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

[Kevin Jardine]

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 <kevinjard... at gmail.com> 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 <paczesi... at gmail.com> wrote:
>
> > Hello Kevin,
>
> > 2010/10/9 Kevin Jardine <kevinjard... at gmail.com>:
>
> > > 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
> > _______________________________________________
> > Haskell-Cafe mailing list
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>
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