Update problem 79

ninety-nine-haskell.cabal

@@ -54,6 +54,7 @@ library
     , psqueues
     , random
     , split
+    , transformers
     , vector
   default-language: Haskell2010
 

package.yaml

@@ -53,6 +53,7 @@ library:
   - random
   - array
   - mtl
+  - transformers
   - psqueues
   - hashable
   - vector

src/Monads.hs

@@ -1,5 +1,4 @@
 {-# LANGUAGE DerivingStrategies #-}
-{-# LANGUAGE FlexibleContexts #-}
 
 module Monads
   ( Operator (..),
@@ -13,9 +12,10 @@ where
 import qualified Control.Monad as M
 import Control.Monad.State (StateT)
 import qualified Control.Monad.State as S
+import Control.Monad.Trans.Maybe (MaybeT)
+import qualified Control.Monad.Trans.Maybe as MbT
 import Control.Monad.Writer (Writer)
 import qualified Control.Monad.Writer as W
-import qualified Data.Maybe as Mb
 import Data.Monoid (Sum (..))
 import qualified Data.Monoid as Md
 
@@ -151,22 +151,25 @@ data Element = Operator Operator | Operand Integer deriving stock (Show, Eq)
 
 type Stack = [Integer]
 
-type Logger = Writer [(Stack, Maybe Operator)]
+type Logs = [(Stack, Maybe Operator)]
 
 type Result = Maybe Integer
 
+type Calculation = MaybeT (StateT Stack (Writer Logs)) Integer
+
 calculatePostfix :: [Element] -> (Result, [(Stack, Maybe Operator)])
 calculatePostfix xs = (res, logs)
   where
-    ((res, _), logs) = W.runWriter $ S.runStateT (calc xs) []
+    ((res, _), logs) = W.runWriter $ S.runStateT (MbT.runMaybeT (calc xs)) []
 
-calc :: [Element] -> StateT Stack Logger Result
-calc [] = S.gets result
-calc (Operand n : xs) = S.get >>= loop xs Nothing . (n :)
-calc (Operator op : xs) =
-  S.get >>= Mb.maybe (return Nothing) (loop xs (Just op)) . runOp op
+calc :: [Element] -> Calculation
+calc elems =
+  S.get >>= case elems of
+    [] -> result
+    (Operand n : xs) -> loop xs Nothing . (n :)
+    (Operator op : xs) -> runOp op M.>=> loop xs (Just op)
 
-loop :: [Element] -> Maybe Operator -> Stack -> StateT Stack Logger Result
+loop :: [Element] -> Maybe Operator -> Stack -> Calculation
 loop xs op s = W.tell [(s, op)] >> S.put s >> calc xs
 
 result :: (MonadFail m) => Stack -> m Integer