Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 106, Issue 38
Message: 12 Date: Wed, 27 Jun 2012 00:19:30 +0200 From: Tillmann Rendel ren...@informatik.uni-marburg.de Subject: Re: [Haskell-cafe] Martin Odersky on What's wrong with Monads Cc: haskell-cafe@haskell.org Message-ID: 4fea3572.5060...@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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 106, Issue 38
On 27/06/2012, at 12:51 PM, John Lato wrote: data Tree a = Leaf a | Branch (Tree a) ( Tree a) deriving (Foldable, Show) While I am familiar with deriving (Show), I am not familiar with deriving (Foldable), which looks rather useful. http://www.haskell.org/ghc/docs/7.4.2/html/users_guide/deriving.html just says With -XDeriveFoldable, you can derive instances of the class Foldable, defined in Data.Foldable. but it provides no details. Would you care to explain more about deriving (Foldable)? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 106, Issue 38
On Wed, Jun 27, 2012 at 9:15 AM, Richard O'Keefe o...@cs.otago.ac.nz wrote: On 27/06/2012, at 12:51 PM, John Lato wrote: data Tree a = Leaf a | Branch (Tree a) ( Tree a) deriving (Foldable, Show) While I am familiar with deriving (Show), I am not familiar with deriving (Foldable), which looks rather useful. http://www.haskell.org/ghc/docs/7.4.2/html/users_guide/deriving.html just says With -XDeriveFoldable, you can derive instances of the class Foldable, defined in Data.Foldable. but it provides no details. Would you care to explain more about deriving (Foldable)? There's not much to explain, DeriveFoldable basically does just that; automatically provide an instance of the Foldable class for a data type. I think the original proposal for DeriveFoldable was from Twan van Laarhoven, http://www.mail-archive.com/haskell-prime@haskell.org/msg02116.html, and there's a little bit of history on GHC's trac, http://hackage.haskell.org/trac/ghc/ticket/2953. The current implementation probably hasn't changed much since Simon PJ's original patch, although there's probably substantial overlap with ghc's generics these days. As for the Foldable class itself, the docs at http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-Foldable.html are pretty good. It lets you perform a number of folds (left, right, monoidal) over arbitrary datatypes, not just lists. I think that's about it. Although I'm not the best person to explain either the DeriveFoldable algorithm or the different uses of folds; maybe someone else would be able to fill in anything I've missed. John L. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 106, Issue 38
On 27/06/2012, at 3:18 PM, John Lato wrote: On Wed, Jun 27, 2012 at 9:15 AM, Richard O'Keefe o...@cs.otago.ac.nz wrote: On 27/06/2012, at 12:51 PM, John Lato wrote: data Tree a = Leaf a | Branch (Tree a) ( Tree a) deriving (Foldable, Show) While I am familiar with deriving (Show), I am not familiar with deriving (Foldable), which looks rather useful. http://www.haskell.org/ghc/docs/7.4.2/html/users_guide/deriving.html just says With -XDeriveFoldable, you can derive instances of the class Foldable, defined in Data.Foldable. but it provides no details. Would you care to explain more about deriving (Foldable)? There's not much to explain, DeriveFoldable basically does just that; automatically provide an instance of the Foldable class for a data type. That was sufficiently obvious, yes. The question remains, ***WHAT*** instance? I think the original proposal for DeriveFoldable was from Twan van Laarhoven, http://www.mail-archive.com/haskell-prime@haskell.org/msg02116.html, which goes into a great deal of detail about what deriving (Functor) does, but none whatsoever about what deriving (Foldable) does. Even for Functor, another 3 or 4 nontrivial examples would be nice. and there's a little bit of history on GHC's trac, http://hackage.haskell.org/trac/ghc/ticket/2953. The current implementation probably hasn't changed much since Simon PJ's original patch, although there's probably substantial overlap with ghc's generics these days. That trac entry contains one sentence that seems to still apply: What is missing is a section in the user manual describing the changes. It refers to section 8.5, which is now 7.5, and there is still no adequate documentation there. As for the Foldable class itself, the docs at http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-Foldable.html are pretty good. Yes, Foldable *is* documented. However, that page says nothing whatever about deriving (Foldable). ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
