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

Tue Jan 6 17:39:57 UTC 2015

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<mailto: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<mailto: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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