I'm reading through Expert F# and I am at the part where the author discusses flexible type constraints. I'm fairly confused by what the # symbol does for you.

Don says:

"F# functions can be declared to accept parameters that are any subtype of a particular type by using type annotations of the form #type, where type is some type that supports subtyping."

The example is:

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let setTextOfControl (c : #Control) (s:string) = c.Text <- s;;

Where this allows any subtype of Control is be used.

However, the following seems equivalent:

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let setTextOfControl (c : Control) (s:string) = c.Text <- s;;

If I instantiate a System.Windows.Forms.Form and pass it to either, it will set the Text property without issue.

What am I not getting?

Thanks!

Justin