[Haskell-cafe] Question, re: type class syntax.

[David Banas]

Thank you, both, for responding.
What I was trying to express was that the particular type of LogTree2, t, would determine the type of SizedVector, a, apropos to the ‘evaluator’ function, for a particular instance. It looks like functional dependencies is the right way to achieve this, because this type checks:

class LogTree2 t a | t -> a where
    evaluator :: LogTree2 t a => t -> SizedVector a -> SizedVector a

-db

On Aug 29, 2015, at 5:39 PM, William Yager <will.yager at gmail.com> wrote:

> Hey David,
> 
> I think there might be a bit of confusion here. When you say "LogTree2 a", that means "a" is an instance of "LogTree2". Maybe you want something like this?
> 
> evaluator :: LogTree2 a => a -> SizedVector a -> SizedVector a
> 
> Some context might help us to understand what you're going for.
> 
> --Will
> 
> 
> On Sat, Aug 29, 2015 at 7:30 PM, David Banas <capn.freako at gmail.com> wrote:
> Thanks, Will.
> Maybe I’ve got the syntax all wrong, but my intent is just to constrain the type ’t’ to be of class 'LogTree2 a’.
> -db
> 
> On Aug 29, 2015, at 5:27 PM, William Yager <will.yager at gmail.com> wrote:
> 
>> If you want the "LogTree2" typeclass to have multiple type arguments (which it looks like you do, from the definition of "evaluator"), you need to define it as 
>> 
>> class (Show a, Num a) => LogTree2 a t where
>> 
>> --Will
>> 
>> On Sat, Aug 29, 2015 at 6:45 PM, David Banas <capn.freako at gmail.com> wrote:
>> Hi all,
>> 
>> I can’t figure out why this is invalid:
>> 
>> class (Show a, Num a) => LogTree2 a where
>>     evaluator :: (LogTree2 a) t => t -> SizedVector a -> SizedVector a
>> 
>> error| ‘LogTree2’ is applied to too many type arguments In the type ‘(LogTree2 a) t => t -> SizedVector a -> SizedVector a’ In the class declaration for ‘LogTree2’
>> 
>> Can anyone help me understand?
>> 
>> Thanks,
>> -db
>> 
>> 
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>> 
> 
> 

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