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Type constraints
f#

I am playing around with type constraints. I am having some difficulty with the syntax of type constraints in some places. Here is an example

open System
open System.Collections.Generic

// Here is a simple string collection
type string_collection = class
val coll : List<string>
member x.get_Item(nm:string) = x.coll.Find(fun y -> y = nm)
end

Now I am trying to extend this to generic types

// Define the INamed interface
type INamed = interface
abstract Name : string with get
end

// All objects in the collection should implement the INamed Interface
type 'a typed_collection = class
val coll : List<'a>
member x.get_Item(nm:string) = x.coll.Find(fun (y :> INamed) -> y.Name = nm)
end

Question: How do I enforce the 'a :> INamed constraint.

Many thanks.

Chris

.

Here's what you need (F# 1.1.11.7). 

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type INamed = interface
    abstract Name : string with get
end

// All objects in the collection should implement the INamed Interface
type 'a typed_collection when 'a :> INamed = class
    val coll : List<'a>
    member x.get_Item(nm:string) = x.coll.Find(fun (y :> INamed) -> y.Name = nm) 
end

However, there are a number of minor glitches with type constraints on class type variables in the current release (we only added support for this in the last release - see the Change ): we're working on fixing them. If you hit them, you can workaround them by using an unconstrained type variable and using "box" instead. 

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type INamed = interface
    abstract Name : string with get
end

// All objects in the collection should implement the INamed Interface
type 'a typed_collection = class
    val coll : List<'a>
   member x.get_Item(nm:string) = x.coll.Find(fun (box y :?> INamed) -> y.Name = nm) 

end
By on 6/23/2006 10:51 AM ()

Din't know what went wrong there... Here's the code:

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open Microsoft.FSharp.Idioms

open System
open System.Collections.Generic

type INamed = interface
    abstract Name : string with get
end

// All objects in the collection should implement the INamed Interface
type 'a typed_collection = class
    val coll : List<'a>
   member x.get_Item(nm:string) = x.coll.Find(fun y -> (box y :?> INamed).Name = nm)  

end
By on 6/28/2006 2:01 PM ()

Hi Don,

I am still getting a syntax error when I copied your code. I am using F# 1.1.11.7. Here is the code that I am looking at.

open Microsoft.FSharp.Idioms
type INamed = interface
abstract Name : string with get
end

// All objects in the collection should implement the INamed Interface
type 'a typed_collection = class
val coll : List<'a>
member x.get_Item(nm:string) = x.coll.Find(fun (box y :?>(syntax error) INamed) -> y.Name = nm)

endIs there a typo somewhere?ThanksChris

By on 6/26/2006 10:53 PM ()

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