Merge pull request #118 from matthewleon/foldr'

foldr': lazy right folds

Author: paf31

src/Data/List/Lazy.purs

@@ -86,6 +86,7 @@ module Data.List.Lazy
   , transpose
 
   , foldM
+  , foldrLazy
 
   , module Exports
   ) where
@@ -716,3 +717,11 @@ foldM f a xs =
          Nothing -> pure a
          Just { head: b, tail: bs } ->
                        f a b >>= \a' -> foldM f a' bs
+
+-- | Perform a right fold lazily
+foldrLazy :: forall a b. Z.Lazy b => (a -> b -> b) -> b -> List a -> b
+foldrLazy op z = go
+  where
+    go xs = case step xs of
+      Cons x xs' -> Z.defer \_ -> x `op` go xs'
+      Nil -> z

test/Test/Data/List/Lazy.purs

@@ -7,7 +7,7 @@ import Control.Monad.Eff (Eff)
 import Control.Monad.Eff.Console (CONSOLE, log)
 
 import Data.Lazy as Z
-import Data.List.Lazy (List, nil, cons, foldl, foldr, foldMap, singleton, transpose, take, iterate, filter, uncons, foldM, range, unzip, zip, length, zipWithA, replicate, repeat, zipWith, intersectBy, intersect, deleteBy, delete, unionBy, union, nubBy, nub, groupBy, group, partition, span, dropWhile, drop, takeWhile, slice, catMaybes, mapMaybe, filterM, concat, concatMap, reverse, alterAt, modifyAt, updateAt, deleteAt, insertAt, findLastIndex, findIndex, elemLastIndex, elemIndex, init, tail, last, head, insertBy, insert, snoc, null, replicateM, fromFoldable, (:), (\\), (!!))
+import Data.List.Lazy (List, nil, cons, foldl, foldr, foldMap, singleton, transpose, take, iterate, filter, uncons, foldM, foldrLazy, range, unzip, zip, length, zipWithA, replicate, repeat, zipWith, intersectBy, intersect, deleteBy, delete, unionBy, union, nubBy, nub, groupBy, group, partition, span, dropWhile, drop, takeWhile, slice, catMaybes, mapMaybe, filterM, concat, concatMap, reverse, alterAt, modifyAt, updateAt, deleteAt, insertAt, findLastIndex, findIndex, elemLastIndex, elemIndex, init, tail, last, head, insertBy, insert, snoc, null, replicateM, fromFoldable, (:), (\\), (!!))
 import Data.List.Lazy.NonEmpty as NEL
 import Data.Maybe (Maybe(..), isNothing, fromJust)
 import Data.Monoid.Additive (Additive(..))
@@ -339,6 +339,10 @@ testListLazy = do
              f acc x = if x >= 10 then Nothing else Just (cons 0 acc)
           in foldM f nil infs == Nothing
 
+  log "foldrLazy should work ok on infinite lists"
+  assert let infs = iterate (_ + 1) 1
+             infs' = foldrLazy cons nil infs
+          in take 1000 infs == take 1000 infs'
 
   log "can find the first 10 primes using lazy lists"
   let eratos :: List Int -> List Int