A variation on the above example:

~> put [x #y ]
parse error: 7-8 in [interactive]: should be ']'
parse error: should be ']'
[tty], line 1: put [x #y ]

That illustrates why such syntax is problematic. It's not enough to support comments on lines without any other expression of syntactically significant tokens. Note that even without the trailing ] in that example it fails using current parsing behavior. Which is another road block to changing the behavior since doing so is something that can only be done in a major release.

Breaking changes should be fine since we're still below 1.0. Interestingly enough it behaves as expected with lambdas:

~> { put 1 # comment }
   }
▶ 1 

@krader1961 I am not sure what's the problem with put [x #y ]. In this case, it is the same as put [x; the closing bracket needs to appear on a second line.

Currently having # inside lists is a syntax error. Making an invalid program valid does not break backward compatibility.

@xiaq, Yes, you're right regarding backward compatibility. I was mislead by the failure of the put [x #y ] example. I expected it to be equivalent to put ['x' '#y'] since I expected # to not be special inside a list.

I widened the scope of the issue a bit. There are in fact more places where comments should be allowed but currently aren't, such as argument lists (in a lambda) and braced lists.