fix #357

tests/comp/Issue357.fr

@@ -0,0 +1,13 @@
+{-- 
+    As outlined in 'https://github.com/Frege/frege/issues/357 #357',
+    the compiler decides that 'myFoldM' should be strict, despite the tail call
+    'Maybe.>>=' returns lazy. This is unfortunate, because the recursion happens
+    on the stack, instead through a Thunk returned by >>=
+-}
+module tests.comp.Issue357 where
+
+myFoldM :: (b -> a -> Maybe b) -> b -> [a] -> Maybe b
+myFoldM f z (x:xs) = f z x >>= \acc -> myFoldM f acc xs
+myFoldM _ z []     = pure z
+
+main = println $ myFoldM (\r _ -> Just $! succ r) 0 [1..5000]