[Haskell-cafe] liftN

[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>
> 
> -- 
> Live well,
> ~wren
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