Question

Why doesn’t the following code compile? Is that a compiler bug or a language feature? What is the best workaround?

type A() as this =
    let a = this.GetTypedObject<int>()    // a
    let b = this.GetTypedObject<string>() // b

    member this.GetTypedObject<'T>() =
        Unchecked.defaultof<'T>

Unchecked.defaultof<‘T> is used for example only, any function or constructor call could be used instead.

Compiler says that code becomes less generic at line (a) and refuses to compile line (b).
I cannot give exact compiler messages because I have them in Russian :).

Turning method GetTypedObject() into a let binding and removing <‘T> ends up with another compiler warning which I don’t like either. The only wokaround I found is to move GetTypedObject<‘T>() into a base class and to make it public. Thanks in advance…

Answer 1

Since type inference works top-to-bottom, it encounters GetTypedObject returning int before it even reaches the method definition and discovers it should be generic. (As Brian points out, members are read prior to let bindings…and there’s a simpler fix for this.) I get the following nasty error:

A use of the function ‘GetTypedObject’ does not match a type inferred elsewhere. The inferred type of the function is Test.A -> Microsoft.FSharp.Core.unit -> ‘a. The type of the function required at this point of use is Test.A -> Microsoft.FSharp.Core.unit -> ‘a This error may be due to limitations associated with generic recursion within a ‘let rec’ collection or within a group of classes. Consider giving a full type signature for the targets of recursive calls including type annotations for both argument and return types.

If the usage appears after the method definition, it works.

type A() =
    member this.GetTypedObject<'T>() =
        Unchecked.defaultof<'T>

    member this.Test() =
      let a = this.GetTypedObject<int>()    // a
      let b = this.GetTypedObject<string>() // b
      ()

Here’s a workaround:

type A() =
  let getTypedObject() = Unchecked.defaultof<_>
  let a : int = getTypedObject()    // a
  let b : string = getTypedObject() // b

  member this.GetTypedObject<'T>() : 'T = getTypedObject()