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.
Another example is given later in the example collection under "random number
generators".
Konrad.
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