Andreas Rossberg wrote:
let a = [], b = [], c = [], d = []
for (int i = 0, f = function(){ return function() { ++i } }; i< 10;
d[i] = function() { return i }, ++i) {
a[i] = f;
b[i] = f();
c[i] = function(){ return i }
}
But note that the environment in b's closures is not the for-loop
environment. Instead, it is a local environment, whose parent is the
for-loop environment. To make that case work, it is not enough to be
able to modify the [[Scope]] environment of closures -- in general,
you'd need to swap out arbitrary parts of an environment chain.
Oww, that's a problem. Copies are hard here... refs can do it, but no
one likes them.
I am fine with either. So far I have favoured
fresh-variables-per-iteration, but the current discussion has raised
some doubts in the back of my mind.
This whole discussion leads me to completely different opinion: what
about to embrace "explicit is better than implicit" for this case, and
allow let/const without initializer to create local copy of an outer
variable?
Then, if you need to capture a value, you write it down (I know this
appeared to avoid capturing by hand in (function (i) { ... })(i), but
reusing normal blocks to do it may work, wouldn't it?
It disallows some of the ninjutsu (init-closures will work with
different i than localized closures), but it will be clearly visible:
let a = [], b = [], c = [], d = [], e = []
for (int i = 0, f = function(){ return function() { ++i } }; i< 10;
d[i] = function() { return i }, ++i) {
a[i] = f; // this
b[i] = f(); // and this
c[i] = function(){ return i } // and this work with loop-level i
{
let i;
e[i] = function () { return i; } // this has local i
}
}
/Andreas
There can be even shorter syntax, like
e[i] = function () let i { return i; } // this has local i
or
e[i] = function () { let i; return i; } // this has local i
whoch can avoid need of {} block, but it must be thought of if it is
possible consistently (the former should, the latter may be questionable).
Herby
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