Michael Shulman wrote:
The frequent occurence of "ListT $ return" in my code when I use the ListT
monad transformer has made me wonder why there isn't a standard typeclass
`MonadList', like those for the other monad transformers, encapsulating
the essence of being a "list-like" monad -- in this case, the ability to
select from a list of things. I quickly wrote one for myself:
class MonadList m where
option :: [a] -> m a
Another use for this class is for selecting a random option:
> instance MonadList SomeMonadWithRandomness where
> option os = pos <- randomRM (0, length os - 1)
> return (os !! pos)
It can also be used for the Nondet monad described in
http://haskell.org/hawiki/NonDeterminism, and as a replacement for the
Parsec combinator 'choice' (which IMHO is a better name). Although msum
might suffice in these cases.
Twan
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