Iavor Diatchki wrote:
I am not clear why you think the current notation is confusing... Could you give a concrete example? I am thinking of something along the lines: based on how "<-" works in list comprehensions and the do notation, I would expect that pattern guards do XXX but instead, they confusingly do YYY. I think that this will help us keep the discussion concrete.
Pattern guards basically are a special-case syntactic sugar for (instance MonadPlus Maybe). The guard foo m x | empty m = bar | Just r <- lookup x m, r == 'a' = foobar directly translates to foo m x = fromMaybe $ (do { guard (empty m); return bar;}) `mplus` (do {Just r <- return (lookup m x); guard (r == 'a'); return foobar;}) The point is that the pattern guard notation Just r <- lookup m x does *not* translate to itself but to Just r <- return (lookup m x) in the monad. The <- in the pattern guard is a simple let binding. There is no monadic action on the right hand side of <- in the pattern guard. Here, things get even more confused because (lookup m x) is itself a Maybe type, so the best translation into (MonadPlus Maybe) actually would be r <- lookup m x Regards, apfelmus