[e-lang] Why aren't guards allowed on forward declarations?

[Kevin Reid]

On Sep 28, 2007, at 10:47, David Hopwood wrote:
> If using backtracking exceptions [*], then the exception caused by  
> trying
> to access a forward-declared variable before it is given a value would
> undo side effects performed in that scope.

There is no such exception. Before it is "given a value" (that is,  
resolved), the value is a promise. It is possible to examine a  
reference and find out that it is a promise.

It would be possible (using a custom slot) to write a forward- 
declaration mechanism that behaves as you describe, but it would be  
less useful, as it would allow only use-before-definition within  
objects and not within data structures.

That is, this would be disallowed:

def foo
def bar

bind foo := makeFoo(bar)
bind bar := makeBar(foo)

This particular case can be rewritten as

def [foo, bar] := [makeFoo(bar), makeBar(foo)]

but in larger cyclic definitions this notation is clunky.

-- 
Kevin Reid                            <http://homepage.mac.com/kpreid/>