cataMの評価順

Question

次のコードで、cataMはツリートップダウンをトラバースすることは可能です（現在の場合はボトムアップではありません）。

私は違っfoldMapを実装する必要がありますが、branchは子供ではありませんtのインスタンスを持っていないので、子供たちの前branchノード自体をどのように処理するかを推測しますか？

module Catatree where 

import Data.Foldable 
import Data.Traversable 
import Data.Monoid 
import Data.Generic 
import Prelude 
import Control.Monad.Writer 
import Control.Monad.Eff (Eff) 
import Control.Monad.Eff.Console (CONSOLE, log, logShow) 

import Data.Functor.Mu (Mu) 
import Matryoshka (class Corecursive, class Recursive, Algebra, AlgebraM, cata, embed, cataM, project) 

data TreeF a t = Leaf | Branch a (Array t) 

type IntTree = Mu (TreeF Int) 

derive instance treeGeneric :: (Generic a, Generic t) => Generic (TreeF a t) 
derive instance treeFunctor :: Functor (TreeF a) 

instance showTree :: (Generic a, Generic t) => Show (TreeF a t) where 
    show = gShow 

instance treeTraversable :: Traversable (TreeF a) where 
    -- traverse :: forall a b m. Applicative m => (a -> m b) -> t a -> m (t b) 
    traverse f Leaf = pure Leaf 
    traverse f (Branch a children) = Branch a <$> traverse f children 
    sequence f = sequenceDefault f 


instance treeFoldable :: Foldable (TreeF a) where 
    foldr f = foldrDefault f 
    foldl f = foldlDefault f 
    -- foldMap :: forall a m. Monoid m => (a -> m) -> f a -> m 
    foldMap f Leaf = mempty 
    foldMap f (Branch a children) = foldMap f children 

evalM :: AlgebraM (Writer (Array String)) (TreeF Int) Int 
evalM Leaf = do 
    tell $ [ "visiting leaf " ] 
    pure 4 
evalM (Branch a children) = do 
    tell $ [ "visiting branch " <> show a ] 
    pure 2 

runM :: forall t. Recursive t (TreeF Int) => t -> Writer (Array String) Int 
runM = cataM evalM 

branch :: forall t. Corecursive t (TreeF Int) => Int -> Array t -> t 
branch i children = embed (Branch i children) 

exp :: IntTree 
exp = branch 3 [(branch 1 []), (branch 2 [])] 

main :: forall eff. Eff (console :: CONSOLE | eff) Unit 
main = do 
    logShow $ runWriter $ runM exp 
    -- outputs (Tuple 2 ["visiting branch 1","visiting branch 2","visiting branch 3"]) 

Answer 1

あなたもマトリョーシカによって提供される機能topDownCataMを探しているように聞こえます。