MGrammar: using tabs to denote blocks

Tue, 02 Dec 2008 19:47:24 Z

Suppose I wanted something like this

if e1:   

  while e2:   

    call()   

else   

  return e3   

call()

Meaning, if you have the same number of whitespace characters as the line above you then you're in the same block. The ":" denotes the end of a block a decrease in indentation denotes an end of a block.

How the heck do I do this??

MGrammar: using tabs to denote blocks

Tue, 02 Dec 2008 23:55:14 Z

So far I'm thinking I can just keep track of the whitespace since the last newline and in the code behind just do the logic check. Does anyone second that idea?

MGrammar: using tabs to denote blocks

Wed, 03 Dec 2008 14:52:12 Z

After messing with this for a while I was able to sort of come up with something:

module count  

{  

    import Language;  

    import Microsoft.Languages;  

    export test;  

    language test  

    {  

        syntax Main = l:Line* => Lines[valuesof(l)];  

        syntax Line  

            = "-" NL  

            | l:L(" ", "-") => Indent[l];  

        syntax L(previous, valid)  

            = previous v:valid NL => v  

            | previous l:L(" ", valid) => Indent[l];  

        token NL = Base.NewLine;  

    }  

}

This doesn't quite enforce indentations (couldn't figure out how to do that) but it does tell you how indented you are in each line. Somehow what I need to do is to use this to determine the start of code blocks... I think I'm on the right trail. I tried to do something like this:

token Indent(i)  

  = " " " "#i..; // at least 1 more space

But that doesn't work, you can't have a parameter be apart of the number constraints. Those appear to be hard coded.

MGrammar: using tabs to denote blocks

Thu, 04 Dec 2008 07:35:37 Z

In the current bits, there is no easy way to do this -- perhaps registering a token event listener for the indenting whitespace may get you closer to doing this manually. For the next drop, we are already looking into generating 'Indent' and 'Unindent' tokens from the lexer. That will make it easier to support scoping by indentation.

MGrammar: using tabs to denote blocks

Sun, 07 Dec 2008 04:32:16 Z

what is a token event listener?

MGrammar: using tabs to denote blocks

Tue, 23 Jun 2009 10:11:05 Z

Hi vbbala,
Are there some news on this front, i.e. are there some new features in current bits which would make it easier to support scoping by indentation? Or are there any planned for next CTP release? I've been looking for 'Indent' and 'Unindent' you mention in current M language specification (May 09 CTP) but found nothing.
Thanks, Martin

-md-

MGrammar: using tabs to denote blocks

Tue, 23 Jun 2009 14:55:45 Z

I don't think it's improved much on this front. There is a new "nest" keyword which I haven't looked into which might make it more feasible. But I think the answer is that you have to not interleave whitespace but instead carefully create tokens for keeping track of indentation.

MGrammar: using tabs to denote blocks

Wed, 24 Jun 2009 10:45:48 Z

Thanks for reply. Since my language is really simple with maximum three levels of indentation, I solved this by simply interleaving all whitespace except new line characters and handling new line characters in syntax/tokens. Nevertheless I still hope M will somehow support scoping by indentation in the future because it is very useful for simple easily readable custom languages.

-md-