On 07/07/2012 05:08, Tyson Whitehead wrote:
PS: To be fully precise, the modified layout decoder in 9.3 would be
L (<n>:ts) i (m:ms) = ; : (L ts n (m:ms)) if m = n = } : (L (<n>:ts) n ms) if n < m L (<n>:ts) i ms = L ts n ms L ({n}:<n>:ts) i ms = { : (L ts n (n:ms)) if n > i (new rule) L ({n}:ts) i (m:ms) = { : (L ts i (n:m:ms)) if n > m (Note 1) L ({n}:ts) i [] = { : (L ts i [n]) if n > 0 (Note 1) L ({n}:ts) i ms = { : } : (L (<n>:ts) i ms) (Note 2) L (}:ts) i (0:ms) = } : (L ts i ms) (Note 3) L (}:ts) i ms = parse-error (Note 3) L ({:ts) i ms = { : (L ts i (0:ms)) (Note 4) L (t:ts) i (m:ms) = } : (L (t:ts) i ms) if m /= 0 and parse-error(t) (Note 5) L (t:ts) i ms = t : (L ts i ms) L [] i [] = [] L [] i (m:ms) = } : L [] i ms if m /= 0 (Note 6)
http://www.haskell.org/onlinereport/syntax-iso.html
As before, the function 'L' maps a layout-sensitive augmented token stream to a non-layout-sensitive token stream, where the augmented token stream includes '<n>' and '{n}' to, respectively, give the indentation level of the first token on a new line and that following a grouping token not followed by '{'.
This time though, we allow the '{n}' '<n>' sequence (before it was supressed to just '{n}'). We also add a new state variable 'i' to track the indentation of the current line. The new rule now opens a grouping over a newline so long as the indentation is greater than the current line.
Upon a less indented line, it will then close all currently open groups with an indentation less than the new line.
It's a little hard to evaluate this without trying it for real to see whether it breaks any existing code. However, unless there are very strong reasons to do so, I would argue against making the layout rule *more* complicated. I find the current rule behaves quite intuitively, even though its description is hard to understand and it is virtually impossible to implement. I now think '\' is too quiet to introduce a new layout context. The pressing need is really for a combination of '\' and 'case', that is single-argument so that we don't have to write parentheses. I think '\case' does the job perfectly. If you want a multi-clause multi-argument function, then give it a name. Cheers, Simon