Suggestion: for...else (or for...empty)

[ceymard]

Seen in other languages,

for obj in container
    # do something
else
    # container was empty, do something else.

or

for obj of container
    # do something
empty
    # container was empty, do something else.

thoughts ?

[satyr]

• How do you compile?
• Use cases?

[ceymard]

Naive implementation :

for x of o
    do_something x
empty
    console.log "o is empty"

=>

var x, _i, _ref, _len;
for (_i = 0, _len = (_ref = o).length; _i < _len; ++_i) {
  x = _ref[_i];
  do_something(x);
}
if (_i == 0)
    console.log("o is empty");

In the case of the in iterator in its simplest form.

for x in o
    do_something x
empty
    console.log "o is empty"

var x, _seen = false;
for (x in o) {
  do_something(x);
  _seen = true;
}
if (!_seen)
   console.log("o is empty");

Use case :

for plugin of plugins
    load_plugin plugin
empty
    throw new Exception "My app is useless without plugins !"

or

for plugin_name in plugins
    load_plugin plugins[plugin_name]
empty
    throw new Exception "My app is useless without plugins !"

[ceymard]

Just read that on the for...else of python. Interesting read : http://psung.blogspot.com/2007/12/for-else-in-python.html

[satyr]

What should it return when else clause is executed?

r =
  for x of a
    x
  else
    y

http://psung.blogspot.com/2007/12/for-else-in-python.html

So Python's isn't like this proposal, but relative to break. Hm.

[ceymard]

What should it return when else clause is executed?

y ?

if the programmer wants to keep the actual for assignation logic, he should write :

r =
   for x of a
       x
   else
       [y]

So Python's isn't like this proposal, but relative to break. Hm.

Yes, wondering too which would be more worthwhile to implement.

[satyr]

Done mostly as proposed; both while and for now accept else clause, run when the loop body wasn't run.

I don't think we should follow Python's, which is unintuitive and complicated to compile.