Negation

[S. Doaitse Swierstra]

On 14 feb 2010, at 09:32, Simon Marlow wrote:

> On 14/02/10 02:21, Lennart Augustsson wrote:
>> I agree, I don't think this is a bug.  If the grammar actually says
>> that this is legal, then I think the grammar is wrong.
>
> As far as I can tell Doitse is correct in that GHC does not  
> implement the grammar, so it's either a bug in GHC or the grammar.   
> To fix it in the grammar would no doubt involve quite a bit of  
> refactoring, I can't immediately see how to do it easily.

This is indeed not easy, and probably one more situation where some  
extra text has to exclude this since I actually think it should not be  
accepted from a language design point of view. How would you explain  
that

weird :: Int -> Int
weird = (if True then 3 else 5+)

is perfectly correct Haskell?

Doaitse



>
> Cheers,
> 	Simon
>
>>
>> On Sun, Feb 14, 2010 at 1:48 AM, John Launchbury<john at galois.com>   
>> wrote:
>>> I don't think this is a bug. I do not expect to be able to unfold  
>>> a definition without some syntactic issues. For example,
>>>
>>> two = 1+1
>>> four = 2 * two
>>>
>>> but unfolding fails (four = 2 * 1 + 1). In general, we expect to  
>>> have to parenthesize things when unfolding them.
>>>
>>> John
>>>
>>>
>>> On Feb 13, 2010, at 11:56 AM, Simon Marlow wrote:
>>>
>>>> On 09/02/10 21:43, S. Doaitse Swierstra wrote:
>>>>> One we start discussing syntax again it might be a good occasion  
>>>>> to
>>>>> reformulate/make more precise a few points.
>>>>>
>>>>> The following program is accepted by the Utrecht Haskell  
>>>>> Compiler (here
>>>>> we took great effort to follow the report closely ;-} instead of
>>>>> spending our time on n+k patterns), but not by the GHC and Hugs.
>>>>>
>>>>> module Main where
>>>>>
>>>>> -- this is a (rather elaborate) definition of the number 1
>>>>> one = let x=1 in x
>>>>>
>>>>> -- this is a definition of the successor function using section  
>>>>> notation
>>>>> increment = ( one + )
>>>>>
>>>>> -- but if we now unfold the definition of one we get a parser  
>>>>> error in GHC
>>>>> increment' = ( let x=1 in x + )
>>>>
>>>> Now that *is* an interesting example.  I had no idea we had a bug  
>>>> in that area. Seems to me that it ought to be possible to fix it  
>>>> by refactoring the grammar, but I haven't tried yet.
>>>>
>>>> Are there any more of these that you know about?
>>>>
>>>> Cheers,
>>>>      Simon
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