[Haskell-cafe] Fwd: [Haskell-beginners] Monad instances and type synonyms

[Steffen Schuldenzucker]

The point in not allowing partially applied type synonym instances is 
that it'd make deciding whether a type is an instance of a class much 
harder.
Cf. here[1] for a similar question with the Category class.

-- Steffen

[1] Attached message. Couldn't find it on the archives..

On 04/14/2013 07:10 AM, Christopher Howard wrote:
>
> I asked this question in Haskell-beginners, but I haven't heard anything
> yet, so I'm forwarding to Cafe.
>
> -------- Original Message --------
> Subject: [Haskell-beginners] Monad instances and type synonyms
> Date: Sat, 13 Apr 2013 17:03:57 -0800
> From: Christopher Howard<christopher.howard at frigidcode.com>
> Reply-To: The Haskell-Beginners Mailing List - Discussion of primarily
> beginner-level topics related to Haskell<beginners at haskell.org>
> To: Haskell Beginners<beginners at haskell.org>
>
> I am playing around with some trivial code (for learning purposes) I
> wanted to take
>
> code:
> --------
> -- SaleVariables a concrete type defined early
>
> -- `Adjustment' represents adjustment in a price calculation
> -- Allows functions of type (a ->  Adjustment a) to be composed
> -- with an appropriate composition function
> type Adjustment a = SaleVariables ->  a
> --------
>
> And put it into
>
> code:
> --------
> instance Monad Adjustment where
>
>    (>>=) = ...
>    return = ...
> --------
>
> If I try this, I get
>
> code:
> --------
> Type synonym `Adjustment' should have 1 argument, but has been given none
> In the instance declaration for `Monad Adjustment'
> --------
>
> But if I give an argument, then it doesn't compile either (it becomes a
> "*" kind). And I didn't want to make the type with a regular "data"
> declaration either, because then I have to give it a constructor, which
> doesn't fit with what I want the type to do.
>
>
>
>
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> Haskell-Cafe at haskell.org
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