class Person:
def __init(fname, lname):
this.fname = fname
this.lname = lname
def greet():
print "Good day, " + fname + ", " + lname
if __name__ == "__main__":
p = Person("saravana", "kumar")
p.greet()
outputs,
hello saravana, kumar
How can this simple python code be ported to haskell? I am keen on learning oops concept like initializing object states and accessing them through methods in the haskell language.
First don't think with an OOPS mind, that will just not work out with Haskell and would cause you pain.
Try to think the problem in terms of the Haskell way,
data Person = Person { fname :: String, lname :: String } deriving Show
greet :: Person -> String
greet p = "Good day, " ++ fname p ++ ", " ++ lname p
Demo in ghci:
λ> greet $ Person "Erik" "Meijer"
"Good day, Erik, Meijer"
See how simple was that ?
greet :: Person -> IO (), greet p = putStr $ "Good day, " ... is still pretty easy to understand what it's doing. (Even you don't entirely understand what IO () means or the exact syntax.)lets try to port it to Haskell literally..
data Person = Person { fname :: String
, lname :: String
}
In your Python code, any Person object can perform an action greet() that does some IO which gets translated to Haskell as:
greet :: Person -> IO ()
greet person = putStrLn $ (fname person) ++ " " ++ (lname person)
then you would write main as:
main :: IO ()
main = greet $ Person "Ben" "Tennyson"
Now, what's wrong in doing it that way ? - Purity.
Haskell idioms provides ways to keep pure and impure code separate. lets try to do it Haskell way...
Create a Category Human whose elements could be a Person/Student/Employee or whatever who all share a common action greet()
class Human a where
greet :: a -> String
We can also create a Show instance for our Person type:
instance Show Person where
show person = (fname person) ++ " " ++ (lname person)
then create a Human instance for Person implementing greet() action:
instance Human Person where
greet person = "Hello, " ++ (show person)
main :: IO ()
main = putStrLn . greet $ Person "Ben" "Tennyson"
-- λ main
-- Hello, Ben Tennyson
Human typeclass is sort of isomorphic to Show.