F#: Oddity with lazy evalution

Question

Given the following code:

let bar =
    lazy(
        printfn "bar"        
        ())

let foo = 
    lazy(
        printfn "foo"
        bar)

In the interactive window (resetting it each time),

When I call let res = foo.Force () I get:

foo
val res : Lazy<unit> = Value is not created.

When I just call foo.Force() , I get:

foo
bar
val it : Lazy<unit> =
  <ToString exception: Object reference not set to an instance of an object.>
    {IsValueCreated = false;
     Value = null;}

In both cases I would have expected it to just print "foo", however the second case prints "foo\nbar". What's going on here?

Answer 1

What version of F# do you have? This sounds maybe like a bug that got fixed; when FSI prints a value of type Lazy<'a>, at some point I think the ToString() to display the value used to force the lazy to print it, whereas now it does not.

Answer 2

The problem seems to be an implementation detail. How does the Lazy reference know when it has calculated a value? Maybe when it's reference is not ().

Having a type Lazy<unit> doesn't make much sense. Try

let bar =
    lazy(
        printfn "bar"        
        42)