[jhc] A possible bug with constraints being perceived as multiparameter type classes

[Roman Cheplyaka]

On 21/04/15 17:09, Roman Cheplyaka wrote:
> On 21/04/15 16:38, Brandon Allbery wrote:
>> On Tue, Apr 21, 2015 at 9:26 AM, Artyom <yom at artyom.me
>> <mailto:yom at artyom.me>> wrote:
>>
>>     |$ Temp/jhc-0.8.2/jhc Temp/jhc1.hs 
>>
>>     jhc Temp/jhc1.hs
>>     jhc 0.8.2 (mydniquipepo-32)
>>     Finding Dependencies...
>>     Using Ho Cache: '/home/yom/.jhc/cache'
>>
>>     Temp/jhc1.hs:1   - Error: Invalid Class. multiparameter classes not yet supported:
>>     (Num,[HsTyTuple [HsTyVar {hsTypeName = a},HsTyVar {hsTypeName = b}]])
>>     |
>>
>>     Am I misunderstanding something, or is it a legitimate bug?
>>
>>     ​
>>
>>
>> Looks multiparameter to me:
>>
>>     instance ... => Num (T a b Int) where
>>
>> with T ~ (,,). Which is what the second line of the error message is
>> telling you, more or less.
> 
> No, it's a multi-parameter type, but a single-parameter class.
> 
> Multi-parameter would be Num a b Int.
> 
> Roman

That said, it's not H98 either (ghc would require FlexibleInstances), so
jhc may be right in rejecting in, just not explaining it properly.

Roman