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
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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