On 24 December 2014 at 06:30, Geoffrey Irving <[email protected]> wrote:
> In python,
>
> closures = []
> for i in range(10):
> closures.append(lambda x:x+i)
>
> makes 10 copies of lambda x:x+9.
Ho-hum. I'm not sure what sense it makes to talk about a language
that doesn't have variable declarations in this context, because if
you insert them where python conceptually does, this example makes
sense. In particular, it allows you to do:
def make_even():
# var even, odd
even = lambda n: True if n == 0 else odd(n - 1)
odd = lambda n: False if n == 0 even(n - 1)
return even
How did even() get a binding to odd before it was set? because it
captured the unbound local from the mutable environment. I think it
would be nonsense for this example /not/ to work. The "mutability
inference", to the extent such a thing exists, is working correctly.
The *other* possibility is to say that the variable introduced by the
loop is fresh, as in the following java:
for ( final int i : someIterable ) {
closures.append(x -> x + i);
}
If you don't require variable declarations, this seems a bit tricky,
because does the loop body also introduce a fresh scope? Java says
yes, but this is awkward in practice.
--
William Leslie
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