RelaxedLayout language extensions should be default for Haskell

[Axel Simon]

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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