Alex unicode trick

[Simon Marlow]

Krasimir is right, it would be hard to use Alex's built-in Unicode 
support because we have to automatically generate the character classes 
from the Unicode spec somehow.  Probably Alex ought to include these as 
built-in macros, but right now it doesn't.

Even if we did have access to the right regular expressions, I'm 
slightly concerned that the generated state machine might be enormous.

Cheers,
	Simon

On 07/01/2014 08:26, Krasimir Angelov wrote:
> Hi,
>
> I was recenly looking at this code to see how the lexer decides that a
> character is a letter, space, etc. The problem is that with Unicode
> there are hundreds of thousands of characters that are declared to be
> alphanumeric. Even if they are compressed into a regular expression
> with a list of ranges there will be still ~390 ranges. The GHC lexer
> avoids hardcoding this ranges by calling isSpace, isAlpha, etc and
> then converting this result to a code. Ideally it would be nice if
> Alex had a predefined macroses corresponding to the Unicode
> categories, but for now you have to either hard code the ranges with
> huge regular expressions or use the workaround used in GHC. Is there
> any other solution?
>
> Regards,
>    Krasimir
>
>
> 2014/1/7 Carter Schonwald <carter.schonwald at gmail.com>:
>> you're probably right, this could be regarded as dead code for ghc 7.8 (esp
>> since alex and happy must be the recent versions to even build ghc HEAD ! )
>>
>>
>> On Tue, Jan 7, 2014 at 2:25 AM, Mateusz Kowalczyk <fuuzetsu at fuuzetsu.co.uk>
>> wrote:
>>>
>>> Greetings,
>>>
>>> When looking at the GHC lexer (Lexer.x), there's:
>>>
>>>> $unispace    = \x05 -- Trick Alex into handling Unicode. See
>>>> alexGetChar.
>>>> $whitechar   = [\ \n\r\f\v $unispace]
>>>> $white_no_nl = $whitechar # \n
>>>> $tab         = \t
>>>
>>> Scrolling down to alexGetChar and alexGetChar', we see the comments:
>>>
>>>
>>>> -- backwards compatibility for Alex 2.x
>>>> alexGetChar :: AlexInput -> Maybe (Char,AlexInput)
>>>>
>>>> -- This version does not squash unicode characters, it is used when
>>>> -- lexing strings.
>>>> alexGetChar' :: AlexInput -> Maybe (Char,AlexInput)
>>>
>>> What's the reason for these? I was under the impression that since
>>> 3.0, Alex has natively supported unicode. Is it just dead code? Could
>>> all the hex $uni* functions be removed? If not, why not?
>>>
>>> --
>>> Mateusz K.
>>> _______________________________________________
>>> ghc-devs mailing list
>>> ghc-devs at haskell.org
>>> http://www.haskell.org/mailman/listinfo/ghc-devs
>>
>>
>>
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