On Feb 5, 2010, at 12:34 , Aran Donohue wrote:

data Binding = Binding Var (Either Value [Value])

representing a variable bound either to a fixed value or that has a list of possible values.

I'd like to perform an operation on say, the fixed-value members of a list of bindings. Data.Either has "partitionEithers"---I'd essentially like to use partitionEithers, but in a way that it "peeks" into the value field of the binding. For the sake of argument, let's say I can't or can't modify Binding to move the Either to the outside.

What would be an idiomatic Haskell way to accomplish this? Currently I've got "liftedPartitionEithers :: [a] -> (a -> Either b c) -> ([a], [a])" which is my own version of partitionEithers that calls a selector first. Another option would be to map each Binding to a new datatype that has the Either on the outside, use partitionEithers, and map back.

Hm. Does it make sense to make this a Functor?

instance Functor Binding where fmap f (Binding v e) = Binding v (f e)

-- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH

On Fri, Feb 5, 2010 at 10:34 AM, Aran Donohue wrote:

What would be an idiomatic Haskell way to accomplish this? Currently I've got "liftedPartitionEithers :: [a] -> (a -> Either b c) -> ([a], [a])" which is my own version of partitionEithers that calls a selector first.

Since you are not using b or c anywhere else, the only thing you care about in that Either is whether it is Left or Right. Which makes it seem much more like a Bool. After this conversion, I can hoogle for your signature. http://haskell.org/hoogle/?hoogle=[a]+-%3E+%28a+-%3E+Bool%29+-%3E+%28[a]%2C[a]%29 Which gives, among other things, Data.List.partition :: (a -> Bool) -> [a] -> ([a],[a]). Without more details about the precise thing you want to accomplish, I don't know what else to say. Many idioms are about the details of the problem, even down to argument order. Luke