[Haskell-cafe] liftN
wren ng thornton
wren at freegeek.org
Wed Oct 16 22:22:46 UTC 2013
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>
--
Live well,
~wren
More information about the Haskell-Cafe
mailing list