While in examining binding article, we start from bind operator, then convert it to do notation. This article start from do, and revert it back to monadic code using a few operator.
I also add Kleiski Fish Operator, that is very useful as a shortcut in a do notation.
Explaining Monad
This tutorial/ guidance/ article is one of some parts.

Overview: Summary.

References: About Monad.

Examining Bind: Bind
>>=
operator. Hello World Example. 
Examining Bind:
<*>
and<$>
operators. Personal Notes. Example using Number. 
Monadic Operator: Fish
>=>
operator.
The first one is overview, then some references. The last three parts is all about Example Code.
Processing Symbol
I you need to know more about operator,
you can just ask hoogle
.
And if you are curious for another operator, you can read this article and have fun.
Monadic Action
Reference about then >>
operator
and bind >>=
operator can be found here.
Function
Think Function in Haskell as Math Equation
Suppose we have this very short function.
greetingFunction :: String
greetingFunction = "Hello " ++ "World"
main = do
putStrLn greetingFunction
Action
Think Action in Haskell as Procedure, Sequence of Command
We can rewrite this as an action
greetingAction :: IO ()
greetingAction = do
putStr "Hello "
putStr "World"
putStrLn ""
main = do
greetingAction
Then Operator
Removing the do
special notation,
we can desugar the greetingAction
using then >>
operator as below.
greetingAction :: IO ()
greetingAction =
putStr "Hello " >>
putStr "World" >>
putStrLn ""
Writing it oneliner, would make this action looks exactly like a function. And in fact, it is just a function.
greetingAction = putStr "Hello " >> putStr "World" >> putStrLn ""
then >>
operator is
just like bind >>=
operator,
except it ignore input.
We need other example containing input.
do
notation does avoid coding horror,
but sooner or later we need to know what is inside.
Bind Operator
Consider this action.
Each command has a Maybe String
result type.
import Data.Maybe
greetingStr :: Maybe String
greetingStr = do
str1 < Just "Hello "
str2 < Just "World"
Just (str1 ++ str2)
main = putStrLn $ fromMaybe "Nothing" greetingStr
We can desugar the action above
into vanilla monadic code without do
:
greetingStr :: Maybe String
greetingStr =
Just "Hello " >>= \str1 >
Just "World" >>= \str2 >
Just (str1 ++ str2)
Or you can make it oneliner vanilla monadic code.
greetingStr = Just "Hello " >>= \str1 > Just "World" >>= \str2 > Just (str1 ++ str2)
Here is another IO example, showing you home directory.
import System.Directory
main = getHomeDirectory >>= putStrLn
There is also good example here
Operation inside Bind
Monads is way to unwrap stuff, do something about it, and wrap the result. Or better, Monad give access for method, to work inside wrapped stuff, without unwrapping it.
This list, will show Monad is overloaded for different types. Every monad has its own implementation.
intList :: [Int]
intList = do
x < [3..5]
[x*2]
main = putStrLn $ show intList
This will display
[6,8,10]
It can be desugared into oneliner vanilla monadic code.
intList = [3..5] >>= \x > [x*2]
The Monad Class
It is clear that sequence command in do notation is
just functions chained together
by bind >>=
operator
or then >>
operator.
Now the next question. What is these two operators has to do with monad ? So here it is the definition of monad in Prelude 4.9.1.0
class Applicative m => Monad m where
(>>=) :: forall a b. m a > (a > m b) > m b
(>>) :: forall a b. m a > m b > m b
m >> k = m >>= \_ > k
return :: a > m a
return = pure
fail :: String > m a
fail s = errorWithoutStackTrace s
Those two are Monad Operators.
Monadic Composition
What other practical used, Monads are good for?
There are many I guess. One of them is Kleiski Arrow.
Kleiski Arrow does function composition, just like .
,
except it perform monadic effects.
Reference about Kleiski Arrow >=>
and <=<
operator can be found here.
I looks like fish, so we can call this fish
operator.
Function
Consider this function
greetStr :: String > String
greetStr text = "Hello " ++ text
We can avoid closing bracket
using function application $
.
main = putStrLn $ greetStr "World"
Or we can join two function
using function composition .
.
main = (putStrLn . greetStr) "World"
Action
Consider this action
greetStrIO :: String > IO String
greetStrIO text = return ("Hello " ++ text)
Since the output is IO String
, while expected input type
from puStrLn
is String
.
The composition have to be Monadic. Kleiski Arrow perform this well.
import Control.Monad
main = (putStrLn <=< greetStrIO) "World"
Or Kleisli Composition, using reversed arrow.
import Control.Monad
main = (greetStrIO >=> putStrLn) "World"
With confidence.
Thank you for Reading.
We shall meet again.
Farewell.