[Haskell-cafe] Re: Polyvariadic functions operating with a monoid

9 Oct 2010

      Another example that also fails to compile (but I cannot see why):

import Data.PolyToMonoid
import Data.Monoid

instance Monoid Int where
    mappend = (+)
    mempty = 0

instance Monoidable Int Int where
    toMonoid = id

main = putStrLn $ show $ unwrap $ polyToMonoid (0::Int) (1::Int)
(2::Int) (3::Int)

In this case, I was expecting a sumOf function.

This gives me:

No instance for (PolyVariadic Int (WMonoid m))
      arising from a use of `polyToMonoid'

Any further suggestions?

On Oct 9, 4:25 pm, Kevin Jardine  wrote:
...
Oleg,
Another puzzle is that:
instance Show a => Monoidable a String where
    toMonoid a = show a
main = putStrLn $ unwrap $ polyToMonoid "" True () (Just (5::Int))
works just fine, but
instance Show a => Monoidable a [String] where
    toMonoid a = [show a]
main = putStrLn $ unwrap $ polyToMonoid [] True () (Just (5::Int))
fails to compile.
Why would that be? My understanding is that all lists are
automatically monoids.
Kevin
On Oct 9, 2:28 pm, Kevin Jardine  wrote:
...
Hi Bartek,
...
Yes, it compiles, but when I try to use polyToMonoid', it turns out
that this function is no longer polyvariadic, unlike the original
polyToMonoid .
...
This may be what Luke meant when he wrote "you lose composability".
...
Even with the extra unwrap function I think that this is pretty cool,
but I would ideally like to hide the unwrap.
...
Kevin
...
On Oct 9, 1:50 pm, Bartek Æwik³owski  wrote:
...
...
Hello Kevin,
...
...
2010/10/9 Kevin Jardine :
...
...
...
I was attempting to turn this into a small library and wanted to avoid
exporting unwrap.
...
...
...
I defined:
...
...
...
polyToMonoid' = unwrap . polyToMonoid
...
...
If you disable MonomorphismRestriction this definition typechecks just
fine. Alternatively, you can ask ghci about the type of "unwrap .
polyToMonoid" and paste that into the type sig.
...
...
regards,
Bartek Æwik³owski
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