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
