[Haskell-cafe] Fwd: [Haskell-beginners] Monad instances and type synonyms
Daniil Frumin
difrumin at gmail.com
Sun Apr 14 13:11:42 CEST 2013
Oh, I see that I'm late to the party, sorry, wasn't able to push my mail for some time
—
On Sun, Apr 14, 2013 at 3:09 PM, Daniil Frumin <difrumin at gmail.com> wrote:
> Maybe you can try "curried" definition:
> type Adjustment = (->) SaleVariables
> I had a similar problem awhile ago.
> Hth
> —
> On Sun, Apr 14, 2013 at 9:11 AM, Christopher Howard <christopher.howard at frigidcode.com="mailto:christopher.howard at frigidcode.com">> 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.
> --
> frigidcode.com
> <Attached Message Part>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20130414/e5c15e56/attachment.htm>
More information about the Haskell-Cafe
mailing list