How to handle side effect with Applicative?

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Output

The applicative equivalent for >> is *>, so you can do

ghci> :m Control.Applicative
ghci> print 5 *> print 7
5
7

Input – a better case for Applicative

import Control.Applicative

data Company = Company {name :: String, size :: Int}
  deriving Show

getCompany :: IO Company
getCompany = Company <$> getLine <*> readLn

Which works nicely for input:

ghci> getCompany >>= print
BigginsLtd
3
Company {name = "BigginsLtd", size = 3}

Notice that since we’re using Applicative for IO, we’re in the IO monad anyway, so can use >>= if we like. The benefit Applicative gives you is the nice syntax.

My favourite is with parsing, so I can do

data Statement = Expr Expression | If Condition Statement Statement
parseStatement = Expr <$> parseExpression <|> 
                 If <$> (string "if" *> parseCondition) 
                    <*> (string "then" *> parseStatement)
                    <*> (string "else" *> parseStatement)

The difference between Applicative and Monad

The difference between Applicative and Monad is that Monad has >>=, which lets you choose what side effect to use based on the value you have.

Using Monad:

don't_reformat_hard_drive :: Bool -> IO ()
don't_reformat_hard_drive yes = if yes then putStr "OK I didn't" 
                                       else putStr "oops!" >> System.IO.reformat "C:/"

maybeReformat :: IO ()
maybeReformat = WinXP.Dialogs.ask "Don't reformat hard drive?" 
               >>= don't_reformat_hard_drive

(There’s no System.IO.reformat or WinXP.Dialogs.ask. This is just an example I found funny.)

Using Applicative:

response :: Bool -> () -> String
response yes () = if yes then "OK I didn't" else "oops!"

probablyReformat = response <$> WinXP.Dialogs.ask "Don't reformat hard drive?"
                            <*> System.IO.reformat "C:\"

Sadly, using Applicative I can’t inspect the Boolean value to determine whether to reformat or not – the side effect order is determined at compile time, in an Applicative, and the hard drive will always be reformatted with this piece of code. I need the Monad’s bind (>>=) to be able to stop the reformat.

Don't reformat hard drive? Yes No

.........your hard drive C: has been successfully reformatted.
"OK I didn't"

More Related Contents:

  1. Good examples of Not a Functor/Functor/Applicative/Monad?
  2. Applicatives compose, monads don’t
  3. Difference between Monad and Applicative in Haskell
  4. How to interpret bind/>>= of the function instance?
  5. Why should Applicative be a superclass of Monad?
  6. Monads with Join() instead of Bind()
  7. Operating on a return from a Maybe that contains “Just”
  8. A Haskell function of type: IO String-> String
  9. IO happens out of order when using getLine and putStr
  10. What is indexed monad?
  11. Distinction between typeclasses MonadPlus, Alternative, and Monoid?
  12. Should do-notation be avoided in Haskell?
  13. Applicative functor evaluation is not clear to me
  14. Why does haskell’s bind function take a function from non-monadic to monadic
  15. Expressing do block using only monadic bind syntax
  16. Why do we need monads?
  17. Defining a new monad in haskell raises no instance for Applicative
  18. What are Alternative’s “some” and “many” useful for?
  19. What is a monad?
  20. Concrete example showing that monads are not closed under composition (with proof)?
  21. How to use (->) instances of Monad and confusion about (->)
  22. Why are side-effects modeled as monads in Haskell?
  23. Converting IO Int to Int
  24. Monoidal Functor is Applicative but where is the Monoid typeclass in the definition of Applicative?
  25. Haskell – depth for each node in binary tree using Reader monad
  26. How to return a pure value from a impure method
  27. How does the ST monad work?
  28. In what sense is the IO Monad pure?
  29. How Monads are considered pure?
  30. Is Haskell’s mapM not lazy?

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