Ken Takata wrote:
> 2016/7/14 Thu 15:43:11 UTC+9 mattn wrote:
> > On Thursday, July 14, 2016 at 12:49:58 PM UTC+9, ZyX wrote:
> > > 2016-07-13 19:17 GMT+03:00 mattn <[email protected]>:
> > > > On Wednesday, July 13, 2016 at 4:11:59 AM UTC+9, Bram Moolenaar wrote:
> > > >
> > > > Well, I wonder this lambda will be useful. At the first, we hoped to
> > > > call statements in lambda. But the implementation you will include into
> > > > vim can't do because it only allow expressions. It's similar to
> > > > python's lambda. python's one doesn't allow statements. So usecase are
> > > > limited to use. I don't have strong opinion but I'm thinking that this
> > > > is an new expresssion or language for the lambda. It will demand to
> > > > learn the new expression for the users.
> > >
> > > Vim has `execute()`. Python-3 has `exec()` function (Python-2 has it
> > > as a statement). Lambdas do not usually allow statements because they
> > > are to be used in contexts which requires return value (e.g. in Python
> > > this is sorted()/list.sort(), defaultdict(), re.sub[n] (BTW, it is
> > > good idea to have `substitute(s, pattern, funcref, flags)` to work
> > > like `substitute(s, pattern, '\=funcref(submatch(0), submatch(1), …)',
> > > flags)`)). Lambdas are also used as a replacement for
> > > `functools.partial` (python)/`function(fref, args, self)` (VimL) in
> > > cases when they do not apply (e.g. when one needs to fix not the
> > > first, but second or other arguments), but this requires closures.
> >
> > execute() doesn't have scope. So:
> >
> > call execute("let a = 1")
> > echo 1
> >
> > This define new variable in global scope. I want anonymous function. If
> > execute() works with the scope, for example "let a = 1" mean "let l:a = 1",
> > It's so great.
>
> Currently "{args -> expr}" has its own scope.
>
> :echo {-> execute("let a = 1")}()
> :echo a
> E121: Undefined variable: a
> E15: Invalid expression: a
>
>
> BTW, because of its own scope, the following code doesn't work:
>
> let list = [ /* some data */ ]
> let l:threshold = 10
> filter(list, {idx, val -> val > l:threshold})
>
> As ZyX pointed out, function() can be used (but redundant):
>
> filter(list, function({th, idx, val -> val > th}, [l:threshold]))
>
> Closure might be useful for this, but it would be the next step.
The question is, once we do implement closures, are all lambda's a
closure or does it need to be defined? In the last case we may wanto to
explore now how that's done.
If a closure is really cheap we may just make all lambda's a closure.
But when it's not cheap, and they are used in something long-living like
a callback, we should make it optional.
I suspect it's not cheap, especially when lambda's live longer. And
they are created in a bigger function (e.g. to start a job).
Instead of keeping the whole context, another way is to explicitly
specify what to take from the context:
{arg, USE var -> expr}
But that will look ugly to anyone using closures elsewhere.
So long as we only use variables from the context, we could use a
prefix, like in your example:
let l:threshold = 5
filter(list, {idx, val -> val > @l:threshold})
But this requires parsing the expression to find out what is used from
the context, which is tricky.
So it might be best to have some way to specify a function or labmda is
a closure. For a function we can just add an argument, like we alrady
have "dict" and "abort".
Some alternatives:
{ arg => expr } (probably too subtle)
{@ arg -> expr }
{ arg CTX => expr }
{{ arg -> expr }}
Can't say any of these is nice. At least none of them mean we have to
change the syntax of lambda.
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