Hi Sebastian,
For example:
[(x, y) | x <- xs | y <- ys] -- Comprehension.
versus
(,) <$> xs <*> ys -- Applicative style.
or
(| (,) xs ys |) -- Idiom brackets in She.
Or am I missing some subtle points here?
Perhaps, you meant [(x, y) | x <- xs , y <- ys] which translates to the following code in the do notation: do x <- xs y <- ys return (x,y) The comprehension notation also features filters. [(x, y) | x <- xs , y <- ys, x == y] which for instances of MonadPlus (i.e., not for all monads) translates to: do x <- xs y <- ys guard (x == y) return (x,y) See Michael Adams' (link is in Max's reply) suggestion on how to translate 'order by' and 'group by' to monadic combinators. There, monadic code gets really unwieldy. The point of the comprehension notation is that (just like of any other notation, including do) it makes certain kinds of programs easier to write and read. As I have already mentioned, the comprehension notation is extremely well suited for writing programs that process collections of values. Maybe this is because the set comprehension notation was hardwired in our brains during high school math. Haskell is widely used for EDSL development. Some EDSLs are about processing collections of values. Monad comprehensions would allow EDSLs to utilise the expressive list comprehension notation that is currently only allowed for lists. Having said that, since the comprehension notation (to some extent) is used in high school math curricula, even a bit mathematically inclined non-computer scientist users of list-based EDSLs may feel at home when offered to program using the comprehension notation. To summarise, IMO programming list-processing applications is more natural in the comprehension notation than in do notation and programming with 'group by' and 'order by' is infeasible in monadic combinators. Of course one could extend do notation further, but I think it is better for the reasons outlined above to generalise the list comprehension notation instead. Of course, I still welcome alternative suggestions. Cheers, George