Monadic compose with discarding (>>) flipped

user_10580513 0
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The definition of (>>) function is following:

(>>) :: Monad m => m a -> m b -> m b

But I would like to achieve this function flipped like following:

I have a function tabulate :: Int -> [Int] -> IO Int which prints the list as a table with the given number of columns and returns a sum of all the list items in the IO monad. After that I want to have an explicit putStr "\n".

If I would use following:

tabulate >> (putStr "\n")

it would discard the result of the tabulate, the other way around it would not print newline after the table. In case of doing this in do:

smth = do
   let a = tabulate
   putStr "\n"
   a

This do would again print newline before the table since the a is evaluated after the putStr.

How would you print newline after the tabulate function?

monads haskell
1 Answers

You can work with (<*) :: Applicative f => f a -> f b -> f a here:

smth :: IO Int
smth = tabulate 14 [2, 5] <* putStr "\n"

This is equivalent to:

smth = do
   a <- tabulate 14 [2, 5]
   putStr "\n"
   return a

It thus first evaluates the IO Int of the tabulate 14 [2, 5], then prints "\n" as actions, but it "returns" the value of the tabulate call, and not that of the putStr call.

user_675790
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