In F#, I have defined an interface with a generic member:
type MyType =
abstract MyMember<'a> : 'a -> int
I would like to write a function that can create an object that implements this interface:
module MyType =
let create f =
{
new MyType with
member __.MyMember(x) = f x
}
This currently fails to compile with the following error message:
This code is not sufficiently generic. The type variable 'a could not be generalized because it would escape its scope.
Is it possible to make this work via inlining and static resolved type parameters (or something similar)? Thanks.
An explanation for the compile error:
Object expressions construct anonymous classes - these are just regular classes, with a compiler generated name. You're essentially creating a closure which captures f.
This is how you'd normally implement it:
type MyType =
abstract MyMember<'a> : 'a -> int
type MyNewType (f) =
interface MyType with
member _.MyMember x = f x
The type's constructor would have a parameter f of type 'a -> int, where 'a would normally be decided by the first usage of MyNewType.
But there's no way to generalize f such that it could be captured as a value that would satisfy the generic constraint of MyMember. Here sufficiently generic means a type of one order above the generic type.
Related
This type definition works:
type Model<'t,'u when 't :> seq<'u> >(query: unit -> 't) = class end
However, as for me the 'u is redundant here, but next definition:
type Model<'t when 't :> seq<_> >(query: unit -> 't) = class end
produces the error:
Anonymous type variables are not permitted in this declaration - F# Compiler (715)
Compact
The most compact form:
type Model<'t>(query:unit -> #seq<'t>) = class end
during instance creation:
Query.users |> Model
produces error:
Type constraint mismatch. The type unit -> (string * int) list
is not compatible with type unit -> 'a
F# Compiler (193)
Probably, because of described here Why are flexible types not allowed in record type definitions?. But anyway the error description is unclear for me, what is wrong in substituting (string * int) list instead of 'a?
Background
The real type Model is a wrapper for a database query, it implements INotifyPropertyChanged and contains mutable state of type Outcome:
type 't Outcome =
| Empty
| Loading
| Success of 't
| Fault of string * string
The #seq<'t> type constraint is needed to detect Empty case with Seq.isEmpty in generic way due to a query can return seq or list or array
As explained by #kvb, the problem is that you cannot have generic constructors - so you can either introduce a new type parameter to the entire class (as you did in your first example), or the return type of query will need to be just seq<'t> (as the compiler forces you to do in your other examples).
If you want to keep things encapsulated in a class, one nice trick is to make the constructor private and add a static Create method which can have the extra generic parameter that you need:
type Model<'t> private(query:unit -> seq<'t>) =
static member Create(query:unit -> #seq<'t>) =
Model(fun () -> query () :> _)
The problem with the definition
type Model<'t>(query:unit -> #seq<'t>) = class end
is that the # introduces an implicit type parameter, but constructors can't have their own type parameters in addition to the class's. For instance, you also can't define something like this:
type IntConverter(conv:'a -> int) = class end
because the constructor can't have its own free type parameter 'a.
However, the good news is that even if you change your definition to
type Model<'t>(query:unit -> seq<'t>) = class end
it's easy to accept an input of type unit -> #seq<'t>:
let model f = Model(fun () -> upcast f())
I guess as long as you are willing to add that helper function you can get somewhat close:
type Model<'t>(query:unit -> seq<'t>) =
class
static member Unwrap<'s when 's :> seq<'t>> (cpar: unit -> 's) =
cpar >> (fun (s: 's) -> (s :> seq<'t>))
static member New (cpar) =
new Model<'t>(Model.Unwrap<_> cpar)
end
At least that will allow you to use something like (I think)
Query.users |> Model.New
As soon as I do something similar with an actual constructor (overload) the compiler says "The type variable 's has been constrained to be type 'seq<'t>'.". So apparently different rules apply to constructors than to static methods (why do I sound surprised).
I'm wish to use F#'s type inference to infer the types of my code simply by the methods/functions that are called.
This CAN be achieved quite nicely by using extension methods of the form...
[<Extension>]
type AttributeExtensions =
[<Extension>]
static member ``BroadcastOffset``<'a, 'b when 'a :> ``BroadcastOffset``<'b>> (this: 'a) =
this.``
when I have code that reads
x.BroadcastOffset()
type inference will kick in and infers that x, must indeed be of type BroadcastOffset<'a>.
A small fly in the ointment though is F#'s own types for example, if I write
x.Value
F# will not infer its an Option<_>, thats understandable, but I can use the same extension trick to get there?
[<Extension>]
type AttributeExtensions =
[<Extension>]
static member Value<'a,'b when 'a :> Option<'b>> (this: 'a) =
this.Value
and that SHOULD in theory mean
x.Value()
should trigger the inference of x being an Option<_>
sadly this ISNT the case, because F# rejects the extension method as an invalid constraint because Option<_> is sealed and thus 'a can only have 1 solution.
True 'a CAN only have 1 solution but, in my book that does NOT make the constraint invalid, it is perfectly valid, just trivial to infer, and because of this overzelous rejection, I am frustrated in making "Value" trigger type inference.
any ideas how to get around it? (apart from changing the F# compiler myself).
(I can of course define a function 'Value' instead, but my intention is to use F# to infer types from "methods", and extension methods fit the requirement where simple functions don't...I have my reasons)
I got compile errors using your code and then factored out what I think you are after into a function, then used the function in the extension. This allowed me to call .Value() and it work out that it was of type 'a option which is what I think you were after?
open System.Runtime.CompilerServices
let v (x : 'a option) = x.Value
[<Extension>]
type AttributeExtensions =
[<Extension>]
static member Value(this) = v(this)
let myFunc o = o.Value()
Hi I just started programming in F# and am stuck on a type issue.
