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

[Nicholls, Mark]

I will post the question again properly tomorrow but your example indeed is almost a good example, but is trivially closed

If the operation on monoid was

[a]->[b]->[c]

That is also closed; ie [c] is also a monoid.... And the typeclass declaration follows the idiom ive suggested

This only works for types of kind *->*

If i wanted to do this over a type of kind * (ignoring the trivial a->a)

Can i express this in a typeclass?

Please ignore until i resubmit this question

Ps i hate phones

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 6 Jan 2015, at 18:10, Tom Ellis <tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk> wrote:
>> 
>> On Tue, Jan 06, 2015 at 05:43:47PM +0000, Nicholls, Mark wrote:
>> 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
> 
> Do you really need a typeclass (or indeed any way) of doing this?  I suspect
> it will not work.  If you consider the multiplication for the monoid of
> concatenation of lists
> 
>   (++) :: [a] -> [a] -> [a]
> 
> you see that its type already implies that it is "closed".
> 
> Tom
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