F# lookup issues on recursive types - f#

I am trying to create two types where one is able to remove itself from the other such as in this example.
type employee (workplace : Job) =
member this.Fire () = workplace.Employees.Remove(this) |> ignore
and Job () =
let employees = new ResizeArray<employee>()
member this.Employees = employees
But this gets me the compile error of "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 not sure what I am doing wrong here. Any help would be appreciated

You can solve the problem even without reordering the declarations - when the F# compiler type-checks the Employee declaration, it doesn't yet know what is the type of workplace.Employees (because the type hasn't been declared yet), so it doesn't know where does the Remove method come from. You can correct that by adding type annotation that specifies that Employees is ResizeArray<Employee>:
type Employee (workplace : Job) =
member this.Fire () =
let emps : ResizeArray<Employee> = workplace.Employees
emps.Remove(this) |> ignore
and Job () =
let employees = new ResizeArray<Employee>()
member this.Employees = employees
However, this example isn't very functional - if you're going to use mutable state (such as ResizeArray), then the state should be hidden as private state of the type (so Jobs could have a Remove method).
In general, declaring recursive type declarations is a bit less comfortable in F# - however, you shouldn't need them that often. Quite frequently, you can use more generic types (i.e. Job may not need to know abou the Employee type).

Try this..
type Job () =
let employees = new ResizeArray<employee>()
member this.Employees = employees
and employee (workplace : Job) =
member this.Fire () = workplace.Employees.Remove(this) |> ignore

Related

How can I declare an object into another class on F#?

I am totally newbie in F# and I am trying to create an object that contains aonother instance from other class. For me in C# it's quite easy, but in F# I have serious problems to understand it.
Here is my code
module Owe
type Owe= class
val Name: string
val Amount: float
new (name, amount) as this =
{
Name = name
Amount = amount
}
end
And this is the class that I want to instace using the Owe object…. but, I can't invoque the Owe attribute.
module Roomate
type Roomate = class
val Name : string
val Owe: module.Owe //Error!
new(name, owe) as this =
{
Name = name
Owe = owe
}
end
I reallay apreciate any advice or solution.
Thanks in advance.
Emma.
You are probably getting an error because module.Owe is not a correct way of referencing a type from another module. You do not need the module keyword here - you just need to give the module name and the class name as in <module-name>.<class-name>. In your case, somewhat confusingly, both the module name and the class name are Owe so you can write Owe.Owe.
Another thing to keep in mind is that, in F#, the order of files matters. This means that you can only reference classes from files that appear earlier in your project.
Finally, I would also add that it is quite common to declare multiple types in one file (and in one module) in F#, so you do not need to move your definitions into separate files. I would also recommend using the lightweitght syntax for declaring classes using implicit constructors:
type Owe(name:string, amount:float) =
member x.Name = name
member x.Amount = amount
type Roomate(name:string, owe:Owe) =
member x.Name = name
member x.Owe = owe
Although in this case, you might not need classes at all and you could probably just use records:
type Owe = { Name:string; Amount:float }
type Roomate = { Name:string; Owe:Owe }

Entity Framework Core : Apply Conversion to all properties of type Option<T> in F#

I'm using Entity Framework Core 2.1 in F#.
I want to setup a generic type conversion for Option types so that EF knows how to handle them.
I found this very helpful post that shows how to setup a generic convertor for Option types.
So here's what I have so far (putting snippits together)
[<Table("Users", Schema = "pm")>]
type [<CLIMutable>] User = {
[<DatabaseGenerated(DatabaseGeneratedOption.Identity)>]
UserID : int64
FirstName : string
LastName : string
LastLoggedInTime : DateTimeOffset option
}
with
static member Configure (mb : ModelBuilder) =
mb.Entity<User>()
.Property(fun p -> p.LastLoggedInTime)
.HasConversion(OptionConverter<DateTimeOffset>()) |> ignore
This is great, except if I add a new property to my type. I will need to add the last three lines of the above code for that property as well. I'm looking for a way to do it for all properties all at once :D
So then I found this helpful SO answer that lead me to the below code
let tt = mb.Model.GetEntityTypes()
|> Seq.map(fun et -> et.GetProperties()) |> Seq.concat
|> Seq.filter(fun p -> p.ClrType = typeof<Option<DateTimeOffset>>)
|> Seq.map(fun p -> mb.Entity(p.DeclaringEntityType.ClrType)
.Property(p.Name)
.HasConversion(OptionConverter<DateTimeOffset>()))
Now the problem that I'm trying to solve is to figure out how to make this code work with Generic types so I don't have to specify the type. I want it to be something like p.ClrType = typeof<Option<T'>> and at the end, something like .HasConversion(OptionConverter<T'>())
Any thoughts?
so going off your initial question "I want to setup a generic type conversion for Option types so that EF knows how to handle them." I am assuming you mean you want EF to properly map an option to a "NULL" column of the corresponding base type, and that you are having problems because EF doesn't know how to handle FSharp.Option?
You could instead just use Nullable in place of Option for your EF types, and it will resolve as it does in C#.
So for instance, your example type becomes:
type [<CLIMutable>] User = {
[<DatabaseGenerated(DatabaseGeneratedOption.Identity)>]
UserID : int64
FirstName : string
LastName : string
LastLoggedInTime : Nullable<DateTimeOffset>
}
For types that can not be Nullable but are Options in your domain (strings for example), you just set them as their base type and it will work just fine.
If you are mapping to F# domain types somewhere else, you can just use Option.ToNullable and Option.OfNullable there, or map None to null if the type cannot be nullable as stated above.
Hope this helps!

