|Version 1 (modified by 11 years ago) (diff),|
This page concerns the possibility of tweaking the layout rules to avoid a common pitfall when
else is used with the
It only documents a problem: no specific solution is proposed. The issue is somewhat related to NondecreasingIndentation.
Strictly speaking, the issue is not tied to the
do-notation, but that is where it (almost?) always shows up.
I have lost count of how many times people (including very experienced programmers) have approached me wondering why on earth code like the following fails to parse:
foo :: Monad m => Bool -> m Int foo x = do if x then return 1 else return 2
When I explain that they have to indent the
else at least one more step, they invariably stare at me in disbelief and vow to henceforth
stay far away from languages where layout has semantic significance. It can actually be somewhat embarassing, and it certainly can
put beginners off.
Might it be possible to handle
else specially to avoid this problem? At least in the context of
Or might it be possible to treat
if as something that opens a kind of explicit layout context (which would
prevent the insertion of a closing
}) that is closed by a matching
- Somewhat related to NondecreasingIndentation
- Would address a layout issue that manifestly trips up a lot of people and which at least one experienced Haskell programmer (me!) finds very annoying.
- No concrete proposal yet
- Not implemented in any system to my knowledge