[Yhc] Yhc chokes on __ identifer

[Isaac]

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Neil Mitchell wrote:
> Hi
> 
>> > Anyone want the -underscore flag to stick around, and not just be the
>> > default which can't be turned off?
>>
>> It depends whether you want to claim Haskell'98 compatibility or not.
>> Haskell-prime is likely to make yhc's default behaviour with underscores
>> the new standard however.
> 
> Can anyone outline exactly which behaviour is which? What the
> differences are? Is one behaviour a superset of the other? Are there
> any incompatabilities?

H'98: underscore is purely considered a lowercase character (except for
`_' which is basically just a keyword).  This allows:
_Bob :: Int; _Bob = 3

default for nhc and yhc, and a likely-to-get-in proposal for
Haskell-prime, is skipping leading underscores in an identifier for the
purpose of determining case.  This allows:
type _Bob = Int

Each example would be a syntax error in the other underscore-treatment.
 It makes no difference except for identifiers that start with
underscores not followed by a lowercase letter.  The proposed/nhc/yhc
default behavior is useful because leading underscore also signifies
"unused", which should be possible to do even for type/constructor/class
names.

Isaac
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