How does fsi.ShowDeclarationValues work?

Question

According to the MSDN documentaion:

When set to false, disables the display of declaration values in the output of the interactive session.

However, the following sample interactive session seems to contradict that summary.

> let x = 42;;

val x : int = 42

> fsi.ShowDeclarationValues <- false;;

val it : unit = ()

> let y = 42;;

val y : int

I was not expecting the last line above.

Have I misunderstood something? Can anyone confirm if this is a bug?

Thanks.

Answer 1

Looks to me that it didn't show the value in the last line, only the name and type of the binding.

Answer 2

Daniel is correct - this disables just printing of the values and not the declarations themselves.

One situation where this is useful is when you define some custom printer for a value that creates a new window as a side-effect (e.g. a value that represents a chart or something you want to display).

For example, you could write this:

// Display all evaluated strings in a message box
fsi.ShowDeclarationValues <- false
fsi.AddPrinter(fun (s:string) -> 
  System.Windows.Forms.MessageBox.Show(s) |> ignore; "")

let a = "foo" // Evaluating this line doesn't show message box
let b = "bar" // (dtto)
a + b         // .. but evaluating this line shows the message box!