[Haskell-beginners] typeclass woes...how to constain a typeclass to be "closed" under an operation....

[akash g]

(S Integer) is not an instance of Bar.  I solved this by doing the following

You can get around this using an empty instance declaration for (S Integer)
like so

instance Bar (S Integer)

Now, the compiler will know that (S Integer) is an instance of this.  It
doesn't matter if there is no definition of inc' as you won't be using it.

However, I don't understand why a class definition will contain itself as a
constraint on one of its method.  By doing so,

That is, whenever you do something like this

class Bar a where
  inc' ::  Bar (S a) => a -> S a

instance Bar Integer where
  inc' x = S x

The resulting expression will also have to be an instance of Bar
(.ie.,instance Bar (S Integer)
)  I just don't think this makes any sense.  Also, tried searching for any
such type signatures in Hoogle, but didn't come across anything like this.



On Tue, Jan 6, 2015 at 11:09 PM, Nicholls, Mark <nicholls.mark at vimn.com>
wrote:

>  I can do the basic Haskell…I’ve done a course even, but as I don’t use
> it… I quickly degenerate into a beginner.
>
>
>
> I think I’ve refound my feet….I will post it to the café and see what
> happens.
>
>
>
> *From:* Beginners [mailto:beginners-bounces at haskell.org] *On Behalf Of *David
> McBride
> *Sent:* 06 January 2015 4:41 PM
> *To:* The Haskell-Beginners Mailing List - Discussion of primarily
> beginner-level topics related to Haskell
> *Subject:* Re: [Haskell-beginners] typeclass woes...how to constain a
> typeclass to be "closed" under an operation....
>
>
>
> I think you've slightly surpassed the skill level of this list.  You might
> try on haskell-cafe, and I would bet you could get an in depth answer on
> stackoverflow.
>
>
>
> On Tue, Jan 6, 2015 at 3:46 AM, Nicholls, Mark <nicholls.mark at vimn.com>
> wrote:
>
> I dont seem to have any replies
>
> Booo
>
> Is my question too stupid? Or boring? Or unintelligable?
>
>
>
> Its quite common in maths to have operations in a theory that are (set)
> closed, i just want to translate that notion to a typeclass
>
>
>
> I have a suggestion that does work
>
>
>
> Class Foo m where
>
>    op :: m a -> m (S a)
>
>
>
> That is closed, but now were working on types of kind • -> •, which is
> another leap of complexity
>
>
>
> Is this the idiom/pattern i should follow? Or can the closure contraint be
> expressed directly?
>
>
>
>
>
> Excuse the spelling, sent from a phone with itty bitty keys, it like
> trying to sow a button on a shirt with a sausage.
>
>
>
>
> On 5 Jan 2015, at 10:54, Nicholls, Mark <nicholls.mark at vimn.com> wrote:
>
>   > {-# LANGUAGE TypeFamilies, MultiParamTypeClasses, FlexibleContexts,
> FlexibleInstances, GADTs #-}
>
>
>
> lets say I have a data type
>
>
>
> > data S a = S a deriving Show
>
>
>
> and a nice simple typeclass
>
>
>
> > class Foo a where
>
> >   inc :: a -> S a
>
>
>
> make Integer an instance and....
>
>
>
> > instance Foo Integer where
>
> >   inc x = S x
>
>
>
> brilliant...
>
>
>
> now lets say I want my typeclass to enforce that any result of inc is also
> of a type that is an instance of the typeclass
>
> I'll follow my nose...which causes me a problem that makes me realise I
> maybe I don't understand whats going on.
>
>
>
> > class Bar a where
>
> >   inc' :: (Bar (S a)) => a -> S a
>
>
>
> follow it to....
>
>
>
> > instance Bar Integer where
>
> >   inc' x = S x
>
>
>
> this WORKS!...even though "S Integer" is not an instance of Bar!...maybe I
> don't understand.
>
>
>
> > -- x = inc' (1 :: Integer)
>
>
>
> This would go BOOM ....”no instance of (Bar (S Integer))"
>
> ...so that makes sense....
>
>
>
> How do I force Haskell to check the constraint before I evaluate an
> expression?
>
>
>
> follow my nose?
>
>
>
> > -- class (Baz (S a)) => Baz a where
>
> > --  inc'' :: a -> S a
>
>
>
> BOOM ...."Cycle in class declaration"....
>
> booo..but I suppose thats understandable....
>
>
>
> so
>
>
>
> a) is my use of typeclasses correct?...or am I missing something?
>
> b) if so, how do I force the compiler to make sure my typeclass operations
> is nicely closed (under the typeclass)?
>
>
>
>
>
>
>
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