On Mon, 4 Jun 2012, Andreas Abel wrote:
Concerning the suggestion that when_ would be in sync with forM_ and whenM_ I'd say: not really. forM_ and whenM_ discard the result of the monadic computation, while when and when_ do not even have such a result. They always just perform some monadic effect and return nothing.
What is whenM_ ? Do you mean mapM_ ?
:: m () -> m b -> b
mapM_ :: (a -> m ()) -> [a] -> m () forM_ :: [a] -> (a -> m ()) -> m () sequence_ :: [m ()] -> m () forever :: m () -> m ()
and many more, like zipWithM_, foldM_, replicateM_.
I would prefer these strict types, too. Alternatively I have wondered in the past whether it might be a good idea to generalize them to:
mapM_ :: Monoid b => (a -> m b) -> [a] -> m b forM_ :: Monoid b => [a] -> (a -> m b) -> m b sequence_ :: Monoid b => [m b] -> m b forever :: Monoid b => m b -> m b
This would still propagate monadic result type () if the final monadic action has result type (). http://www.haskell.org/pipermail/haskell-cafe/2009-January/054243.html
Sorry, but I think all these function have been given their maximal general type
==> to be able to ignore a result of a monadic computation
==> without further noise.
Since the addition of 'void' the noise has become acceptable for me. I would follow a kind of "separation of concerns". Ignoring results is one step and performing forM_, when etc. is the second step.
In my opinion, the types of when and unless are not general enough, an that is, imho, just an accident of history. Because it is the type that inferred for the shortest definition, which is
when cond m = if cond then m else return ()
Please reevaluate my proposal to change to
when :: Bool -> m a -> m () unless :: Bool -> m a -> m ()
in the light of the above arguments.
Last time I asked the question, what application you do have in mind. Is your code cluttered with void's or do you just need it occasionally?