Defining member as event handler in F#

This is probably asked several times but I just can't find an example.
My goal is to define an event handler for an event and the handler should be a member of the class. In other words I don't want to use function since I need to access instance variables and members
The latest variation I've tried:
namespace A
type ValueList<'TValueItem when 'TValueItem :> IValueItem>() =
inherit System.Collections.ObjectModel.ObservableCollection<'TValueItem>()
// This is causing error: The value or constructor 'ValueList_CollectionChanged' is not defined
let collectionChangedHandler = new System.Collections.Specialized.NotifyCollectionChangedEventHandler(ValueList_CollectionChanged)
// Constructor code
do base.CollectionChanged.AddHandler(collectionChangedHandler)
// Handles collection changed events for data items
member this.ValueList_CollectionChanged(sender : obj, e : System.Collections.Specialized.NotifyCollectionChangedEventArgs) =
// The code I want to run goes here
...
Or is this maybe a completely wrong approach?
Looks like you're looking for the self-identifier syntax:
type ValueList<'TValueItem when 'TValueItem :> IValueItem>() as this =
The as this (or any other identifier in place of this) allows to refer to the instance being constructed from the constructor.
You could then change your other lines to use the identifier:
let collectionChangedHandler = new System.Collections.Specialized.NotifyCollectionChangedEventHandler(this.ValueList_CollectionChanged)
do this.CollectionChanged.AddHandler(collectionChangedHandler)
For this to be valid as-is, the ValueList_CollectionChanged method also needs to be in curried form:
member this.ValueList_CollectionChanged (sender : obj) (e : System.Collections.Specialized.NotifyCollectionChangedEventArgs) =
As an alternative to using curried arguments, you can use a lambda to transform the arguments where the handler is instantiated, e.g. .NotifyCollectionChangedEventHandler(fun sender e -> this.(...).

F# mutable with no initial value for MEF

I have a bunch of modules that export an IModule interface. So in the main program I have no problems
...
let mutable modules = Seq.empty
[<ImportMany>]
member x.Modules
with get():IEnumerable<Lazy<IModule, IModuleData>> = modules
and set(a) = modules <- a
...
But now I need to expose an interface back to those modules. So each module will import a single interface
...
let mutable parent:IParent = ?
[<Import>]
member x.Parent
with get():IParent = parent
and set(a) = parent <- a
...
So my problem is how do I go about creating my mutable "parent" when I have no initial value for it? Also, is this the appropriate way to expose an API back to component parts?
Using Unchecked.defaultof<_> should do the trick, but it means that you're circumventing the F# type system, which may be a dangerous thing to do - the system tries to prevent you from accidentally dereferencing null values (and getting NullReferenceException).
Types that are declared in F# don't have null as a proper value, which is an attempt to eliminate the usual errors caused by null. The clean F# approach is to use option types to represent the fact that a value is missing:
let mutable parent:option<IParent> = None
[<Import>]
member x.Parent
with get():IParent =
match parent with
| Some p -> p
| None -> failwith "TODO: Throw some reasonable exception here!"
and set(a) = parent <- Some(a)
If you just want to say that IParent can have a null value (perhaps because you need to use it in some C# code that will ignore the F# restriction anyway), then you can mark the type definition using a special attribute that allows using null with the type.
[<AllowNullLiteral>]
type IParent =
abstract DoStuff : unit -> unit
Then you can write let mutable parent:IParent = null. The benefit of this approach is that you can also easily check whether a value is null (using just if parent <> null then ...) which is not that obvious when you use Unchecked.defaultof<_>.
let mutable parent = Unchecked.defaultof<IParent>
should do the trick.
Following up on what Tomas explained, you should probably put your imports directly into your constructor. That will allow your code to be a bit more idiomatic.

why is the implementation of my abstract member not public

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#).

Resources