foldlM/foldrM doesn't work with Maybe and other pure monads #357
Description
The following code results in StackOverflowError:
import frege.data.Foldable (foldlM)
main = do
let m :: Maybe Int
m = foldlM (\r _ -> pure $! succ r) 0 [1..5000]
println mException in thread "main" java.lang.StackOverflowError
at Main$$Lambda$17/930990596.<init>(Unknown Source)
at Main$$Lambda$17/930990596.get$Lambda(Unknown Source)
at Main.lambda$null$1(Main.java:85)
at frege.prelude.PreludeBase.lambda$$$excl$18(PreludeBase.java:16226)
at frege.run8.Thunk.call(Thunk.java:231)
at Main.lambda$null$2(Main.java:95)
at frege.run8.Thunk.call(Thunk.java:231)
at frege.prelude.Maybe$IMonad_Maybe.ƒ$gt$gt$eq(Maybe.java:496)
at frege.data.Foldable$1Let$10255.f$tick$8372(Foldable.java:1583)
at frege.data.Foldable.lambda$foldlM$47(Foldable.java:1593)
at frege.run8.Thunk.call(Thunk.java:231)
at frege.prelude.Maybe$IMonad_Maybe.ƒ$gt$gt$eq(Maybe.java:497)
at frege.data.Foldable$1Let$10255.f$tick$8372(Foldable.java:1583)
at frege.data.Foldable.lambda$foldlM$47(Foldable.java:1593)
at frege.run8.Thunk.call(Thunk.java:231)
at frege.prelude.Maybe$IMonad_Maybe.ƒ$gt$gt$eq(Maybe.java:497)
...
foldrM has a similar problem. Either, [] cause StackOverflowError, too.
Meanwhile, IO doesn't suffer from this:
import frege.data.Foldable (foldlM)
main = do
let m :: IO Int
m = foldlM (\r a -> pure $! succ r) 0 [1..5000]
println =<< m5000
Workaround
UPDATE Having an undefined case changes the return type of Java method to Lazy, so the following seem to work for both finite and infinite lists (only for Maybe):
foldlM' :: (b -> a -> Maybe b) -> b -> [a] -> Maybe b
foldlM' f z (x:xs) = f z x >>= \acc -> foldlM' f acc xs
foldlM' _ z [] = pure z
foldlM' _ _ _ = undefined
main = do
println $ foldlM' (\r _ -> Just $! succ r) 0 [1..100000]
println $ foldlM' (\_ _ -> Nothing) 0 [1..]Just 100000
Nothing