group keyword with TransformListComp

[Neil Mitchell]

Hi,

The question of syntax is always something that provokes intense
discussion. My interest (and concern) with the choice of syntax in
this case is three-fold:

1) I want HLint to turn on as many Haskell extensions as possible when
parsing, with the trade off that they don't break a massive number of
existing programs. For each extension I have weighed up the number of
programs that will get broken (because the extension steals previously
valid syntax), vs the number that will be parsed (how popular the
extension is). I've erred on the side of enabling extensions, so am
happy to accept changes in meaning for a!b and $(a). Almost all
extensions get turned on, with the exception of TransformListComp,
which even breaks the source code to HLint itself - something no other
extension does. It seems the syntax stolen is far too much, especially
when compared to other extensions.

2) I want to use TransformListComp in HLint. In particular, I have a
nested pile of group/sort surrounding a list comp that would be
perfect for TransformListComp. However, as it stands, I can't even
enable the extension without that file breaking. I could qualify all
the uses of group, but that seems too much hassle. I'd really like to
enable the extension (without modification), and then start converting
each group-like list-comp one at a time.

3) We'll never be able to make TransformListComp enabled in future
versions of Haskell' since we break too many programs. It's an
extension that is destined forever to remain only an extension. That
seems like a lonely life for an extension.

Thanks

Neil

On Sat, Jun 27, 2009 at 11:52 AM, Max Bolingbroke<omega.theta at gmail.com> wrote:
> Hi,
>
> I agree this is annoying. It was hard to find syntax which was both
> meaningful and currently unused, so we settled on this instead. As
> Simon says, suggestions are welcome!
>
> Note that group *should* be parsed as a special id, so you can still
> import D.L qualified and then use dot notation to access the function.
>
> Cheers,
> Max
>
>
> 2009/6/21 Neil Mitchell <ndmitchell at gmail.com>:
>> Hi,
>>
>> The TransformListComp extension makes group a keyword. Unfortunately
>> group is a useful function, and is even in Data.List. Thus,
>> Data.List.group and TransformListComp are incompatible. This seems a
>> very painful concession to give up a nice function name for a new
>> extension. Is this intentional? Here's an example:
>>
>>> $ cat GroupKeyword.hs
>>> {-# LANGUAGE TransformListComp #-}
>>> module GroupKeyword where
>>>
>>> a = map head $ group $ sort [1..100]
>>> $ ghci GroupKeyword.hs
>>> GHCi, version 6.10.2: http://www.haskell.org/ghc/  :? for help
>>> Loading package ghc-prim ... linking ... done.
>>> Loading package integer ... linking ... done.
>>> Loading package base ... linking ... done.
>>> [1 of 1] Compiling GroupKeyword     ( GroupKeyword.hs, interpreted )
>>>
>>> GroupKeyword.hs:4:15: parse error on input `group'
>>> Failed, modules loaded: none.
>>> Prelude>
>>
>> There are some places I'd like to use TransformListComp, but I often
>> want to use group in the same module.
>>
>> Thanks
>>
>> Neil
>>
>