[Haskell-cafe] monoid pair of monoids?

[Daniel Feltey]

Is "Socket2 a b" any different from the pair (a,b)?

Assuming Socket2 looks roughly like the following:

> import Data.Monoid
> data Socket2 a b = Socket2 (a,b)

Then if both a and b are instances of Monoid we can make Socket2 a b into
an instance of Monoid the same way we make (a,b) into a Monoid.


> instance (Monoid a, Monoid b) => Monoid (Socket a b) where
>     mempty = Socket2 (mempty, mempty)
>     Socket2 (a, b) `mappend` Socket2 (w, x) = Socket2 (a `mappend` w, b `mappend`
x)

You were only missing the restriction that both types a and b must be
instances of Monoid in order to make Socket a b into an instance of Monoid.



Dan Feltey



On Thu, Dec 20, 2012 at 8:40 PM, Christopher Howard <
christopher.howard at frigidcode.com> wrote:

> In my current pondering of the compose-able objects them, I was thinking
> it would be useful to have the follow abstractions: Monoids, which were
> themselves tuples of Monoids. The idea was something like so:
>
> code:
> --------
> import Data.Monoid
>
> instance Monoid (Socket2 a b) where
>
>   mempty = Socket2 (mempty, mempty)
>
>   Socket2 (a, b) `mappend` Socket2 (w, x) = Socket2 (a `mappend` w, b
> `mappend` x)
>
> data Socket2 a b = Socket2 (a, b)
> --------
>
> However, this does not compile because of errors like so:
>
> code:
> --------
> Sockets.hs:9:21:
>     No instance for (Monoid a)
>       arising from a use of `mempty'
>     In the expression: mempty
>     In the first argument of `Socket2', namely `(mempty, mempty)'
>     In the expression: Socket2 (mempty, mempty)
> --------
>
> This makes sense, but I haven't figured out a way to rewrite this to
> make it work. One approach I tried was to encode Monoid constraints into
> the data declaration (which I heard was a bad idea) but this didn't
> work, even using forall. Also I tried to encode it into the instance
> declaration, but the compiler kept complaining about errant or illegal
> syntax.
>
> --
> frigidcode.com
>
>
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20121220/65df582e/attachment.htm>