[Haskell-beginners] Return type from class method

[Thomas Jakway]

You could hide Prelude and define it yourself in a different module but 
that would be a pretty bad idea.  Everyone who wanted to use it would 
have to import your module qualified and refer to it as MyModule.+, 
which defeats the point of making it (+) and not `myAdditionFunction` in 
the first place.

Haskell deliberately doesn't allow overloading.  Having (+) return 
something other than Num would be extremely confusing.


On 10/07/2017 05:07 AM, PATRICK BROWNE wrote:
> Hi,
> Is there a way rewriting the definition of (+) so that testPlusArg 
> returns a (Moving Double). My current intuition is that the signature 
> [(+)  ::  a -> a  -> a] says that the type should be the same as the 
> arguments. And indeed (:t testPlus) confirms this. But the type of  
> testPlusArg is a Double.
>  Can I make it (Moving Double) ?
> Thanks,
> Pat
>
> {-# LANGUAGE FlexibleInstances #-}
> {-# LANGUAGE TypeSynonymInstances #-}
> module Moving where
> data  Time  = Time Double
> type Moving v  = Time -> v
>
> class  Number a where
>  (+)  ::  a -> a  -> a
>
> instance Number  (Moving Double) where
>  (+) a b = \t -> ((a t) Prelude.+ (b t))
>
> a,b ::  Moving Double
> a (Time x) = 2.0
> b (Time x) = 2.0
> testPlus ::(Moving Double)
> testPlus = (a Moving.+ b)
> testPlusArg = (a Moving.+ b) (Time 2.0)
>
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