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

Bind on monad is a better example, its specified to be closed by the typeclass, but agajn operares on types of kind *->* if i have a function that orbits through multiple types (via type family) of kind * then i have to aggregate them under some wrapper type of kind *->* to define the set closed operation on the wrapper Hmmmmm As you can see i dont do much haskell, and im trying to get my head around how to map simple mathematical concepts into it 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 18:53, Nicholls, Mark wrote:

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.

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On 6 Jan 2015, at 18:10, Tom Ellis 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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe CONFIDENTIALITY NOTICE

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