add EsSi solution (#30)

Author: d1skort

MonadsAndEffects/3.3/essi.hs

@@ -0,0 +1,38 @@
+module Essi where
+
+import Control.Monad (when)
+import Control.Monad.Trans (lift)
+import Control.Monad.Trans.State
+import Control.Monad.Trans.Except
+
+
+tickCollatz :: State Integer Integer
+tickCollatz = do
+  n <- get
+  let res = if odd n then 3 * n + 1 else n `div` 2
+  put res
+  return n
+
+
+type EsSi = ExceptT String (State Integer)
+
+
+runEsSi :: EsSi a -> Integer -> (Either String a, Integer)
+runEsSi = runState . runExceptT
+
+
+go :: Integer -> Integer -> State Integer Integer -> EsSi ()
+go lower upper s = do
+  n <- lift get
+  let next = execState s n
+  lift $ put next
+  when (next >= upper) (throwE "Upper bound")
+  when (next <= lower) (throwE "Lower bound")
+
+
+go2 :: Integer -> Integer -> State Integer Integer -> EsSi ()
+go2 lower upper s = do
+  lift s
+  n <- lift get
+  when (n >= upper) (throwE "Upper bound")
+  when (n <= lower) (throwE "Lower bound")