Haskell's layout indentation?

[Kile-Asmussen]

Haskell uses a peculiar form of significant whitespace.

When a 'layout' keyword appears in the source code, the next non-whitespace source element determines the indentation; even if it appears on the same line as the keyword. Subsequent lines are parsed based on whether they are of greater, lesser, or the same indentation.

layoutkeyword thing
              same_level_thing_item_in_layout
                 deeper_level_thing_part_of_above_thing
              same_level_thing_item_in_layout
          shallower_level_thing_that_ends_layout

layoutkeyword
    thing
    same_level_thing_item_in_layout
       deeper_level_thing_part_of_above_thing
    same_level_thing_item_in_layout
  shallower_level_thing_that_ends_layout

AFAICT, this form of matching isn't possible. Perhaps a way to match on the current column? A way to unconditionally push strings?

[dragostis]

I think you could make it work with same level and deeper level of indentation, but I don't really understand how the shallower indentation works. Is it same_level - 4 * k > 0?

[Kile-Asmussen]

Sorry, let me clarify in the OP

ETA: basically, a layout starts at the first non-space item, and every subsequent line indented to the same column is an item in the layout. Every deeper-indented line is a part of the above item, and the layout ends at the first shallower indented line.

[dragostis]

This is definitely possible. It's not the most trivial of approaches, but I find it quite elegant. The idea behind this is that you know how long layoutkeyword is in terms of spaces, so you can pop it out of the stack then push whitespace instead.

[Kile-Asmussen]

Yeah, the problem is that the layout keywords constitute expressions themselves, so they can be preceded by arbitrary tokens.

Maybe I can peek on the line if it contains a keyword, but I don’t understand how to “push whitespace instead”

ETA there’s also the issue of the indentation level being given on the same line as the layout keyword.

[dragostis]

Are the arbitrary tokens on the same line?

[shika-blyat]

This is definitely possible. It's not the most trivial of approaches, but I find it quite elegant. The idea behind this is that you know how long layoutkeyword is in terms of spaces, so you can pop it out of the stack then push whitespace instead.

The link (https://pest.rs/?bin=jgwd2#editor) seems dead, do you have the grammar somewhere ? I'm also interested about it