On Sat, 21 Apr 2012, Andreas Abel wrote:

While your comments about not ignoring results of functions may be right, in the particular case of when and unless the situation is different. Since both are basically an if-then without an else, the expression *cannot* yield a result, just execute the monadic effect. Thus, confusion is unlikely.

I don't understand this argument. The current type of 'when' is

when :: (Monad m) => Bool -> m () -> m ()

thus it makes sure, that the enclosed action has no result that could be ignored by accident. If the function would be

when_ :: (Monad m) => Bool -> m a -> m ()

then you could write

when_ b (system "command")

and thus miss the ExitCode of 'system' by accident.

I have defined myself when_ and unless_ but I think it is silly, the library functions could be more general.

Since you think that the current 'when's type is silly, I would like to know what are your examples where you want to ignore the result of the enclosed action?