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 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 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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