On Sat, Mar 22, 2014 at 05:51:55PM +0100, martin wrote:
Hello all,
when I groupBy a list in order to cluster elements which satisfy some sort of equality, I usually have to sort the list first, which requires Ord. However groupBy itself does not require Ord, but just Eq.
How can I groupBy a List whose elements are only instances of Eq but not of Ord?
Well, you can just call groupBy on it. =) But I assume you mean that you still want to group together all equal elements. You could do something like groupEq [] = [] groupEq (a:rest) = (a:as) : groupEq bs where (as,bs) = partition (==a) rest and you could easily generalize it to take a binary predicate, like groupBy, as well. If you want to get a little fancier and avoid the explicit recursion, you can use Data.List.Split.chop (from the 'split' package), which provides a generic way of recursively processing a list: chop :: ([a] -> (b,[a])) -> [a] -> [b] like so: import Data.List.Split (chop) import Control.Arrow (first) groupEq = chop (\(a:rest) -> first (a:) (partition (==a) rest)) -Brent