[Haskell-cafe] liftN

[Arjun Comar]

This is slightly off-topic, but you might be interested in the
monad-layers<http://hackage.haskell.org/package/layers-0.1/docs/Documentation-Layers-Overview.html>package
that obviates the need for a function like liftN.

Thanks,
Arjun


On Thu, Oct 17, 2013 at 1:44 PM, Wvv <vitea3v at rambler.ru> wrote:

> Thank you!
>
> Unfortunately, liftN always take same number of arguments.
> So, polyvariadic "magic" don't work here.
>
>
> wren ng thornton wrote
> > On 10/16/13 5:06 PM, Wvv wrote:
> >> I try to write a function "liftN", but I'm not satisfied with result.
> >>
> >> lift :: (MonadTrans t) => Monad m => m a -> t m a
> >>
> >> liftN n
> >>      | n < 1       = error "liftN: n<1"
> >>      | n == 1     = lift
> >>      | otherwise = lift . (liftN (n-1))
> >>
> >> 1) I know(?), that it is impossible to write liftN now: typechecker
> can't
> >> decide which signature it is.
> >
> > You should check out the solutions for encoding polyvariadic functions,
> > e.g.
> >
> > *
> > <
> http://community.haskell.org/~wren/wren-extras/dist/doc/html/wren-extras/Data-List-ZipWithN.html&gt
> ;
> > * <http://okmij.org/ftp/Haskell/polyvariadic.html#polyvartype-fn>
> > * <
> http://paczesiowa.blogspot.com/2010/03/generalized-zipwithn.html>
> > *
> > <
> http://hackage.haskell.org/package/TypeCompose-0.9.9/docs/Data-Zip.html&gt
> ;
> >
> > --
> > Live well,
> > ~wren
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>
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>
>
>
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