Steven Blenkinsop wrote:
Can you clarify the syntax for function types with lifetime
parameters? I saw this in the meeting transcript:
&fn<'a>(f: &fn(&'a T))
&fn(f: &fn<'a>(&'a T))
I think I've filled in blanks that were left correctly. I'm guessing
the that the first case forces the lifetime 'a to be known when you
call the function, and the second case allows the lifetime 'a to be
determined inside the body of the function as needed on each call to
f. I'm pretty sure that the <'a> in both cases is new syntax, since I
don't think you can put type parameters on function values. Is this
correct?
This is exactly correct and yes <'a> would be new syntax.
The first form (with 'a bound on the outer fn) is pretty close to
useless, I imagine, because the outer fn has no way to know what the
lifetime 'a is and moreover it has no pointers with that lifetime. This
implies that it could invoke its argument `f` with pointers whose
lifetime is 'static. So the first function is, for all intents and
purposes, equivalent to
&fn(f: &fn(&'static T))
I am not 100% sure if this pattern arises in other cases where it might
be more important, but I can't imagine what such a case would be.
Niko
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