Haskell : >>= : Use result of first IO action in second

By using (>>=), we can use the result of first IO action in second IO action.

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

Sample.hs

square :: Integer -> Integer
square x = (x*x)

main = putStrLn "Enter a number"
      >> readLn
      >>= \x -> putStrLn ("Square of " ++ (show x) ++ " is " ++ show (square x))

$ ghc Sample.hs
[1 of 1] Compiling Main             ( Sample.hs, Sample.o )
Linking Sample ...
$ 
$ ./Sample
Enter a number
10
Square of 10 is 100

All the statements in the do block are internally combined using >>, >>=.

Sample.hs

main = 
 do 
   putStrLn "'Enter a Number"
   a <- readLn
   print (a==10)

Above code snippet is translated like below.

Sample.hs

main = 
 do 
   putStrLn "'Enter a Number" >> readLn >>= \a -> print (a==10)

Previous                                                 Next                                                 Home