I have this function:
member private this.UpdateStats<'T when 'T :> StatisticsBase>(passed: bool, stats: 'T) =
//more stuff..
stats
I'm calling it like this:
this.UpdateStats<GroupStats>(true, GroupStats(Id = Guid.NewGuid()))
The compiler says:
The parameter has been used in a way that constrains it to always be GroupStats.
GroupStats inherit from StatisticsBase. What do I have to do to make the function usable for all entities that inherit from StatisticsBase?
types:
[<AllowNullLiteral>]
type StatisticsBase() =
member val Id = String.Empty with get,set
[<AllowNullLiteral>]
type GroupStats() =
inherit Stats()
StatisticsBase actually inherits from a C# type and is used in a repository, but I can reproduce the error with the code above
After much back and forth, we have been able to ascertain that your actual non-working code is like this (tip for the future: provide more information):
type SomeType() =
member this.M2() =
this.M<GroupStats>(true, GroupStats())
member private this.M<'T when 'T :> Stats>(x: bool, t: 'T) =
t
This code will indeed produce the described error. This is because F# type inference works top-to-bottom, left-to-right. But there is an exception: classes (and other mutually recusrive definition groups) get two passes of type inference - first signatures, then bodies.
When the compiler first comes across the body of M2, it determines that method M must return GroupStats. Later, when the compiler comes across the body of M, it sees that M's return value is the same as parameter t, which means that M must return 'T. But since the compiler already knows, from examining the body of M2, that M must return GroupStats, it follows that 'T must be GroupStats.
This doesn't happen if M is defined before M2: in this case, type inference will first encounter the body of M and correctly determine its return type to be 'T, which will then match up with the body of M2, and the will be no problem.
From the above, two solutions can be formulated: first, you could define M before M2. Second, you could just explicitly specify the return type of M:
member private this.M<'T when 'T :> Stats>(x: bool, t: 'T): 'T =
t
This way, its return type will become known after the first pass, and the problem disappears.
According to the 3rd example on this site http://msdn.microsoft.com/en-us/library/dd233224.aspx, F# lists can contain objects of different types as long as both types derive from the same super-type. However, I can't get the cons (::) operator to add subtypes to a list of the supertype
module test
type A() =
member this.x = 2
type B() =
inherit A()
member this.y = 4
let mutable myList : A list = []
myList <- [B()] // Valid
myList <- B()::myList // Invalid: This expression was expected to have type A but here has type B
Is there a reason I can't use :: to append to the list?
F# does not always insert upcasts (conversion to a base type) automatically, so you have to insert an explicit cast that turns the B value into a value of type A.
Note that F# distinguishes between upcasts - casts to a base class (which are always correct) and downcasts - casts to a derived class (which may fail).
You can either use the upcast keyword or you can use the expr :> Type notation. In both cases, the compiler can fill in the required target type, so you can just write:
myList <- (upcast B())::myList
myList <- (B() :> _)::myList
I am trying to write a typed abstract syntax tree datatype that can represent
function application.
So far I have
type Expr<'a> =
| Constant of 'a
| Application of Expr<'b -> 'a> * Expr<'b> // error: The type parameter 'b' is not defined
I don't think there is a way in F# to write something like 'for all b' on that last line - am I approaching this problem wrongly?
In general, the F# type system is not expressive enough to (directly) define a typed abstract syntax tree as the one in your example. This can be done using generalized algebraic data types (GADTs) which are not supported in F# (although they are available in Haskell and OCaml). It would be nice to have this in F#, but I think it makes the language a bit more complex.
Technically speaking, the compiler is complaining because the type variable 'b is not defined. But of course, if you define it, then you get type Expr<'a, 'b> which has a different meaning.
If you wanted to express this in F#, you'd have to use a workaround based on interfaces (an interface can have generic method, which give you a way to express constraint like exists 'b which you need here). This will probably get very ugly very soon, so I do not think it is a good approach, but it would look something like this:
// Represents an application that returns 'a but consists
// of an argument 'b and a function 'b -> 'a
type IApplication<'a> =
abstract Appl<'b> : Expr<'b -> 'a> * Expr<'b> -> unit
and Expr<'a> =
// Constant just stores a value...
| Constant of 'a
// An application is something that we can call with an
// implementation (handler). The function then calls the
// 'Appl' method of the handler we provide. As this method
// is generic, it will be called with an appropriate type
// argument 'b that represents the type of the argument.
| Application of (IApplication<'a> -> unit)
To represent an expression tree of (fun (n:int) -> string n) 42, you could write something like:
let expr =
Application(fun appl ->
appl.Appl(Constant(fun (n:int) -> string n),
Constant(42)))
A function to evaluate the expression can be written like this:
let rec eval<'T> : Expr<'T> -> 'T = function
| Constant(v) -> v // Just return the constant
| Application(f) ->
// We use a bit of dirty mutable state (to keep types simpler for now)
let res = ref None
// Call the function with a 'handler' that evaluates function application
f { new IApplication<'T> with
member x.Appl<'A>(efunc : Expr<'A -> 'T>, earg : Expr<'A>) =
// Here we get function 'efunc' and argument 'earg'
// The type 'A is the type of the argument (which can be
// anything, depending on the created AST)
let f = eval<'A -> 'T> efunc
let a = eval<'A> earg
res := Some <| (f a) }
res.Value.Value
As I said, this is a bit really extreme workaround, so I do not think it is a good idea to actually use it. I suppose the F# way of doing this would be to use untyped Expr type. Can you write a bit more about the overall goal of your project (perhaps there is another good approach)?