[Haskell-cafe] liftN

[wren ng thornton]

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