SYB: can a map over the result of listify be rewritten with a gfoldl?

Question

Can I use SYB's gfoldl to do the map over the result of listify in one go?

Consider for example the following code:

extractNums :: Expr -> [Int]
extractNums e = map numVal $ listify isNum e

  where isNum :: Expr -> Bool
        isNum (Num _) = True
        isNum _       = False

        numVal :: Expr -> Int
        numVal (Num i) = i
        numVal _       = error "Somehow filter did not work?"

I do not like that in the numVal function I have to consider the different data constructors of the Expr type while I am only interested in the Num constructor. I rather would replace isNum and numVals with something like the vals function below:

    vals :: [Int] -> Expr -> [Int]
    vals xs (Num x) = x : xs
    vals xs _       = xs

Can this be done with gfoldl? How?

Answer 1

Function listify is defined as

-- | Get a list of all entities that meet a predicate
listify :: Typeable r => (r -> Bool) -> GenericQ [r]
listify p = everything (++) ([] `mkQ` (\x -> if p x then [x] else []))

which is similar to filter. We could create an alternative that resembles mapMaybe which combines map and filter you need into one:

import Data.Generics
import Data.Generics.Schemes
import Data.Maybe (maybeToList)
import Data.Typeable

listify' :: (Typeable t) => (t -> Maybe r) -> GenericQ [r]
listify' f = everything (++) ([] `mkQ` (maybeToList . f))

Then your example could be expressed as

numVal :: Expr -> Maybe Int
numVal (Num i) = Just i
numVal _       = Nothing

test :: Expr -> [Int]
test = listify' numVal