.
I don't believe it's possible to have abstract inline methods. Even if you could, your code wouldn't work, because your interface definition promises that users can call it with any 'a, but your implementation places a static member constraint on 'a - in a hypothetical world where F# supported abstract inline methods, the declaration of the method on the interface would also need to include the constraint.
In any case, to see why it's not possible for F# to support abstract inline methods, consider what inline means: the code that you write to implement the method will be essentially copied and pasted into the call site. However, with an abstract method, you don't know the concrete type that is defining the implementation of the method, so there's no way to figure out at compile time what code you're supposed to be inlining!
I think the correct answer is interface implementations may not be inlined. I'm not sure why it's allowed in the interface definition.
I know I must be missing something really obvious here. B.GetInstance().Call() generates the error: Lookup on object of indeterminate type based on information prior to this program point. A type annotation may be needed prior to this program point to constrain the type of the object. This may allow the lookup to be resolved.
I'm using v1.9.9.9.
type A() =
member x.Call() = B.GetInstance().Call()
and B() =
static member GetInstance() = new B()
member x.Call() = ()
I just discovered that this works: (B.GetInstance() :> B).Call()
Any idea why the cast is necessary?
Frequently when you've got a recursive set of methods whose types to infer, F# needs help. A more pleasant alternative would be to annotate the definition of B.GetInstance:
type A() =
member x.Call() = B.GetInstance().Call()
and B() =
static member GetInstance() : B = new B()
member x.Call() = ()
I believe that the reason you run into this problem is that F# tries to solve all inferred types on all methods in A and B simultaneously (because they are defined as mutually recursive types), and this leads to problems, but perhaps someone from the F# team will weigh in.
The quick summary is that in a recursive group (e.g. members in one type, or members of recursive types like we have here) F# reads the declarations in left-to-right top-to-bottom order, followed by the definitions in left-to-right top-to-bottom order. So in this instance when it reaches the definition of A.Call, it has not yet read the definition of B.GetInstance and therefore does not (yet!) know that the return type of GetInstance will be B.
Keith's answer nails it for this situation, you can provide a type annotation to specify the return type of GetInstance in its declaration.
See
Forcing F# type inference on generics and interfaces to stay loose
for a deep discussion of what's going on here.
Note also that in your original attempt, you don't need to "cast" (the potentially dynamic operation, using :>), instead you can just "annotate" (statically declare a type, using :) to get it to compile. But makes more sense to put the type annotation in the method declaration for GetInstance (generally, prefer addition annotations to method signatures instead of arbitrary places inside bodies).