This is slightly off-topic, but you might be interested in the
monad-layershttp://hackage.haskell.org/package/layers-0.1/docs/Documentation-Layers-Over...package
that obviates the need for a function like liftN.
Thanks,
Arjun
On Thu, Oct 17, 2013 at 1:44 PM, Wvv
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> ; * <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> ;
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