Is there an F# cast function with type inference like Seq.cast?

2
votes

So I have some COM-types with hard-to-remember, long, unwieldy names, so I'd rather not have to type them out when casting from object if I can avoid it. With Seq.cast it'll infer the required type and cast as necessary.

Here's a simplified version with int instead:

> let o = 1 :> obj;;
val o : obj = 1

> let inc x = x+1;;
val inc : int -> int

> inc o;;

  inc o;;
  ----^

stdin(15,5): error FS0001: This expression was expected to have type
    int    
but here has type
    obj

Okay, makes sense. So we cast it:

> inc (o :?> int);;
val it : int = 2

However, if I cast it with Seq.cast I wouldn't need to explicitly write the type:

> inc ([o] |> Seq.cast |> Seq.head);;
val it : int = 2

Is there a function that works like cast below?

> inc (o |> cast);;
val it : int = 2

Is there an F# cast function with type inference like Seq.cast?

2
castingf#type-inference
You perhaps expect way too much of Seq.cast: something as simple as inc ([2.0] |> Seq.cast |> Seq.head) will throw InvalidCastException. - Gene Belitski

2 Answers

6
votes

You can use the 'unbox' and 'box' operators to take advantage of type inference

inc (unbox o)
4
votes

As Leaf mentioned, box and unbox work for conversions to/from obj. For other types you can use the upcast or static cast operators (:>) for upcasting and the downcast or dynamic cast (:?>) operators for downcasting. A wildcard can be used in place of an explicit type, for example: x :?> _.