On Tue, Nov 2, 2010 at 7:58 AM, Konrad Hinsen <konrad.hin...@fastmail.net>wrote:
> On 02.11.2010, at 11:45, Sunil S Nandihalli wrote:
>
> > following is the extract from the monads example ...
>
> It looks quite modified and no longer returns pairs! Here is the original:
>
> (with-monad sequence-m
> (defn pairs [xs]
> ((m-lift 2 #(list %1 %2)) xs xs)))
>
> > ; Another way to define pairs is through the m-seq operation. It takes
> > ; a sequence of monadic values and returns a monadic value containing
> > ; the sequence of the underlying values, obtained from chaining together
> > ; from left to right the monadic values in the sequence.
> > (with-monad sequence-m
> > (defn pairs [xs]
> > (m-seq (list xs xs))))
> >
> > can somebody help me understand what is happening in the second one .. ?
> > when would it be more appropriate to use m-seq instead of m-lift ..
>
> In most real-life cases you wouldn't have that choice, as m-lift and m-seq
> do very different things. It's just for making pairs that either one is
> fine.
>
> One example for the use of m-seq is given right after the pairs example:
>
> (with-monad sequence-m
> (defn ntuples [n xs]
> (m-seq (replicate n xs))))
>
> This is a generalization from pairs to n-tuples. You couldn't do this using
> m-lift applied to the list function because m-lift requires a fixed arity.
This wouldn't work?:
(with-monad sequence-m
(defn ntuples [n xs]
(apply (m-lift n list) (replicate n xs))))
(If m-lift is a macro that requires the arity arg to be known at
macroexpansion time:
(with-monad sequence-m
(defn ntuples [n xs]
(apply (eval `(m-lift ~n list)) (replicate n xs))))
instead. Icky, mind you.)
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