Message: 12 Date: Wed, 27 Jun 2012 00:19:30 +0200 From: Tillmann Rendel Subject: Re: [Haskell-cafe] Martin Odersky on "What's wrong with        Monads" Cc: haskell-cafe@haskell.org Message-ID: <4FEA3572.5060200@informatik.uni-marburg.de> Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Hi,

MightyByte wrote:

...

Of course every line of your program that uses a Foo will change if you switch to IO Foo instead.

But we often have to also change lines that don't use Foo at all. For example, here is the type of binary trees of integers:

  data Tree = Leaf Integer | Branch (Tree Integer) (Tree Integer)

A function to add up all integers in a tree:

  amount:: Tree -> Integer   amount (Leaf x) = x   amount (Branch t1 t2) = amountt1 + amountt2

All fine so far. Now, consider the following additional requirement: "If the command-line flag --multiply is set, the function amount computes the product instead of the sum."

In a language with implicit side effects, it is easy to implement this. We just change the third line of the amount function to check whether to call (+) or (*). In particular, we would not touch the other two lines.

How would you implement this requirement in Haskell without changing the line "amount (Leaf x) = x"?

Why would you do that even in an imperative language? The program logic turns into a spaghetti mess, and it's much harder to test. I would write Tree like this: data Tree a = Leaf a | Branch (Tree a) ( Tree a) deriving (Foldable, Show) and instead of an "amount" function I would use a fold. If you like, you can use "ala" from Control.Newtype *Main Data.Foldable Data.Monoid> let t1 = Branch (Leaf 1) (Branch (Leaf 4) (Leaf 5)) :: Tree Int *Main Data.Foldable Data.Monoid> ala Sum foldMap t1 10 *Main Data.Foldable Data.Monoid> ala Product foldMap t1 20 Now the amount calculation can be something like amount :: Num a => Tree a -> IO a amount tree = multFlag >>= \b -> if b then ala Product foldMap tree else ala Sum foldMap tree although I probably wouldn't actually write it unless it was called in more than one place. There are other ways to write it too; the important part is that checking the configuration is completely separate from the tree traversal. Plus, if it changes again (now there's another flag that says to ignore values == n or something, you can use the same fold, just change the function that's passed to it (or the monoid instance if you're using that). Plus, this type of code is much simpler to debug, test, and maintain than imperative-style magic functions. Cheers, John