Change foldM type signature to more closely match foldl (#165)

* Change foldM type signature to more closely match foldl

Author: milesfrain

src/Data/List.purs

@@ -760,6 +760,6 @@ transpose ((x : xs) : xss) =
 --------------------------------------------------------------------------------
 
 -- | Perform a fold using a monadic step function.
-foldM :: forall m a b. Monad m => (a -> b -> m a) -> a -> List b -> m a
-foldM _ a Nil = pure a
-foldM f a (b : bs) = f a b >>= \a' -> foldM f a' bs
+foldM :: forall m a b. Monad m => (b -> a -> m b) -> b -> List a -> m b
+foldM _ b Nil = pure b
+foldM f b (a : as) = f b a >>= \b' -> foldM f b' as

src/Data/List/Lazy.purs

@@ -727,12 +727,12 @@ transpose xs =
 --------------------------------------------------------------------------------
 
 -- | Perform a fold using a monadic step function.
-foldM :: forall m a b. Monad m => (a -> b -> m a) -> a -> List b -> m a
-foldM f a xs =
+foldM :: forall m a b. Monad m => (b -> a -> m b) -> b -> List a -> m b
+foldM f b xs =
     case uncons xs of
-         Nothing -> pure a
-         Just { head: b, tail: bs } ->
-                       f a b >>= \a' -> foldM f a' bs
+         Nothing -> pure b
+         Just { head: a, tail: as } ->
+                       f b a >>= \b' -> foldM f b' as
 
 -- | Perform a right fold lazily
 foldrLazy :: forall a b. Z.Lazy b => (a -> b -> b) -> b -> List a -> b

src/Data/List/NonEmpty.purs

@@ -299,5 +299,5 @@ zip = zipWith Tuple
 unzip :: forall a b. NonEmptyList (Tuple a b) -> Tuple (NonEmptyList a) (NonEmptyList b)
 unzip ts = Tuple (map fst ts) (map snd ts)
 
-foldM :: forall m a b. Monad m => (a -> b -> m a) -> a -> NonEmptyList b -> m a
-foldM f a (NonEmptyList (b :| bs)) = f a b >>= \a' -> L.foldM f a' bs
+foldM :: forall m a b. Monad m => (b -> a -> m b) -> b -> NonEmptyList a -> m b
+foldM f b (NonEmptyList (a :| as)) = f b a >>= \b' -> L.foldM f b' as