Support commas in first indentation column

[arybczak]

Consider the following code:

data T = T {
  foo :: String
, bar :: String
}

After putting the closing bracket you get the error message from the title.

[gracjan]

haskell-indentation finds first line that starts at column zero above the line you are trying to indent. So it sees only this fragment:

, bar :: String
}

and of course gets confused. What is the idea of putting commas in the first column? For now use one of the following styles:

data T = T {
    foo :: String
  , bar :: String
  }

or

data T = T
  { foo :: String
  , bar :: String
  }

or

data T = T {
  foo :: String,
  bar :: String
  }

[arybczak]

For new code sure, but the problem is that it breaks if such code is already there. This is valid Haskell and indentation mode should work with it.

[mgsloan]

Agreed, to me it's crazy that an indentation mode would throw an error and not have a fallback behavior. Like syntax highlighting, this stuff should be a "best effort" thing, and not a syntax parser that that attempts to validate your Haskell (particularly if it doesn't accept valid Haskell...).

[gracjan]

I agree that haskell-indentation should never throw a parser error. I do not agree that haskell-indentation should throw and swallow exceptions. haskell-indentation should do something sensible with any stream of characters, original authors stood on the position that only valid Haskell should be supported. We need to backtrack that decision.

[mgsloan]

As a temporary solution, how about catching exceptions and printing them, and then falling back on some simple indentation strategy like "same column as start of current line".