[Haskell] Proposal: Allow "\=" for field update in record update
syntax
Keean Schupke
k.schupke at imperial.ac.uk
Fri Mar 4 17:44:09 EST 2005
Benjamin Franksen wrote:
>On Friday 04 March 2005 16:32, Keean Schupke wrote:
>
>
>>robert dockins wrote:
>>
>>
>>>Is that really how this is done? That doesn't seem like it can be right:
>>>
>>>instance X (a b) -- single parameter class where 'a' has an arrow kind
>>>
>>>is very different from:
>>>
>>>instance X a b -- multiple parameter class
>>>
>>>I would expect a type constructed with 'appT' to correspond to the
>>>first declaration, and not to the second.
>>>
>>>
>>Yup, thats how it is done, I have some complex working TH that generates
>>multi parameter classes with fundeps
>>instances etc... and I can say for definite it all works fine:
>>
>>For the above examples
>>
>>appT X (appT a b) -- X is applied once (to a applied to b)
>>
>>appT (appT X a) b -- X is applied twice first to a then to b
>>
>>
>
>But this has nothing to do with the instance question. I agree with Robert
>that
>
> AppT a b
>
>definitely sounds like type constructor application. How can this help with
>multi parameter class instances? Consider:
>
> class Bogus a b
> instance Bogus Int Char
>
>How do you express the /instance/ in TH? Using AppT? AppT would make sense for
>
> instance Show a => Show [a] where ...
>
>where one would express the '[a]' in TH as
>
> AppT LiftT (VarT mkName "a")
>
>
That would be:
(using 6.4 syntax)
AppT (AppT (ConT (mkName "Bogus")) (ConT ''Int)) (ConT ''Char)
If the instance is using type variables:
instance Bogus Int a
you get:
AppT (AppT (ConT (mkName "Bogus")) (ConT ''Int)) (VarT (mkName "a"))
Keean.
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