[xg15]

There would at least be some ambiguity with single-value lists otherwise:

If

  numbers: 1, 2, 3

was a list, what would

  numbers: 1

be?

[jampekka]

You could have it like in Python tuples where 1 is a scalar and 1, is a tuple.

[xg15]

True, but I think that wouldn't be any more intuitive or error safe than the :: syntax.

[renox]

Error safe, the : or :: I kind of doubt it, I think that finding a typing mistake (two : instead of 1) won't be easy..