Monads and Decorator pattern

Question

I'm new to functional programming. While following a tutorial on Monads, I got to think it as the analogy of Decorator pattern in OOP. Am I correct or is there any design pattern that closely resembles Monads ?

Answer 1

Monads are definitely not decorators. There isn't a standard OOP pattern in common use that is a direct analogy of monads. Though you can implement monads in OOP just fine, see e.g.:

• http://logicaltypes.blogspot.sg/2011/09/monads-in-java.html

The best Clojure-based monad tutorial I know of is this video series by Brian Marick:

• https://vimeo.com/20717301

I suggest watching this through - it's a pretty good introduction with lots of examples.

Answer 2

you can start thinking about monads as about "overridable semicolons", due to the known syntax-sugar in Haskell

in general, that does mean you can have same control structure (code block) doing different things depending on which monad is currently used

qick example in Haskell

import Data.Maybe
import Data.List


funcMaybe x = do
  z <- x
  return $ z * z

funcList x = do
  z <- x
  return $ z * z

runMaybe = funcMaybe ( Just 5 )

runList = funcList [ 5, 6 ]

being executed in GHCI, it will prompt

ghci> runMaybe 
Just 25
ghci> runList 
[25, 36]

as you can see, the same code block produces different results - List in one case and Maybe in another case, wrapped into appropriate data structure