[Haskell] Types of when and unless in Control.Monad

[Henning Thielemann]

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?