Does Haskell have foldlM'?

Question

How does one fold over a monad strictly? Data.Foldable has the strict foldl' and the monadic foldlM , but no strict foldlM' ? Is the strictness somehow defined by the monad itself? If so, how does one work out what it is?

Imagine I must determine whether the product of an enormous list of ring elements is zero, but my ring isn't an integral domain, ie it contains zero devisors. In this case, I should tail recursively foldl my multiplication *** over the list, but return False the moment the product becomes zero, rather than waiting on the full product.

safelist :: [p] -> Bool
safelist [] = True
safelist (x:xs) = snd $ foldl' f (x,True) xs
   where  f (u,b) v = (w, b && w /= Zero)  where  w = u *** v

I could perhaps simplify this code slightly using the Maybe monad's foldlM but doing so seemingly lacks the required strictness.

Answer 1

There's no such standard function, but it's easy to define:

foldM' :: (Monad m) => (a -> b -> m a) -> a -> [b] -> m a
foldM' _ z [] = return z
foldM' f z (x:xs) = do
  z' <- f z x
  z' `seq` foldM' f z' xs

This is just the standard foldM , but with the same seq ing in it that foldl' does (compared to foldl ). It's probably not defined anywhere standard because it's not likely to be all that useful: for most monads, (>>=) is "strict" in the sense you need to use a left-fold without overflowing the stack; this is only useful when your excessive thunks are in the returned values themselves, but a useful application of foldM will perform some monadic computation with the value from the last step, making that unlikely.

I think your code is simple enough as it is; I doubt foldM' would make it any more elegant.