Sticking with the Person example, we might want to use a dynamically-bound function to specify a different greeting message for different values of Person:

data Person = Person { name :: String , greet :: Person -> String }

anne = Person "Anne" (\p -> "Howdy, " ++ name p ++ ".")
bob = Person "Bob" (\p -> "Hi, " ++ name p ++ "! I'm Bob")
carol = Person "Carol" (\_ -> "'Sup?")

main = do
    putStrLn $ anne `greet` bob
    putStrLn $ bob `greet` carol
    putStrLn $ carol `greet` anne

You could even get really crazy and construct the greeting function at runtime:

main = do
    putStr "Enter a name: "
    n <- getLine
putStrL"Enter a greeting: "
greeting <- getLine

let user = Person n (\p -> concat [greeting, ", ", name p, "!"])
bob = Person "Bob" (\p -> concat ["Hello, ", name p, "!"])

    putStrLn $ bob `greet` user
    putStrLn $ user `greet` bob

$ runhaskell test.hs
Enter a name: Morbo
Enter a greeting: I WILL DESTROY YOU
Hello, Morbo!
I WILL DESTROY YOU, Bob!

On Tue, 5 May 2015 at 08:38 Shishir Srivastava <shishir.srivastava@gmail.com> wrote:

Hi Brandon,

Thanks for your response. My example was just a bad turnout which I conjured up using tutorials and playing with it.

I was going to follow up my question with the possible practical use of why and where someone would use such a construct to wrap a function inside a new data-type.

For all that matters I could have used 'length' function directly to get the same output.

I appreciate that you already have given the practical example but anything more basic for beginners to highlight the usage would be helpful.

Thanks,
Shishir

On Tue, May 5, 2015 at 4:28 PM, Shishir Srivastava <shishir.srivastava@gmail.com> wrote:
Thanks Matthew - that was crisp !

Cheers,
Shishir

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