On 21.01.2011, at 09:41, Axel Simon wrote:
Hi,
following the discussion on the ghc mailing list, I propose that the new "RelaxedLayout" extension that is proposed for ghc 7.1 should be incorporated into the language standard.
Sorry, this should read "NondecreasingIndentation". Axel
The motivation is that ghc <7.1 have silently implemented the "RelaxedLayout" by default and that removing this feature will break many programs without any gain except that ghc complies to the standard. Hence, it would be better that the standard is adjusted, as this would imply no changes to current ghc compilers nor user programs.
As I'm not familiar with the technical details, I can only give an idea about what "RelaxedLayout" does. Haskell 2010 would parse
foo = do some computation trace "hello" $ do other computation
such that "other computation" belongs to the first "do", i.e.
foo = do { some computation ; trace "hello" $ do ; other computation }
which leads to a parse error since the second "do" not followed by any instructions. The relaxed layout extension opens a new block after a "do" if the next line commences on in the same column as the current block. Thus, it would parse the example above as:
foo = do { some computation ; trace "hello" $ do { other computation } }
Observations: - the proposal has been implemented in ghc for years - it does not break any existing programs (as far as I can see) - incorporating this proposal into the standard involves no implementation effort for the ghc compiler nor any program that compiles with ghc
Cheers, Axel
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