Re: [Haskell-cafe] What's the advantage of writing Haskell this way?

From: John Ky Sent: Monday, May 30, 2011 8:01 AM

Hi all,

I'm trying to learn about enumerators by reading this paper and came across some code on page 2 that I found hard to digest, but I think I finally got it:

Hi John. These programs should behave identically, and I think your version should be preferred. This first code uses some class methods like mconcat, but it seems to always be used on the list in Chunks, so it will only ever use the definition for list, which is equivalent to what you wrote directly in the second code. The result may not be useful, but to understand this more thoroughly you might try parametrizating the definition of Stream so the use of more general operators actually means something. Perhaps data Stream m a =   Chunks (m a)   | EOF I think you would want Monad and MonadPlus on m.

import Data.Monoid

...

data Stream a = Chunks [a] | EOF deriving (Show, Eq)

instance Monad Stream where return = Chunks . return Chunks xs >>= f = mconcat (fmap f xs) EOF >>= _ = EOF

instance Monoid (Stream a) where mempty = Chunks mempty mappend (Chunks xs) (Chunks ys) = Chunks (xs ++ ys) mappend _ _ = EOF

I guess, it shows my lack of experience in Haskell, but my question is, why is writing the code this way preferred over say writing it like this:

import Data.Monoid

...

data Stream a = Chunks [a] | EOF deriving (Show, Eq)

instance Monad Stream where return x = Chunks [x] Chunks xs >>= f = mconcat (fmap f xs) EOF >>= _ = EOF

instance Monoid (Stream a) where mempty = Chunks [] mappend (Chunks xs) (Chunks ys) = Chunks (xs ++ ys) mappend _ _ = EOF

Cheers,

-John

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