Syntax propertize could mark layout blocks

[gracjan]

The existing haskell-syntax-propertize could mark first token after a where and other layout tokens as beginning of a layout block. Then whenever a first token on a line comes in it could check to which layout block it belongs (if any).

This should be good enough to implement #899.

I'm not sure what kind of marker should that be. Marker? Overlay? What else Emacs has?

[gracjan]

Note that this might not be that easy without lexical analysis, for example:

let x, y :: Int
     x = 5
     y = 7 in x + y

Here the in keyword end the layout.

[gracjan]

Oh boy:

Note 5.
The side condition parse-error(t) is to be interpreted as follows: if the tokens generated so far by L together with the next token t represent an invalid prefix of the Haskell grammar, and the tokens generated so far by L followed by the token "}" represent a valid prefix of the Haskell grammar, then parse-error(t) is true.

From: https://www.haskell.org/onlinereport/syntax-iso.html#layout

So basically cannot do lexical analysis while not doing parsing. Ugh.

[gracjan]

Seems this is impossible without significant parsing machinery. Closing with sadness and tears.