In F# how can I produce an expression with a type

问题:

I'm working with an api that requires a value of type Func. (Specifically, I'm trying to call ModelMetadataProviders.Current.GetMetadataForType().

How can I construct that value in F#?

回答1:

When calling a method that takes any delegate of the Func you shouldn't need to explicitly create the delegate, because F# implicitly converts lambda expressions to delegate type (in member calls). I think that just calling the method with lambda function should work (if it doesn't, could you share the error message?)

Here is a simple example that demonstrates this:

type Foo() = 
  member x.Bar(a:System.Func<obj>) = a.Invoke()

let f = Foo()
let rnd = f.Bar(fun () -> new Random() :> obj)

In your case, I suppose something like this should work:

m.GetMetadataForType((fun () -> <expression> :> obj), modelType)

Note that you need explicit upcast (expr :> obj), to make sure the lambda function returns the right type (obj). If you want to assign the lambda function to a local value using let, then it won't work, because implicit conversion works only when it is passed as an argument directly. However, in that case, it makes the code a bit nicer.

回答2:

You can normally pass in any () -> obj and it will be automatically converted to Func<obj>. You may need to wrap your fun with Func<obj>:

> let d : Func<obj> = Func<obj>(fun () -> box "hello");;

val d : Func<obj>

回答3:

let f = new System.Func<obj>(fun() -> printfn "ok"; new obj())